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Articles

Dynamical analysis of an integrated pest management predator–prey model with weak Allee effect

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Pages 218-244 | Received 22 May 2018, Accepted 21 Feb 2019, Published online: 19 Mar 2019

Abstract

In this paper, a pest management predator–prey model with weak Allee effect on predator and state feedback impulsive control on prey is introduced and analysed, where the yield of predator released and intensity of pesticide sprayed are assumed to be linearly dependent on the selected pest control level. For the proposed model, the existence and stability of the order-1 periodic orbit of the control system are discussed. Meanwhile, with the aim of minimizing the input cost in practice, an optimization model is constructed to determine the optimal quantity of the predator released and the intensity of pesticide sprayed. The theoretical results and numerical simulations indicated that the number of pests can be limited to below an economic threshold and displays periodic variation under the proposed control strategy. In addition, it indicated in numerical simulations that an order-2 periodic orbit exists for some certain parameters.

1. Introduction

Food losses due to pests and plant diseases are nowadays one of the major threats to food security, particularly in large parts of the developing world. As reported by the United Nations, the world population in 2014 was estimated at 7.2 billion, with an approximate yearly growth of 82 million, a quarter of which occurs in the least developed countries [Citation35]. This unprecedented amount of people in the world poses serious challenges for food producers and policy-makers, specially regarding the minimization of crop losses due to pests and plant diseases, which have been estimated to be as high as 40% of the world production [Citation22]. This issue has been a matter of active research for many decades, where the main challenge lies in the unavoidable trade-off between pest reduction, financial costs, effects on human health and environmental impact. Spraying chemical pesticide can kill part of the agricultural pests and control the rapid growth of pests, however, the extensive use and unreasonable abuse of pesticides destroy the structure of the agricultural ecosystem, reduce biodiversity, and lead to the residue of chemical composition in crops, threatening the health of human beings. In addition, the long-term use of pesticides also leads to pest resistance, weakens or reduces the ability of natural enemy to control pests, results in frequent and even more rampant pests, which falls into the vicious cycle that it is the more drug treatment the more difficult to govern. A large number of facts show that the side effects of chemical pesticides have challenged the single method of pest control by chemical pesticides. In fact, natural enemies of the agricultural pests also play an important role in control the quantity pests [Citation3,Citation10,Citation19,Citation36]. Therefore, the problem of pest control has necessarily to be addressed in an integrated manner, which has motivated the development of various integrated approaches, such as Integrated Pest Management (IPM) [Citation2,Citation22,Citation37].

IPM's basic principle consists in the judicious and coordinated use of multiple pest control mechanisms (e.g. biological control, cultural practices, selected chemical methods, etc.) in ways that complement one another, maintaining pest damage below acceptable economic levels, while minimizing hazards to humans, animals, plants and the environment. IPM has been proved to be more effective than the classic methods both experimentally [Citation36,Citation38] and theoretically [Citation31,Citation41]. The key concept for the implementation of a pest control programme in an IPM framework is that of economic injury level, which means the lowest pest population density that will cause economic damage. In general, it is assumed that a number of pest control mechanisms is available, for instance biological methods, cultural practices, natural enemies, habitat management, synthetic pesticides, etc. The basic decision rules rely on a predefined economic injury level and an economic threshold, which gives the pest population density above which control actions must be taken so as to prevent the pest population from reaching the economic injury level. An IPM-based pest control scheme, in its simplest form, will then require that whenever the amount of pests is less than the economic threshold only ecologically benign control measures are applied, i.e. those that enhance natural control. If natural control is not capable of preventing the pest population from reaching the economic injury level, then synthetic pesticides come into play, nevertheless, in adequate combination with environmentally friendly control measures so as to minimize the amount of pesticides released into the underlying ecosystem. In practice, however, to develop and implement an IPM-based pest control programme sustainable both in ecological and economic terms is by no means trivial tasks.

Integrated pest management may cause a radical change in biological population due to the variety of manual intervention. This phenomenon also occurs in many dynamical systems due to abrupt changes at certain instants during the evolution process. To describe these phenomena in mathematics, impulsive differential equations are a powerful tool. The research on the theory of impulsive state feedback control dynamic systems (ISFCDS) has made a great progress in recent years, and the basic theory of impulsive semi-dynamical systems, as well as the criteria for checking the existence and stability of periodic solutions of impulsive semi-dynamical systems are presented [Citation4–6,Citation8,Citation9]. Besides the theoretical study aspect of impulsive semi-dynamical systems, in application aspect, Tang et al. made a pioneer work in pest management predator–prey model with state-dependent impulse [Citation28,Citation29,Citation32]. Since then many scholars have introduced ISFCDS in predator–prey system to model the pest control action [Citation12–14,Citation20,Citation23,Citation26,Citation27,Citation30,Citation33,Citation39,Citation42,Citation43,Citation46,Citation47]. Among these studies, Jiao et al. investigated the Allee effect on a single population model with state-dependent impulsively unilateral diffusion [Citation14]. The Allee effect is a phenomenon in biology characterized by a correlation between population size or density and the mean individual fitness (often measured as per capita population growth rate) of a population or species [Citation11,Citation15,Citation17]. Many scholars investigated the rich dynamical behaviours of predator–prey model with Allee effect [Citation1,Citation7,Citation16,Citation18,Citation21,Citation24,Citation34,Citation40,Citation44,Citation45,Citation48]. To consider that the predators are difficult to seek spouses when the species has a low population density, the Allee effect on predator is introduced to model phenomenon. Thus in this work, an integrated pest management predator–prey model with Allee effect is presented by introducing the biological control threshold and the chemical control threshold, where the pest control level is selected between the two control thresholds.

The rest of the paper is organized as follows. In Section 2, the mathematical model is formulated and some preliminaries are presented. The dynamic properties of the free system are introduced in Section 3, and then followed by the control system including the existence and stability of the order-1 periodic orbit. Existence depends on the method of successive functions, and stability is by the limit method of successor point sequences and analogue of Poincaré criterion. What't more, an optimization problem is formulated to minimize the total cost in the pest control. In Section 4, the numerical simulations are carried out to verify the theoretical results. And a brief conclusion is presented in Section 5.

2. Model formulation and preliminaries

2.1. Mathematical model

Let x(t) and y(t) denote the population of the pest and the predator at time t, respectively. The intrinsic rate of increase of the pest is assumed to follow the Logistic type, i.e. r(1x(t)/K), where r is the birth rate, K is the environmental carrying capacity for the pest in absent of predator. For the species without environmental carrying capacity constraint, K can be chosen as a larger positive constant. The restriction from the predator is proportion to the pest, i.e. bx(t)y(t). The conversion rate of predators after predating pests is similar to weak Allee effect, i.e. y(t)/y(t)+m, where m is Allee effect constant. And d is the death rate of predator. The model can be formulated as follows: (1) dx(t)dt=rx(t)1x(t)Kbx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+mdy(t).(1) Considering the harm of the pest on crops, let xZX denote the pest slight harmful threshold (or biological control level) and xZD(xZX<xZD<K) denote the pest economic injury threshold (or chemical control level). To achieve the control effect, assume that the yield releases of the predator at xZX and xZD are τmax and τmin(τmax>τmin0), respectively. Suppose that the chemical control strength at xZD is pmax to the pest and qmax to the predator. To determine the optimal control threshold, an integrated pest control threshold xT is assumed to be between xZX and xZD(xZXxTxZD). Once the population of pests reaches control level xT, The corresponding pest management strategy, a certain yield of releases of predators τ(xT) and a certain strength of insecticide spraying p(xT),q(xT), should be adopted. And τ(xT),p(xT),q(xT) are linearly dependent on pest control level xT, which are as follows: (2) τ(xT)=τmax(τmaxτmin)xTxZXxZDxZX,p(xT)=pmaxxTxZXxZDxZX,q(xT)=qmaxxTxZXxZDxZX.(2) According to the effect of insecticide, the strength of insecticide spraying to the pest p(xT) is equal or greater than that to the predator q(xT).

Based on the above control strategy, an integrated pest management predator–prey model with Allee effect is obtained in the following: (3) dx(t)dt=rx(t)1x(t)Kbx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+mdy(t),x<xT,Δx(t)=p(xT)x(t)Δy(t)=q(xT)y(t)+τ(xT)x=xT.(3)

2.2. Preliminaries

For the state-dependent impulsive differential equations (4) dxdt=P(x,y)dydt=Q(x,y)ifΦ(x,y)0,Δx=α(x,y)Δy=β(x,y)ifΦ(x,y)=0,(4) where Φ(x,y)=0 describes the states at which the control strategy is taken on, α and β describe the effects of the control strategy and (x,y)ΩR2. P(x,y) and Q(x,y) are arbitrarily derivative with respect to (x,y)Ω; α and β are linearly dependent on x and y, i.e. Φx,Φy,αx,αy,βx,βy are constant, let's denote M={(x,y)|Φ(x,y)=0} and N={(x,y)|x=x+α(x,y),y=y+β(x,y),(x,y)M}.

Definition 2.1

Successor function [Citation8]

Suppose that the impulse set M and the phase set N are both line. Define the coordinate in the phase set N as follows: denote the point of intersection Q between N and x-axis as O, then the coordinate of any point A in N is defined as the distance between A and Q, and denoted by a. Let C denote the point of intersection between the trajectory starting from A and the impulse set M, and B denote the phase point of C after impulse with coordinate b. Then we define B as the successor point of A, and the successor function fsor at A is defined fsor(A)=yByA=ba, which is also continuous on N.

Remark 1

If there exists a point L(xL,yL)N such that fsor(L)=0, then the orbit starting from L forms a periodic orbit.

Assume that there exists an order-1 periodic orbit in system (Equation2), and let γL=LLLˆ denoted the order-1 periodic orbit starting from L, its parameter equation is denoted by x=φ(s), y=ϕ(s), expressed with the arc length s starting from L, where 0ssL. Since the arc length s is a function, i.e. s=s(t), where 0tTL, then the solution x=ξ(t)φ(s(t)) and y=η(t)ϕ(s(t)) is a TL order-1 periodic solution.

Lemma 2.2

Stability Criterion [Citation25,Citation33]

The TL period-1 solution x(t)=(ξ(t),η(t)) of the model (Equation2) is orbitally asymptotically stable if the convergency ratio ργL is less than one, where ργLf+I[(1+βy)ΦxβxΦy]+g+I[(1+αx)ΦyαyΦx]fIΦx+gIΦyexp0+Tfx+gy(ξ(t),η(t)),fI(gI) represents the value of f(g) at L(ξ(TL),η(TL))M, and f+I(g+I) represents the value of f(g) at L(ξ(0+),η(0+))N.

3. Dynamic properties of the control system

System (Equation1) is called the free system, and system (Equation3) is called control system. In this section, the dynamic properties of the free system will be discussed firstly, and then followed by the control system.

3.1. Dynamic properties of the free system

For the free system, the following result holds:

Lemma 3.1

The solutions of system (Equation1) is positive and bounded.

Proof.

Let (x(t),y(t)) be a solution of system (Equation1) with the initial condition (x(0)>0,y(0)>0), by the equations of system (Equation1), there is x(t)=x(0)exp0trrx(s)Kby(s)ds>0,y(t)=y(0)exp0tcx(s)y(s)y(s)+mdds>0. Thus the solutions of system (Equation1) with the initial condition (x(0)>0,y(0)>0) is positive.

Let x=(x(t),y(t)) be a solution of system (Equation1) starting from (x0,y0), where x0K. Define Γ1(x,y)xK, then dΓ1dtΓ1=0=bKy<0, which means that the orbit of system (Equation1) goes across Γ1 from right to left. In addition, let ymax=1+cbdmaxx[0,k]rx1xK+dx.

Define Γ2(x,y)(c/b)x+yymax. Then dΓ2dt|Γ2=0=cbxr1xKby+ycxyy+md=cbxr1xKby+cxy2y+md(ymaxcbx)=cbxr1xK+dcxy+cxy2y+mdymaxcbxr1xK+ddcbmaxx[0,k]rx1xK+dx<0. which means that the orbit of system (Equation1) goes across Γ2 from above to below. Thus, there exists an area Σ0{(x,y)|0xK,0yymax(c/b)x} such that x=(x(t),y(t))Σ0 for tT, where T>0 is a large number. This completes the proof.

Lemma 3.2

[Citation18]

If 0<m<A12/A2 and K>d/c, then there exist four equilibria: O(0,0),R(K,0),E1(x1,y1) and E2(x2,y2) in system (Equation1), where x1=Krc+dKA12A2m2cr,y1=r(1x1/K)b,x2=Krc+dK+A12A2m2cr,y2=r(1x2/K)b,A1=rc+dK,A2=4bcdrK. The equilibria O(0,0), E2(x2,y2) are hyperbolic saddle, R(K,0) is a stable node. The equilibrium E1(x1,y1) is stable if H1 holds: H1:

one of the following conditions holds:

  1. 4r>d>0,d/c<K<4r/c;

  2. 4r3<d<4r,K=4r/c>maxd/c,dr/c(dr);

  3. d=4r/3,K=dr/c(dr);

  4. 0<dr,mA12ξ02/A2;

  5. r<d<4r/3,4r/c<K<dr/c(dr),mA12ξ02/A2;

  6. d>4r,dr/c<K<K2;

  7. 4r/3<d<4r,dr/c(dr)<K<K2;

  8. 0<m<m1 or m2<m<A12/A2,r/2<dr,4r/c<K<K2;

  9. 0<m<m1 or m2<m<A12/A2,0<d<r/2,K>4r/c;

  10. 0<m<m1 or m2<m<A12/A2,r<d<4r/3,4r/c<K<dr/c(dr).

  11. 0<m<m2,r14,12<d<4r,K>4r/c;

  12. 0<m<m2,0<r<14,12<d<min{4r,r},K>4r/c;

  13. 0<m<m2,0<r<14,max12,r<d<4r,K>K2;

  14. 0<m<m2,0<r<14,max12,r<d<r,K=K2;

  15. 0<m<m2,0<r<14,maxr,r<d<4r/3,K=K2;

  16. 0<m<m2,d>4r,K>K2;

where K2d(d+r)+dd(4r+d)c(2dr),r(8r+1)(4r1)+(14r)(128r2+28r+1)16(14r),ξ1c(rd)+drKcdKK(K4rc),ξ2c(rd)+drK+cdKKK4rcξ0c(rd)+drK,m1=A12ξ22A2,m2A12ξ12A2.

Lemma 3.3

[Citation18]

If H2:0<m<A12/A2, d/c<K4r/c holds, then system (Equation1) doesn't have limit cycle and closed orbit in R+={(x,y)|x>0,y>0}.

Lemma 3.4

[Citation18]

The system (Equation1) has at least one stable limit cycle if H3 holds: H3: if one of the following conditions holds:

  1. (r,c,b,d,K,m)Hp2,0<m<A12ξ12/A2] and |mA12ξ12/A2|1;

  2. (r,c,b,d,K,m)Hp5,m>A12ξ22/A2 and |mA12ξ22/A2|1;

  3. (r,c,b,d,K,m)Hp7,A12ξ22/A2<m<A12/A2 and |mA12ξ22/A2|1;

where Hp2{(r,c,b,d,K,m):0<d<r2,K>4rc,m=A12ξ12A2,Q>0},Hp5{(r,c,b,d,K,m):0<d<r2,K>4rc,m=A12ξ22A2,Q>0},Hp7{(r,c,b,d,K,m):r14,r2<d<4r,K>4rc,m=A12ξ22A2,Q>0}, Qrd2y1bK2x1m4+rx1bK2y15d2+r2x12bKy15drx13b2K2dm2y12rx13b2K2dm4+bcd2y13K3m2x1+bd3y13K3m27r2x13dm2Kcy126r2x13bcKy13m22r2x13bcKy12m3+4rx13bc2K2y15+8rx13bc2K2y14m4rx12bcK2y15d12rx12bcK2y14dm+4rx13bc2K2y13m2+4r2x12bKy14dm+5r2x12bKy13dm2+2r2x12bKy12dm32rx13b2K2dm3y12bcd2y12K3m3x1+2bd3y12K3m34y1r2x13dm3Kc+2r2x12d2m2Ky124y1r2x12d2m3K+2rd3m4K2x1+5rd2y13bK2x1m2+2rd2y12bK2x1m310rx12bcK2y13m2d+4rx1bK2y14d2m2r2x13bcKy156r2x13bcKy14m+2cx14y12m2r3+2cx14y1m3r32bcx12y12m3dK2r.

The vector diagram of system (Equation1) is illustrated in Figure .

Figure 1. The vector diagram of system (Equation1)(a) the case of H1,H2, where r=0.5,b=0.03,c=0.01,d=0.1,m=4,K=28. (b) the case of H3, where r=0.5,b=0.03,c=0.01,d=0.1,m=21.5,K=280.

Figure 1. The vector diagram of system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) )(a) the case of H1,H2, where r=0.5,b=0.03,c=0.01,d=0.1,m=4,K=28. (b) the case of H3, where r=0.5,b=0.03,c=0.01,d=0.1,m=21.5,K=280.

Theorem 3.5

For a suitable selection of parameter values, there exists a homoclinic curve determine by the stable and unstable manifold of the point E2(x2,r(1x2/K)/b) (Figure  (a)).

Proof.

Consider the point (x,y)E1E2¯, then dx/dt=0,dydt>0, we have that the direction of the vectors at points under the line y=r(1x/K)/b is pointing to the above.

Next, let W+s(E2) and W+u(E2) be the left stable manifold and the upper unstable manifold of the saddle point E2, respectively. Then because we have by theorem 1 that Σ0 is an invariant region, the orbits cannot cross the horizontal line y=ymax(c/b)x towards the above. The trajectories determined by the upper unstable manifold W+u(E2) cannot cross the trajectory determined by the left stable manifold W+s(E2). Moreover, the αlimit of the manifold W+s(E2) could be the origin or at infinity in the upper direction of the yaxis. On the other hand, the ωlimit of the upper unstable manifold W+u(E2) must be

  1. the equilibrium E1, whenever this point is an stable node; or

  2. the equilibrium (K,0), whenever this point is an stable node,

then there exists a set of parameter values for which W+u(E2) intersects W+s(E2) and a homoclinic curve is generated. In this case, the point E2 is the ωlimit of the upper unstable manifold W+u(E2).

3.2. Dynamic properties of the control system

We discuss existence,uniqueness and stability of the order-1 periodic orbit of system (Equation3) based on the qualitative analysis of system (Equation1). Denote p(xT)pT, q(xT)qT, τ(xT)τT. The strength of insecticide spraying to the pest is denoted as pT. The larger pT is, the greater the strength is.

3.2.1. Existence of the order-1 periodic orbit

Denote Q as Q((1pT)xT,τT). The intersection points between the isocline x˙/x=0 and the lines x=(1pT)xT and x=xT are denoted as A((1pT)xT,yA) and B(xT,yB), where yA=r(1(1pT)xT/K)/b, yB=r(1xT/K)/b=yxT. Denote A(xT,yA) and Q(xT,yQ) as the intersection points between x=xT and the trajectory starting from A and Q respectively. Obviously, we obtain yQ<yA.

We denote the pulse set as M={(x,y)|x=xT,y0}, and the phase set as N{(x,y)|x=(1pT)xT,yτT}.

The case for H1,H2

System (Equation1) has two positive equilibria: E1,E2, and E1 is stable, E2 is a saddle point, but it has no limit cycle in Σ0 in the case of H1,H2. The trajectory starting from the point A interests the impulsive set M at point A, and then jumps to the point A+. Define τ¯1rb1(1pT)xTK(1qT)yA. According to the magnitude of x1,x2 and XT, the following three cases are discussed:

  • Case I: xZXxTx1

Theorem 3.6

If xZXxTx1, then system (Equation3) has at least one order-1 periodic solution when τTτ¯1, it has a unique order-1 periodic solution when τT>τ¯1.

Proof.

To prove the existence of order-1 periodic solution, we need find a point LN such that fsor(L)=0. According to the magnitudes between τ¯1 and τT, two cases will be discussed.

(a) τTτ¯1. Then the orbit AAA+ˆ is an order-1 periodic orbit when τT=τ¯1. The successor function of point A satisfies fsor(A)=yA+yA<0 when τT<τ¯1. Choosing Q as the point in the phase set N with yQ=τT, thus fsor(Q)=yQ+yQ>0. By the continuity of fsor, there exists at least a point LAQ¯N such that fsor(L)=0, so system (Equation3) has at least one order-1 periodic solution which passes through point L, as shown in Figure (a).

Figure 2. The existence of order-1 periodic solution of system (Equation3) when xZXxTx1, (a)τTτ¯1; (b)τT>τ¯1.

Figure 2. The existence of order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when xZX≤xT≤x1, (a)τT≤τ¯1; (b)τT>τ¯1.

(b) τT>τ¯1, then the successor function of point A satisfies fsor(A)=yA+yA>0, and then fsor(A+)=(1qT)yA++τTyA+<(1qT)yA+τTyA+=0. Let R((1pT)xT,yR)AA+¯ such that |yRyR+|<fsor(A),yRyA<qTfsor(A). So we have fsor(R)=yR+yR>yA+(1qT)fsor(A)yR=yA+qTfsor(A)yR>0. According to the continuity of fsor, there exists at least a point LRA+¯N such that fsor(L)=0, so the system (Equation3) has an order-1 periodic solution which passes through point L, as shown in Figure (b).

In the following part, we discuss the uniqueness of the order-1 periodic solution to system (Equation3). Select two points L1,L2RA+¯N arbitrarily, without loss of generality, we assume xL1<xL2. Then there exist trajectories lL1 and lL2 passing L1 and L2, and they cross the impulsive set M at L1 and L2, respectively. L1 must be located on the right of L1+, we have xL1>xL2. After impulsive effect, L1 and L2 jump to the phase set N at L1+ and L2+, respectively. Hence, xL1+>xL2+. The successor functions of L1 and L2 are fsor(L1) and fsor(L2), then fsor(L1)fsor(L2)=xL1+xL1(xL2+xL2)=xL1+xL2++(xL2xL1)>0, which means the successor function fsor is monotonically decreasing in the phase set. Thus, there exists a unique order-1 periodic solution.

  • Case II: x1<xTx2

In this case, there exists a trajectory l0 of the system (Equation3) which is tangency to the impulsive set M at point B(xT,yB) and intersects the isocline x˙/x=0 at point C0 with xC0<xB=xT. If there are intersection points between the trajectory l0 and the phase set N, denoted by C1 and C2 with yC1<yC2.

Theorem 3.7

(1) If x1<xTx2and0<(1pT)xTxC0, then system (Equation3) has an order-1 periodic solution. (2) If x1<xTx2and(1pT)xT>xC0, then system (Equation3) has an order-1 periodic solution when τTτ¯2yC1(1qT)yB or τTτ¯3yC2(1qT)yB.

Proof.

(1) If x1<xTx2 and 0<(1pT)xTxC0, then system (Equation3) has at least one order-1 periodic solution when τTτ¯1, it has a unique order-1 periodic solution when τT>τ¯1. The proof is similar to that of Theorem 3.6 and omitted thereby, as illustrated in Figure (a,b).

Figure 3. The existence of order-1 periodic solution of system (Equation3) when x1<xTx2, (a) 0<(1pT)xTxC0 and τTτ¯1; (b) 0<(1pT)xTxC0 and τT>τ¯1; (c) (1pT)xT>xC0.

Figure 3. The existence of order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when x1<xT≤x2, (a) 0<(1−pT)xT≤xC0 and τT≤τ¯1; (b) 0<(1−pT)xT≤xC0 and τT>τ¯1; (c) (1−pT)xT>xC0.

(2) If x1<xTx2 and (1pT)xT>xC0, as shown in Figure (c). Then there is fsor(C1)0 when τTτ¯2, note that fsor(Q)>0, thus there exists LQC1¯ such that fsor(L)=0. When τ¯2<τT<τ¯3, the trajectory will tend to the equilibrium point E1 by the stability of point E1. So the system (Equation3) has no order-1 periodic solution. When τTτ¯3, there is fsor(C2)0 and fsor(C2+)<0, thus there exists LC2C2+¯ such that fsor(L)=0, the proof of the uniqueness is similar to that of Theorem 3.6.

  • Case III: x2<xTmin{xZD,K}

Assume that l1,l2,l3,l4 are dividing lines of the saddle point E2(x2,y2), where l1,l2 are the stable flows of E2(x2,y2), l3,l4 are the unstable flows of E2(x2,y2). Let C0(xC0,yC0) denote the intersection point between the isocline x˙/x=0 and the stable flow l2 with xC0<xE2, as illustrated in Figure . According to the magnitude of (1pT)xT,x2 and xC0, there exist three cases:

Figure 4. The existence of order-1 periodic solution of system (Equation3) when x2<xTmin{xZD,K}, (a) 0<(1pT)xTxC0 and τTτ¯1; (b) 0<(1pT)xTxC0 and τT>τ¯1; (c) (1pT)xT>xC0.

Figure 4. The existence of order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when x2<xT≤min{xZD,K}, (a) 0<(1−pT)xT≤xC0 and τT≤τ¯1; (b) 0<(1−pT)xT≤xC0 and τT>τ¯1; (c) (1−pT)xT>xC0.

(a) (1pT)xTxC0, the case is similar to that of Theorem 3.6, as illustrated in Figure (a,b);

(b) xC0<(1pT)xT<x2, denote C1 and C2 as the intersection points between x=(1pT)xT and stable flow l2 with yC1<yC2. And the unstable flow l3 intersects x=xT at point C1. There is fsor(C1)<0 when τT<τ¯4yC1(1qT)yC1, note that fsor(Q)>0, thus there exists LQC1¯ such that fsor(L)=0. When τ¯4<τT<τ¯5yC2(1qT)yC1, the trajectory will tend to the equilibrium point E1 by the stability of point E1. So system (Equation3) has no order-1 periodic solution. When τT>τ¯5, there is fsor(C2)>0 and fsor(C2+)<0, thus there exists LC2C2+¯ such that fsor(L)=0, as shown in Figure (c).

(c) x2(1pT)xT<xTxZD, assume that the unstable flows l3 and the stable flows l1 intersect x=(1pT)xT,x=xT at points A1,B1 and A2,B2, respectively. Suppose that the isocline x˙/x=0 and y˙/y=0 intersect the line x=(1pT)xT,x=xT at points A,B and C,D, respectively.

Then there must exist τ1,τ2 such that (1qT)yB1+τ1=yA,(1qT)yB1+τ2=yA2. It can be deduced that the point B1 is mapped into A if τT=τ1 and it is mapped into A2 if τT=τ2. Therefore, applying the definition of the homoclinic cycle, we obtain that the trajectories A2E2~,E2B1~ together with the impulsive line B1A2¯ constitute a homoclinic cycle for the system (Equation3) when τT=τ2.

Theorem 3.8

When x2(1pT)xT<xTxZD, there exist τ1,τ2 such that for any τT(τ1,τ2), the order-1 homoclinic cycle disappears and system (Equation3) bifurcates a unique order-1 positive periodic solution.

Proof.

Denote H as the image point of B1, for any τT(τ1,τ2), there is yH=(1qT)yB1+τT((1qT)yB1+τ1,(1qT)yB1+τ2)=(yA,yA2), and H is between A and A2. Assume that the trajectory starting from the point H firstly intersects the line x=xT at point H and then jumps into H+ because of the impulsive effect. On account that any two trajectories of autonomous systems cannot intersect, so yH>yB1. Thus, yH+>yH and the successor function satisfies fsor(H)=yH+yH>0. In another aspect, we take a point A1ϵ((1pT)xT,yA1ϵ) where yA1ϵ=yA2ϵ and ϵ>0 is sufficiently small. Assume that the trajectory with the initial point A1ϵ firstly intersects x=xT at a point A1ϵ and then jumps into A1ϵ+ when the impulse occurs. A1ϵ is next to point B1 which can be inferred from the continuous dependence of the solution concerning the initial value. Then fsor(A1ϵ)=yA1ϵ+yA1ϵ<0. So there must exist a point LA1ϵH¯ such that fsor(L)=0. Therefore, system (Equation3) has at least a positive order-1 periodic solution and the geometrical interpretation is as shown in Figure .

Figure 5. The existence of order-1 periodic solution of system (Equation3) when x2(1pxT)xT<xTxZD,τT(τ1,τ2).

Figure 5. The existence of order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when x2≤(1−pxT)xT<xT≤xZD,τT∈(τ1,τ2).

Next, the uniqueness of the positive order-1 periodic solution will be proved. Take any two points L1,L2A1ϵH¯ with yL1<yL2. Suppose that the respective trajectories with these two starting points are L1L1L1+,L2L2L2+, where L1+,L2+ are the first successor points for L1 and L2, respectively. We obtain yL2<yL1 from yL1<yL2. Then yL2+=(1qT)yL2+τT<(1qT)yL1+τT=yL1+. Thus fsor(L1)fsor(L2)=yL1+yL1(yL2+yL2)=(yL2yL1)+(yL1+yL2+)>0 which implies that fsor is a monotone decreasing function. Notably, L1 and L2 are arbitrary in the line A1ϵH¯. Therefore, there exists a unique point L such that fsor(L)=0, and it follows from the property of the Poincare´ map that the bifurcated positive periodic solution is unique.

According to Theorem 3.5, when the homoclinic curve disappears, it has two situations by the position relationship between the stable flow l2 and the unstable flow l4.

(i) If the unstable flow l4 is outside of the stable flow l2, three kinds of situations (see Figure (a)): τTτ¯6=yA2(1qT)yA, there exists an order-1 periodic solution from the proof of Theorem 3.6(a); τ¯1<τT<τ¯6=yA2(1qT)yA, there exists a unique order-1 periodic solution from the proof of Theorem 3.8; τT>τ¯6, there exists a unique order-1 periodic solution from the proof of Theorem 3.6(b) and omitted thereby.

Figure 6. The existence of order-1 periodic solution of system (Equation3) when x2(1pT)xT<xTmin{xZD,K}, (a)the unstable flow l4 is outside of the stable flow l2; (b)the stable flow l2 is outside of the unstable flow l4.

Figure 6. The existence of order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when x2≤(1−pT)xT<xT≤min{xZD,K}, (a)the unstable flow l4 is outside of the stable flow l2; (b)the stable flow l2 is outside of the unstable flow l4.

(ii) If the stable flow l2 is outside of the unstable flow l4, the stable flow l2 intersects the phase set N at point F as t. There are four kinds of situations (see Figure (b)):τTτ¯1, it is similar to that of Theorem 3.6(a); τ¯1<τTτ¯6=yA2(1qT)yA, it is similar to that of Theorem 3.8; τ¯6<τTτ¯7=yF(1qT)yA, then by the stability of point E1, we know the trajectory will approach point E1; τT>τ¯7, there exists a unique order-1 periodic solution.

The case for H3

System (Equation1) has two positive equilibria: E1,E2, E1 is unstable, E2 is a saddle point, and it has a stable limit cycle around point E1 in Σ0 in the case of H3. Denote the limit cycle by Γ0. The limit cycle intersects the isoclinic line x˙/x=0 at points S1(h1,yS1),S2(h2,yS2), and h1<h2.

When xZX<xT<h1<xZDorxZX(1pT)xT<h1<xTh2<xZD, system (Equation3) has an order-1 periodic solution from the proof of Theorem 3.6.

When xZX(1pT)xT<xT<x2<xZD system (Equation3) has an order-1 periodic solution or the trajectories of system (Equation3) tend to the limit cycle after finite times impulsive effects at most for t+ by Theorem 3.7.

When xZX(1pT)xT<x2<xT<xZD<K, the case is similar to that of case III (1) and (2).

When min{x2,xZX}<(1pT)xT<xT<xZD<K, the case is similar to that of case III (3).

Theorem 3.9

If h1<(1pT)xT<xT<h2, then system (Equation3) has at last four kinds of order-1 periodic solutions (the case for τT>τ¯1).

Proof.

When x<x1, the horizontal ordinate of the orbit passing through the isocline x˙/x=0 increase with the increase of time, i.e. dx/dt>0. Therefore, when h1<(1pT)xT<xT<h2, there must exist a point L1M such that system (Equation3) has an order-1 periodic solution which consists of L1, the phase point LN of L1 and the orbit between them (Figure (a,b and d)).

Figure 7. Four kinds of order-1 periodic solutions of system (Equation3) when h1<(1pT)xT<xT<h2.

Figure 7. Four kinds of order-1 periodic solutions of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when h1<(1−pT)xT<xT<h2.

Since the limit cycle is table, then the orbits starting from the point in the limit cycle tend to the limit cycle. For the point L1(xT,yL1) close to the isocline y˙=0 sufficiently, the trajectory passing through the point L1(xT,yL1) can intersect the line y=(yL1qTτT)/pTxTx+(1(qT/pT))yL1+(τT/pT) of the impulsive function Δy=qTy+τT at two points for t, then system (Equation3) has an order-1 periodic solution, see Figure (c). Particularly, the trajectory between L and L1 can revolve round the equilibrium E1 several cycles, which resembles Figure (c). This completes the proof.

3.2.2. Stability analysis of the order-1 periodic orbit

Denote γL=LLLˆ as the order-1 periodic orbit of system (Equation3). Analysing the stability of the possible periodic orbit, the results are as follows.

Theorem 3.10

The order-1 periodic orbit γL of system (Equation3) is orbitally asymptotically stable if x2(1pT)xT<xTxZDandτT(τ1,τ2).

Proof.

Denote H+ as H1, we need prove the sequence {yH2k} is monotonically increasing and {yH2k1} is monotonically decreasing, where H2k,H2k1(k=1,2,3,) are successor points of H (Figure ). On account of yL<yH1<yA1ϵ, we obtain yH<yH2<yL. Repeating the process, we have yL<yH3<yH1<yA1ϵ,yH<yH2<yH4<yL. By induction, we get that {yH2k} is monotonically increasing and {yH2k1} is monotonically decreasing. For any point IHA1ϵ¯, assume that IHL¯, i.e. yH<yI<yL. Clearly, there must exist a positive integer i such that yH2i<yI<yH2(i+1). Assume that the iterates of I are given by {I1,I2,I3,I4,}. Thus, there are the following inequalities: yL<yH2(i+1)+1<yI1<yH2i+1<yA1ϵ,yH<yH2(i+1)<yI2<yH2(i+2)<yL,yL<yH2(i+2)+1<yI3<yH2(i+1)+1<yA1ϵ,yH<yH2(i+1)<yH2(i+2)<yI4<yH2(i+3)<yL.

Figure 8. The stability of the order-1 periodic solution of system (Equation3) when τT(τ1,τ2).

Figure 8. The stability of the order-1 periodic solution of system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) when τT∈(τ1,τ2).

By the use of resembling arguments to those in the statement of {yH2k} and {yH2k1}, we can obtain inductively that {yI2k} is monotonically increasing and {yI2k1} is monotonically decreasing. In addition, limkyI2k1=yL,limkyI2k=yL. Thus, the trajectory starting from point I will eventually attract to L. The order-1 periodic orbit γL of system (Equation3) is orbitally asymptotically stable for any point IHA1ϵ¯.

Corollary 3.11

The order-1 periodic orbit γL of system (Equation3) is orbitally asymptotically stable if xZXxTx1andτT>τ¯1.

Theorem 3.12

The order-1 periodic orbit γL of system (Equation3) is orbitally asymptotically stable if 0+TLcmξη(η+m)2rξKdt<lnyByL[(1qT)yL+τT]yA.

Proof.

Let x=(ξ(t),η(t)) be the order-1 periodic solution. In system (Equation3), there are f(x,y)=rx(1(x/K))bxy,g(x,y)=y((cxy)/(y+m)d),Φ(x,y)=xxT,α(x,y)=pTx,β(x,y)=qETy+τT. fx=r2rxKby,gy=cxy2+2cmxy(y+m)2d,Φx=1,Φy=0,αx=pT,αy=0,βx=0,βy=qT,fI=r1xT(1xTK)bxTyL,f+I=r1(1pT)xT1(1pT)xTKb(1pT)xT[(1qT)yL+τT]. In addition, 0+TLfx+gy(ξ(t),η(t))dt=0+TLr12r1ξKbη+cξη2+2cmξη(η+m)2ddt=ln11pTyL(1qT)yL+τT+0+TLcmξη(η+m)2rξKdt. Thus, exp0+TLfx+gy(ξ(t),η(t))dt=11pTyL(1qT)yL+τTexp0+TLcmξη(η+m)2rξKdt. By Lemma 2.2, there is ργL=(1qT)yL(1qT)yL+τT(1qT)yL+τTyAyByLexp0+TLcmξη(η+m)2rξKdt. Thus if ργL<1, 0+TLcmξη(η+m)2rξKdt<lnyByL[(1qT)yL+τT]yA, i.e. the order-1 periodic orbit γL is orbitally asymptotically stable.

3.2.3. Optimal pest control level

To determine the optimum frequency of chemical control and optimum yield of releases of the predator, the optimal pest control level has to be determined. Let suppose the unit cost of releases of the predator be a1, a2 be the unit cost of the chemical control including the price of chemical agent and the price of the damage to the environment. Reducing the cost per unit time is our purpose. Denote Ycost as the total cost in one period, it is a function of the chemical strength p(xT) and the yield of releases of the predators τ(xT). Then the total cost is Ycost(xT)=a1τ(xT)+a2p(xT)xT. To obtain the optimal control threshold, we consider the unit control cost, i.e. Pcost=Ycost/T(xT). Thus the following optimization model are constructed (5) minPcost=Ycost(xT)T(xT)s.t.xZXxTxZD,(5) where τ(xT) and p(xT) is defined by Equation (Equation2). By solving the optimization model (Equation5), we can obtain the optimal pest control level xT. Correspondingly, the optimum yield of the releases of the predator τ=τ(xT), the optimum chemical control strength p=p(xT) and the optimum frequency of the chemical control T=T(τ,p) can be obtained. Notably, the optimum pest control level xT is dependent on the ratio of σa1/a2.

4. Numerical simulations and optimization

4.1. Numerical simulations

To verify the theoretical results obtained in above sections, let r=0.5, c=0.01, b=0.03, d=0.1, m=4, K=28. Through calculation, there is m<A12/A2=21.4881, d/c=10<K. The positive equilibrium points are E1(15.2852,7.5684) and E2(22.7148,3.1459). Also, there is 4r=2>d=0.1>0, d/c=10<K=28<4r/c=200. It is easily know from Lemmas 3.2 and 3.3 that system (Equation3) has no limit cycle and the positive equilibrium point E1 is stable. Assume that the biological control level is 30%K of the environmental carrying capacity, i.e. xZX=30%K=8.4, and the chemical control level is 30%K of the environmental carrying capacity, i.e. xZD=60%K=16.8. The yield of releases of the predator at xZX is τmax=4, τmin=10%τmax=0.4. And the chemical control strength at xZD are pmax=1xZX/xZD=0.5, qmax=40%pmax=0.2.

For the case of xZXxTx1, the phase portrait, time series of prey density and predator density can be seen in Figure , where there exists an order-1 periodic solution for xT=9.8<x1=15.2852 with period T=8.3333. By Corollary 3.11 the order-1 periodic solution is asymptotically stable.

Figure 9. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=35%K=9.8, pT=0.0833, qT=0.0333 and τT=3.4. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 9. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=35%K=9.8, pT=0.0833, qT=0.0333 and τT=3.4. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

Figure  shows the order-1 periodic solution with period T=4.3478 for xT=14<x1=15.2852. Figure  shows the phase portrait, time series of prey density and predator density starting from the initial point (x0,y0)=(15,15) for (1pT)xT=10.5875<x1=15.2852<xT=15.4. With pest control level xT increasing, τT decreases and the order-1 periodic solution moves from high to low.

Figure 10. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=50%K=14, pT=0.3333, qT=0.1333 and τT=1.6. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 10. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=50%K=14, pT=0.3333, qT=0.1333 and τT=1.6. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

Figure 11. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=55%K=15.4, pT=0.4167, qT=0.1667 and τT=1. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 11. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,15). Control parameters: xZX=30%K, xZD=60%K, xT=55%K=15.4, pT=0.4167, qT=0.1667 and τT=1. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

For the case of xZX=50%K, xZD=95%K, xT=90%K=25.2, (1pT)xT=14.5883<x1=15.2852<x2=22.7148<xT=25.2 The trajectory starting from the initial point (15,3) tends to an order-1 periodic solution (Figure ).

Figure 12. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,3). Control parameters: xZX=50%K, xZD=95%K, xT=90%K=25.2, pT=0.4211, qT=0.1684 and τT=0.8. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1,E2 are presented in red and green asterisk, respectively.

Figure 12. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,3). Control parameters: xZX=50%K, xZD=95%K, xT=90%K=25.2, pT=0.4211, qT=0.1684 and τT=0.8. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1,E2 are presented in red and green asterisk, respectively.

There is x1=15.2852<(1pT)xT=17.2418<x2=22.7148<xT=25.2 when xZX=60%K, xZD=95%K, xT=90%K=25.2, where an order-1 periodic solution can be seen in Figure .

Figure 13. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(14,2). Control parameters: xZX=60%K, xZD=95%K, xT=90%K=25.2, pT=0.3158, qT=0.1263 and τT=0.9143. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1,E2 are presented in red and green asterisk, respectively.

Figure 13. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(14,2). Control parameters: xZX=60%K, xZD=95%K, xT=90%K=25.2, pT=0.3158, qT=0.1263 and τT=0.9143. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1,E2 are presented in red and green asterisk, respectively.

The phase portrait, time series of prey density and predator density can be seen in Figure  for xZX=85%K, xZD=95%K, xT=95%K=26.6, and we can get x2=22.7148<(1pT)xT=23.799<xT=26.6.

Figure 14. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(14,1). Control parameters: xZX=85%K, xZD=95%K, xT=95%K=26.6, pT=0.1053, qT=0.0421 and τT=0.4. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E2 are presented in green asterisk.

Figure 14. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(14,1). Control parameters: xZX=85%K, xZD=95%K, xT=95%K=26.6, pT=0.1053, qT=0.0421 and τT=0.4. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E2 are presented in green asterisk.

Let r=0.5,c=0.01,b=0.03,d=0.1,m=21.5,K=280. then 0<d=0.1<(r/2)=0.25,K>(4r/c)=200,m>(A12ξ22/A2)=21.4870,Q>0,|m(A12ξ22/A2)|1. We have that the positive equilibrium points are E1(24.1158,15.2312) and E2=(265.8842,0.8402). It is easily know from Lemma 3.4 that system (Equation3) has a stable limit cycle. In Figures , we can find that the system (Equation3) has four kinds of order-1 periodic solution for h1<(1pT)xT<xT<h2, where S1(h1,yS1),S2(h2,yS2) are intersect points of the limit cycle and the isoclinic line y˙=0 and h1<h2. Theorem 3.9 only gives the existence of four kinds of order-1 periodic solution for h1<(1pT)xT<xT<h2.

Figure 15. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,13.47). Control parameters: xZX=1%K, xZD=20%K, xT=8%K=22.4, pT=0.35, qT=0.14 and τT=8.0211. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 15. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,13.47). Control parameters: xZX=1%K, xZD=20%K, xT=8%K=22.4, pT=0.35, qT=0.14 and τT=8.0211. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

Figure 16. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15.01,15.42). Control parameters: xZX=1%K, xZD=20%K, xT=10%K=28, pT=0.45, qT=0.18 and τT=6.8842. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 16. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15.01,15.42). Control parameters: xZX=1%K, xZD=20%K, xT=10%K=28, pT=0.45, qT=0.18 and τT=6.8842. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

Figure 17. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(21.55,16.37). Control parameters: xZX=1%K, xZD=40%K, xT=10%K=28, pT=0.225, qT=0.09 and τT=2.7731. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 17. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(21.55,16.37). Control parameters: xZX=1%K, xZD=40%K, xT=10%K=28, pT=0.225, qT=0.09 and τT=2.7731. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

For the case of xZX=1%K, xZD=20%K,τmax=12, for example xT=8%K=22.4, in this case, h1<(1pT)xT=18.2<xT=22.4<x1=24.1158<h2, there exists an order-1 periodic solution, which is presented in Figure . With xT increasing, i.e. xT=10%K=28, an order-1 periodic solution starting from the initial point (15.01,15.42) can be seen in Figure  for h1<(1pT)xT=15.4<x1=24.1158<xT=28<h2.

For the case of xZX=1%K, xZD=40%K, xT=10%K=28, τmax=3.5, the phase portrait, time series of prey density and predator density can be seen in Figure , in this case the trajectory starting from (21.55,16.37) tends to an order-1 periodic solution, where h1<(1pT)xT=21.7<x1=24.1158<xT=28<h2.

For the case of xZX=10%K, xZD=50%K, xT=15%K=42, τmax=5, it can be observed that there exists an order-1 periodic solution from Figure , and with a simply calculation, h1<x1=24.1158<(1pT)xT=37.8<xT=42<h2.

Figure 18. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,13). Control parameters: xZX=10%K, xZD=50%K, xT=15%K=42, pT=0.1, qT=0.04 and τT=4.4375. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 18. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(15,13). Control parameters: xZX=10%K, xZD=50%K, xT=15%K=42, pT=0.1, qT=0.04 and τT=4.4375. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

For a suitable selection of parameter values, i.e. xZX=5%K, xZD=50%K, xT=10%K=28, τmax=5, then the existences of order-2 periodic solutions can be seen in Figure , which means that the system (Equation3) has complex dynamical behaviours for h1<(1pT)xT<xT<h2.

Figure 19. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(25,13.4). Control parameters: xZX=5%K, xZD=50%K, xT=10%K=28, pT=0.1, qT=0.04 and τT=4.5. The solution of the free system (Equation1) is represented in red dotted lines, the solution of the system (Equation3) is presented in blue full line and E1 is represented in red asterisk.

Figure 19. The phase portrait (a), time series of prey density (b) and predator density (c) starting from (x0,y0)=(25,13.4). Control parameters: xZX=5%K, xZD=50%K, xT=10%K=28, pT=0.1, qT=0.04 and τT=4.5. The solution of the free system (Equation1(1) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t).(1) ) is represented in red dotted lines, the solution of the system (Equation3(3) dx(t)dt=rx(t)1−x(t)K−bx(t)y(t),dy(t)dt=cx(t)y(t)y(t)y(t)+m−dy(t),x<xT,Δx(t)=−p(xT)x(t)Δy(t)=−q(xT)y(t)+τ(xT)x=xT.(3) ) is presented in blue full line and E1 is represented in red asterisk.

In the case of r=0.5,b=0.03,c=0.01,d=0.1,m=4,K=28, the period T of the order-1 periodic orbit is dependent on the pest control level xT, as shown in Figure (a), and the cost per unit time Ycost/T is presented in Figure (b), where the unit cost of the chemical control a2 is assumed to be 1000 and the unit cost of culturing the predator a1 is 5000. The optimum pest level to take control is xT=53.57%K=15, the optimum yield of releases of the predator τ=3.4172, the optimum chemical control strength p=0.3928 and the optimum frequency of the chemical control is T=16.6874. However, it should be noted that the optimum pest level to take control xT is dependent on the ratio of a1/a2, the larger the ratio a1/a2, the upper the optimum pest level xT, as illustrated in Figure .

Figure 20. The change in the order-1 periodic orbit's period T and the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=4, K=28.

Figure 20. The change in the order-1 periodic orbit's period T and the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=4, K=28.

Figure 21. The change in the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=4, K=28 with a1/a2=0.1,1,5,100.

Figure 21. The change in the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=4, K=28 with a1/a2=0.1,1,5,100.

In the case of r=0.5,b=0.03,c=0.01,d=0.1,m=21.5,K=280, the period T of the order-1 periodic orbit is dependent on the pest control level xT, as shown in Figure (a), and the cost per unit time Ycost/T is presented in Figure (b), where the unit cost of the chemical control a2 is assumed to be 1000 and the unit cost of culturing the predator a1 is 10,000. The optimum pest level to take control is xT=13.75%K=38.5, the optimum yield of releases of the predator τ=6.5349, the optimum chemical control strength p=0.6375 and the optimum frequency of the chemical control is T=14.2625. However, it should be noted that the optimum pest level to take control xT is dependent on the ratio of a1/a2, the larger the ratio a1/a2, the upper the optimum pest level xT, as illustrated in Figure .

Figure 22. The change in the order-1 periodic orbit's period T and the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=21.5, K=280.

Figure 22. The change in the order-1 periodic orbit's period T and the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=21.5, K=280.

Figure 23. The change in the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=21.5, K=280 with a1/a2=1,2.5,10,100.

Figure 23. The change in the cost per unit time Ycost/T on the pest control level xT for r=0.5, b=0.03, c=0.01, d=0.1, m=21.5, K=280 with a1/a2=1,2.5,10,100.

5. Conclusion

In this paper, a predator–prey model with state feedback impulsive control and Allee effect on predator is established and analysed. The results indicate that the existence of order-1 periodic solution (especially homoclinic cycle) for H1,H2. If H3 holds, the existence of order-k(k1) periodic solution is complex. When h1<(1pT)xT<xT<h2, system (Equation3) has at last four kinds of order-1 periodic solutions and order-2 periodic solution. But there exist some troubles in the proof of the existence of order-2 periodic solution, which will be our next work. In order to minimize the control cost, the optimization model is constructed based on the order-1 periodic solution, and the optimal control threshold is obtained by numerical simulation. Simulation results show that the pest control strategy proposed is effective, and the optimal quantity of natural enemies released and the intensity of pesticide sprayed is determined according to the control threshold given by optimization. The state dependent control can be transformed into periodic control, and thus avoid monitoring the population size of the predator.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work is supposed by the National Natural Science Foundation of China [Nos.: 11671346,11371306,11401068], the Project for Science and Technology Open Cooperation of Henan Province [172106000071], Nanhu Scholars Program of XYNU [2016].

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