Forecasting the influence of temperature increase on the development of the Colorado potato beetle [Leptinotarsa decemlineata (Say)] in the Wielkopolska region of Poland

Abstract

The Colorado potato beetle (Leptinotarsa decemlineata) is probably the most dangerous pest species of potatoes. This study assessed the influence of expected climate warming on the number of Colorado potato beetle generations in the Wielkopolska region of Poland using two simulation models, SimLep 1 and NumoGen 1. Two types of data sets were utilised in the simulations: real meteorological data recorded in Winna Góra (52.2N; 17.4E) in the period 2002–2011 and virtual data obtained after transformation of the real data to include an increase in the recorded temperature values between 1 and 6°C. The threat to potato crops from Colorado potato beetle was found to increase in response to expected climate change. A temperature increase of 2°C above that recorded during the last few years would allow the development of two full generations of the beetle, and further temperature increases may even permit the occurrence of three generations.

Keywords:

• Colorado potato beetle
• global warming
• simulation

Introduction

The Intergovernmental Panel on Climate Change (IPCC, 2007) projects increases in global mean surface temperatures of 1.1–6.4°C by the end of the twenty-first century. Similar to the global temperature the average temperature over Europe is also projected to continue increasing. Summer temperatures are projected to increase by up to 7°C in Southern Europe and 5°C in Northern Europe in the period 2080–2100 compared with the 1961–1990 average (van der Linden & Mitchell, 2009). It means that maximal temperature increase in Poland is projected to be about 6°C. Such changes in climate are likely to affect many natural processes that are temperature-dependent (Deutsch et al., 2008; Inouye, 2008). Insects are cold-blooded organisms and hence temperature is one of the most important environmental factors influencing their development (Petzoldt & Saeman, 2006). Temperature increases may therefore result in alterations in the geographical distribution of different insect species, variability in overwintering and population growth rates, increases in the number of generations, extension of the developmental period, effects on crop–pest synchrony, changes in interspecies interactions and increased risk of invasion by migrant pests (Porter et al., 1991). The close relationship between temperature and the development of cold-blooded organisms explains why insects are appropriate model candidates on which to study the effects of climate change (Chen et al., 2011).

The influence of temperature on the speed of development of a single generation in multivoltine species has been studied by many researchers (Collier et al., 1991; Ungerer et al., 1999; Mezeyová et al., 2008; Amiri et al., 2010). The objects of these studies have usually been insects of economic importance.

The Colorado potato beetle (Leptinotarsa decemlineata (Say)) is the most destructive insect pest of potatoes in North America and Europe (Ferro et al., 1985). If left uncontrolled, the pest can cause serious yield losses (Cranshaw & Radcliffe, 1980; Hare, 1980; Ferro et al., 1983). Experiments conducted in Poland have revealed yield losses caused by Colorado potato beetle amounting to 20% (Wójtowicz & Jörg, 2006) or 40% (Sidorczyk, 1965). Thus the beetle is also a serious pest in potato production in Poland (Przybysz et al., 1996; Pawińska & Mrówczynski, 2000; Węgorek et al., 2001). Overwintered adults emerge from the soil in the spring and search for food plants. Eggs are deposited on foliage in late spring. Larvae emerge and pass through four instars that feed on potato foliage. Mature larvae burrow into the soil and pupate. Approximately two weeks later, adults of the new generation emerge and begin feeding on the maturing potato plants. When ready to diapause, beetles either enter the soil within potato fields or move out in search of peripheral overwintering sites (Węgorek, 1959). The number of beetle generations depends on temperature and amounts to four in the warmest areas where it is found, where the development cycle takes 30 days, to one full or even a partial generation in colder regions. Thus in Poland and in Germany, where the mean annual air temperature is about 8 and 9°C, respectively, there is one full generation of Colorado potato beetle per year, whilst in Italy there are two and three generations in regions where the mean annual air temperature exceeds 14 and 16°C, respectively (Ferrari et al., 1993).

The close connection between air temperature and the number of pest generations (Anon, 1994) means that there is an urgent need to define the impact of predicted climate warming on the development of Colorado potato beetle. This has already been studied in Germany, Lithuania, Czech Republic and Austria (Šmatas et al., 2008; Wittchen & Freier, 2008; Kocmánková et al., 2010). The present study examined the influence of predicted temperature increases on the development of Colorado potato beetle in the Wielkopolska region of Poland, with the overall aim of determining the potential response of the pest to climate change and allowing future management policy to be formulated accordingly.

Methods

The study was carried out using two simulation models: SimLep 1 and NumoGen 1. SimLep 1 was originally developed in the former German Democratic Republic based on the literature data and the results of experiments conducted in the field and in the laboratory. The model was validated in the periods 1975–1981 and 1983–1985 (Kurth & Rossberg, 1986). According to the actual development, SimLep 1 presents the following results: date of first occurrence of the hibernating beetles, date of first occurrence of egg clusters, abundance of egg clusters, abundance of young larvae, abundance of old larvae and date of first occurrence of young adult beetles (Jörg et al., 2007). Detailed information on the sources used in construction of the SimLep 1 algorithms can be found in Kurth et al. (1984). However, due to the fact that in Polish climate conditions the life cycle of Colorado potato beetle simulated by the SimLep 1 model began 3–4 weeks earlier than the observed life cycle, we abandoned the direct use of this model to predict the development of the pest. Instead, we used the results generated by SimLep 1 to estimate the effect of increasing temperature on the onset of the beetle egg-laying period, based on simulations using meteorological data recorded between 2002 and 2011 and virtual data obtained after raising the actual temperature by 1–6°C. The following relationship was obtained:

1

where y=acceleration of the onset of egg-laying (days), and x=temperature increase (1–6°C).

This equation was then incorporated into the model NumoGen 1, where it was used to determine the egg-laying date of the pest in conditions where the recorded temperature had increased by 1–6°C. The procedure for determining the date of egg-laying at elevated temperatures was performed after inputting data of the observed date of occurrence of this developmental stage in ambient thermal conditions. NumoGen 1 simulates the development of Colorado potato beetle from the occurrence of egg clusters to the beginning of the winter diapause and helps in estimation of the effect of temperature on the number of beetle generations. The model was developed for the purposes of the present study based on scientific reports describing the influence of temperature on successive stages of Colorado potato beetle, and publications on the effects of photoperiod on the pest diapause. Data presented by Larczenko (1958) were used to simulate the development of the pest from egg to adult form, and data presented by Alyokhina and Ferro (1999) were applied to estimate the dates of egg-laying by female beetles of the second or third generation. The beginning of winter diapause was determined on the basis of data reported by Tauber et al. (1988), who found that all females reared at a photoperiod between 10:14 and 14:10 (L:D) entered diapause. This information was used to determine the end of the simulation period, with NumoGen 1 setting the date of beetle diapause based on day length in Winna Góra, Poland. The information about day length is available on the internet (http://calendar.zoznam.sk/sunset-pl.php). The beginning of the simulation was determined by the date of egg-laying on monitored potato crops or the date calculated by the NumoGen 1 model using the SimLep1 results. The first option was used for calculations based on meteorological data recorded in Winna Góra and the second when the temperature was increased by 1–6°C. Sample data are presented in Table I. Table II presents the parameters used in NumoGen 1. The NumoGen 1 model was validated in two ways, both of which concerned the development of the first generation of the beetle. One approach involved comparing the results generated by NumoGen 1 against the results of field observations in 2009–2011 (Table III), while the other approach involved comparing the results produced by the NumoGen 1 and SimLep 1 models (Table IV). This method of validation required the models to be synchronised, which was achieved as follows. Simulations using the model SimLep1 were carried out first, and the date of egg-laying determined by that model was then used as input to NumoGen 1. The simulations were conducted using actual data recorded at Winna Góra in the period 2002–2011 and values obtained after transformation of the recorded data to reflect a temperature increase of 1–6°C.

Table I. Sample data set (year-month-day).

Date	Hour	T	T+1	T+2	T+3	T+4	T+5	T+6
2002-04-01	0	9.7	10.7	11.7	12.7	13.7	14.7	15.7
2002-04-01	1	9.5	10.5	11.5	12.5	13.5	14.5	15.5
2002-04-01	2	9.4	10.4	11.4	12.4	13.4	14.4	15.4
2002-04-01	3	9.3	10.3	11.3	12.3	13.3	14.3	15.3
2002-04-01	4	9.2	10.2	11.2	12.2	13.2	14.2	15.2
2002-04-01	5	9.1	10.1	11.1	12.1	13.1	14.1	15.1
2002-04-01	6	9	10	11	12	13	14	15
2002-04-01	7	10.2	11.2	12.2	13.2	14.2	15.2	16.2
2002-04-01	8	11.3	12.3	13.3	14.3	15.3	16.3	17.3
2002-04-01	9	12.5	13.5	14.5	15.5	16.5	17.5	18.5
2002-04-01	10	13.8	14.8	15.8	16.8	17.8	18.8	19.8
2002-04-01	11	15	16	17	18	19	20	21
2002-04-01	12	16.3	17.3	18.3	19.3	20.3	21.3	22.3
2002-04-01	13	16.2	17.2	18.2	19.2	20.2	21.2	22.2
2002-04-01	14	16.2	17.2	18.2	19.2	20.2	21.2	22.2
2002-04-01	15	16.2	17.2	18.2	19.2	20.2	21.2	22.2
2002-04-01	16	15.2	16.2	17.2	18.2	19.2	20.2	21.2
2002-04-01	17	14.1	15.1	16.1	17.1	18.1	19.1	20.1
2002-04-01	18	13.1	14.1	15.1	16.1	17.1	18.1	19.1
2002-04-01	19	12.8	13.8	14.8	15.8	16.8	17.8	18.8
2002-04-01	20	12.4	13.4	14.4	15.4	16.4	17.4	18.4
2002-04-01	21	12.1	13.1	14.1	15.1	16.1	17.1	18.1
2002-04-01	22	12	13	14	15	16	17	18
2002-04-01	23	11.8	12.8	13.8	14.8	15.8	16.8	17.8
T – hourly temperature [°C] registered at Winna Góra on 1 April 2002. Hour standard (0–23, 0 = midnight); T+1–T+6 – virtual data obtained after temperature increase of 1–6°C.

Table II. Parameters used in NumoGen 1 model.

Source	Parameter	Value
Łarczenko (1958)	Degree days from egg to adult with temperature threshold(s)	330 11.5°C
Alyokhin and Ferro (1999)	Degree days from pupation to egg-laying with temperature threshold	51 10°C
Tauber et al. (1988)	Critical photoperiod for diapause induction	14:10 (L:D)
Source = results of Łarczenko, Alyokhin and Ferro, and Tauber et al. used for the development of NumoGen 1 model; Parameter = parameter names used for the development of NumoGen 1 model; Value = value of the parameters used in the NumoGen 1 model.

Table III. Comparison of NumoGen 1 model results with field observations.

	Results of pest monitoring			Model results
Year	Egg-laying date^1	Date of adult form appearance^1	Number of days^1	Egg-laying date^2	Date of adult form appearance^2	Number of days^2	Difference [days]
2009	3.07	10.08	38	3.07	16.08	44	6
2010	11.06	20.07	39	11.06	20.07	39	0
2011	24.05	5.07	42	24.05	12.07	49	7
Egg-laying date^1=the date of egg laying by Colorado potato beetle registered on potato plantation at Winna Góra; Date of adult form appearance^1=the date of appearance of Colorado potato beetle adult form registered on potato plantation at Winna Góra; Number of days^1=number of days between date of egg laying and the date of adult form of Colorado potato beetle appearance registered at Winna Góra; Egg-laying date^2=the date of egg laying by Colorado potato beetle generated by NumoGen 1 model; Date of adult form appearance^2=the date of appearance of Colorado potato beetle adult form generated by NumoGen 1 model; Number of days^2=number of days between date of egg laying and the date of adult form of Colorado potato beetle generated by NumoGen 1 model; Difference = the result of subtracting the results of field observations from those generated by computer simulations.

Table IV. Comparison of simulation results generated by NumoGen 1 and SimLep 1 on the basis of meteorological data registered at Winna Góra.

	SimLep 1			NumoGen 1
Year	Egg-laying date^1	Date of adult form appearance^1	Number of days^1	Egg-laying date^2	Date of adult form appearance^2	Number of days^2	Difference [days]
2002	3.05	27.06	55	3.05	28.06	56	1
2003	6.05	3.07	58	6.05	29.06	54	−4
2004	5.05	*		5.05	23.07	79
2005	22.05	10.07	49	22.05	8.07	47	−2
2006	8.05	6.07	59	8.05	5.07	58	−1
2007	13.05	2.07	50	13.05	29.06	47	−3
2008	14.05	8.07	55	14.05	7.07	54	−1
2009	9.05	16.07	68	9.05	17.07	69	1
2010	21.05	14.07	54	21.05	13.07	53	−1
2011	9.05	30.06	52	9.05	29.06	51	−1
Egg-laying date^1=the date of egg laying by Colorado potato beetle generated by SimLep 1 model; Date of adult form appearance^1=the date of appearance of Colorado potato beetle adult form generated by SimLep 1 model; Number of days^1=number of days between date of egg laying and the date of adult form of Colorado potato beetle generated by SimLep 1 model; Egg-laying date^2=the date of egg laying by Colorado potato beetle generated by NumoGen 1 model; Date of adult form appearance^2=the date of appearance of Colorado potato beetle adult form generated by NumoGen 1 model; Number of days^2=number of days between date of egg laying and the date of adult form of Colorado potato beetle generated by NumoGen 1 model; Difference = the result of subtracting the results of SimLep 1 from those generated by NumoGen 1; *Simulation did not generate information about appearance of adult beetles.

Results

Comparison of results of simulations based on real data and the data obtained after raising the recorded temperature by 1–6°C confirmed the stimulating effect of temperature on the rate of beetle development. On the basis of real data, the number of days required for the development of the Colorado potato beetle first generation from egg to adult amounted on average to 46 days (Figure 1). Raising the temperature by 1°C resulted in a shortening of the period by 3 days, to 43 days. Subsequent modifications involving an increase in temperature of 2, 3 and 4°C gave a total generation period of 39, 36 and 33 days, respectively, i.e., a decrease of 3–4 days per 1°C increase. Further increasing the temperature by 5 and 6°C gave a smaller (1 day/°C) decrease in the generation period, to 31 and 30 days, respectively. The relationship between temperature increase and development of the first generation of the beetle is expressed using the following equation:

2

Figure 1.  Influence of temperature increase on the number of days needed to complete the first, second and third generations of Colorado potato beetle. T1–T6 refer to temperature increase of 1–6°C. The number of days expressed on the y-axis was achieved using the following equations: for first generation y=17.7572171454938 + exp(3.35092368334087+(−0.152325086952484)*x); x=temperature increase (1–6°C); for second generation y=21.07043775382 + exp(2.92595137352898+(−0.269914662094477)*x); x=temperature increase (1–6°C); for third generation y= − 497.148817944526 + exp(6.27873052566205+(−0.00411496277771)*x); x=temperature increase (4–6°C).

where y=number of days needed for the development of the first generation of the Colorado potato beetle and x=temperature increase (1–6°C).

Simulations performed on the basis of real data collected in the period 2002–2011 showed that the date of appearance of adult forms of the first generation ranged from 11 July to 5 August (Table V). The observed dates of appearance of first-generation adult beetles in the period 1–28 July were used in calculations conducted after raising the temperature by 1°C. With an increase in temperature of 2 and 3°C, the dates of appearance of first-generation adults were 23 June–20 July and 17 June–12 July, respectively. The corresponding dates produced by simulations performed on data obtained after a real temperature increase of 4, 5 and 6°C were 13 June–6 July, 8 June–1 July and 6–28 June, respectively.

Table V. Date of appearance of first, second and third generation of Colorado potato beetle adults resulted in simulations using NumoGen 1 model.

		Year
Temperature increase [°C]	No. of gener.	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011
T	1	18.07	21.07	5.08	16.07	11.07	19.07	26.07	29.07	22.07	12.07
	2					20.08
	3
T+1	1	11.07	12.07	28.07	10.07	9.07	14.07	16.07	22.07	17.07	1.07
	2	20.08	16.08		24.08	10.08	24.08				19.08
	3
T+2	1	6.07	3.07	20.07	3.07	6.07	3.07	8.07	16.07	12.07	23.06
	2	10.08	7.08	25.08	10.08	1.08	13.08	15.08	25.08	15.08	7.08
	3
T+3	1	27.06	26.06	12.07	27.06	3.07	24.06	2.07	12.07	8.07	17.06
	2	1.08	31.07	15.08	30.07	28.07	3.08	5.08	16.08	7.08	28.07
	3
T+4	1	22.06	19.06	6.07	23.06	30.06	19.06	28.06	6.07	4.07	13.06
	2	27.07	24.07	9.08	23.07	24.07	24.07	30.07	8.08	31.07	19.07
	3	24.08	21.08			24.08					24.08
T+5	1	19.06	14.06	1.07	20.06	27.06	14.06	24.06	1.07	1.07	8.06
	2	18.07	19.07	4.08	16.07	20.07	17.07	25.07	1.08	25.07	12.07
	3	16.08	13.08		18.08	17.08	17.08	23.08		24.08	14.08
T+6	1	15.06	11.06	27.06	16.06	24.06	12.06	21.06	28.06	28.06	6.06
	2	13.07	14.07	29.07	13.07	16.07	13.07	19.07	25.07	21.07	7.07
	3	8.08	7.08	23.08	9.08	8.08	9.08	14.08	23.08	16.08	5.08
T – temperature registered at Winna Góra [°C]; T+1–T+6 – virtual value of temperature obtained after temperature increase of 1–6°C; No. of gener.=number of generation of Colorado potato beetle obtained in simulations using NumoGen 1.

Thus the simulations showed that the average date of Colorado potato beetle adult appearance based on data recorded at Winna Góra was 21 July. Raising the temperature by 1°C resulted in an acceleration of the Colorado potato beetle first generation appearance by 7 days, while a simulated increase in temperature of 2 and 3°C gave an acceleration of 14 and 20 days. When the temperature was increased further by 4, 5 and 6 °C, the acceleration in appearance of first-generation adults amounted to 25, 29 and 32 days.

The relationship between temperature increase and acceleration of the first-generation adult appearance is expressed using the following equation:

3

where y=acceleration of the first-generation adult appearance (days) and x=temperature increase (1–6°C).

The calculations also revealed a close relationship between temperature and development of the second generation of Colorado potato beetle. The number of days required for the development of the second generation, estimated on the basis of simulations of a recorded temperature increase of 1°C, amounted to 36 days (Figure 1). Raising the temperature by 2 and 3°C shortened the period until the development of the second beetle generation to 32 and 29 days, respectively. A temperature increase of 4, 5 and 6°C shortened the period required by a further 1–2 days, to 28, 26 and 25 days, respectively. The relationship between temperature increase and development of the second beetle generation is expressed using the following the equation:

4

where y=number of days needed for the development of the second generation of the Colorado potato beetle and x=temperature increase (1–6°C).

Raising the temperature also affected the date of appearance of the second-generation adults of Colorado potato beetle (Table V). The results of calculations conducted on the basis of data obtained after a temperature increase of 1°C showed that the second-generation adults appeared in the period 10–24 August. When the temperature was increased by 2 and 3°C, the dates ranged from 1 to 25 August and 28 July to 16 August, respectively. As an effect of a 4, 5 and 6°C increase in temperature, the corresponding dates were 19 July–9 August, 12 July–4 August and 7–29 July, respectively. The average data of appearance date of the second-generation adults achieved in calculations when the temperature was increased by 1°C was 18 August. Raising the temperature by 2 and 3°C accelerated the appearance of Colorado potato beetle second-generation adults by 11 and 19 days, respectively (Figure 2). As a result of data transformation consisting of a temperature elevation of 4, 5 and 6°C, the period for appearance of the second-generation adults was shortened to 26, 33 and 37 days, respectively.

Figure 2.  Relationship between temperature increase and acceleration (expressed in days) of the appearance of adults of the first, second and third generation of Colorado potato beetle. T1–T6 refer to temperature increase of 1–6°C. The number of days expressed on the y-axis was achieved using the following equations: for first generation y= − 0.438095*x**2 + 8.05*x–0.240476; x=temperature increase (1–6°C); for second generation y= − 0.723214*x**2 + 12.45774*x+ − 11.5500; x=temperature increase (2–6°C); for third generation y= − 0.5*x**2 + 14.5*x−50.0; x=temperature increase (4–6°C).

The relationship between temperature increase and acceleration of the second-generation adult appearance is expressed using the following equation:

5

where y=acceleration of the second-generation adult appearance (days) and x=temperature increase (2–6°C).

The simulations also showed a relationship between temperature and development of a third generation of Colorado potato beetle. Calculations using the real and virtual data obtained after a temperature increase of 1–3°C showed that development of a third generation of the beetle was not possible but with an increase in temperature of at least 4°C, the simulations generated dates for appearance of third-generation adults. The average number of days required for the development of the third generation of the Colorado potato beetle after a temperature increase of 4, 5 and 6°C was 27, 26 and 23 days, respectively (Figure 1).

The relationship between temperature increase and development of the third pest generation was as follows:

6

where y=number of days needed for the development of the third generation of the Colorado potato beetle and x=temperature increase (4–6°C).

Simulations performed with a 4, 5 and 6°C increase in mean temperature showed that the period of the third-generation beetle appearance ranged from 21 to 24 August, 17 to 24 August and 12 to 23 August, respectively (Table V). The average date of appearance of the third generation adults following an increase in temperature of 4, 5 and 6°C was 23, 15 and 7 August, respectively. This means that the acceleration of appearance of the third-generation adult as a consequence of a temperature increase of 5 and 6°C in comparison with that achieved after a temperature increase by 4°C amounted to 10 and 19 days, respectively (Figure 2). The relationship between temperature increase and acceleration of third-generation adult appearance was:

7

where y=acceleration of the third-generation adult appearance (days) and x=temperature increase (4–6°C).

The analyses conducted using the model NumoGen 1 also allowed the impact of anticipated changes in climate conditions on the number of generations of the beetle to be estimated. Nine out of ten simulations performed on the basis of real data showed the appearance of only one generation of the beetle (Table VI). Following a simulated increase in temperature of 1°C, 60% of the results indicated the possibility of two generations of Colorado potato beetle. The remaining data enabled two generations as a result of increasing the temperature by 2°C. Third-generation appearance was conditioned by increasing the temperature recorded in 2002, 2003, 2006 and 2011 by 4°C. Similar results were achieved when the temperature recorded in the years 2005, 2007 and 2010 was increased by 5°C. However, the simulations using the meteorological data recorded in 2004 and 2009 generated the appearance of three pest generations only after raising the temperature by 6°C. The results of simulations allowed the relationship between temperature increase and the number of Colorado potato beetle generations to be determined as:

8

Table VI. Comparison of mean temperature in April–August with results of simulation using the NumoGen 1 model.

	Temperature increase [°C]
	T		T+1		T+2		T+3		T+4		T+5		T+6
Year	April–August	No. of gener.	April–August^+1	No. of gener.	April–August^+2	No. of gener.	April–August^+3	No. of gener.	April–August^+4	No. of gener.	April–August^+5	No. of gener.	April–August^+6	No. of gener.
2002	17.7	1	18.7	2	19.7	2	20.7	2	21.7	3	22.7	3	23.7	3
2003	16.3	1	17.3	2	18.3	2	19.3	2	20.3	3	21.3	3	22.3	3
2004	15.0	1	16.0	1	17.0	2	18.0	2	19.0	2	20.0	2	21.0	3
2005	15.9	1	16.9	2	17.9	2	18.9	2	19.9	2	20.9	3	21.9	3
2006	16.4	2	17.4	2	18.4	2	19.4	2	20.4	3	21.4	3	22.4	3
2007	16.1	1	17.1	2	18.1	2	19.1	2	20.1	2	21.1	3	22.1	3
2008	15.6	1	16.6	1	17.6	2	18.6	2	19.6	2	20.6	3	21.6	3
2009	15.6	1	16.6	1	17.6	2	18.6	2	19.6	2	20.6	2	21.6	3
2010	15.4	1	16.4	1	17.4	2	18.4	2	19.4	2	20.4	3	21.4	3
2011	16.1	1	17.1	2	18.1	2	19.1	2	20.1	3	21.1	3	22.1	3
T – temperature registered at Winna Góra [°C]; T+1–T+6 – virtual data obtained after temperature increase of 1–6°C; April–August = mean value of temperature registered from 1 April till 31 August at Winna Góra; April–August^+1=mean value of data resulting in 1°C increase of temperature registered from 1 April till 31 August at Winna Góra; April–August^+2=mean value of data resulting in 2°C increase of temperature registered from 1 April till 31 August at Winna Góra; April–August^+3=mean value of data resulting in 3°C increase of temperature registered from 1 April till 31 August at Winna Góra; April–August^+4=mean value of data resulting in 4°C increase of temperature registered from 1 April till 31 August at Winna Góra; April–August^+5=mean value of data resulting in 5°C increase of temperature registered from 1 April till 31 August at Winna Góra; April–August^+6=mean value of data resulting in 6°C increase of temperature registered from 1 April till 31 August at Winna Góra; No. of gener.=number of generation of Colorado potato beetle obtained in simulations using NumoGen 1.

where y=number of Colorado potato beetle generations and x=temperature increase (1–6°C).

Equation 8 is demonstrated graphically in Figure 3.

Figure 3.  Influence of temperature increase on the number of Colorado potato beetle generations achieved in simulations using NumoGen 1. T1–T6 refer to temperature increase of 1–6°C. Numbers 1.2–3.0 refer to the number of Colorado potato beetle generations resulted in calculation using the following equation: y= − 0.008333*x*2 + 0.353571*x+1.17619, where y=number of Colorado potato beetle generations and x=temperature increase (1–6°C).

Discussion

Our computer simulations indicated a growing threat to potato from Colorado potato beetle. An increase of 1–2°C in actual temperature recorded at Winna Góra (52.2N; 17.4E) in recent years would create opportunities for the development of two full generations of the insect in the Wielkopolska region of Poland. An increase in temperature of at least 4°C could result in three generations. The results obtained corresponded well with the actual development of Colorado potato beetle recorded in regions characterised by a higher temperature than that in the Wielkopolska region. For example, in the regions of Campania in Italy and in Caucasus, where the mean temperature recorded in the period April–August amounts to 21 and 20.6°C, respectively, three generations of Colorado potato beetle occur. Two full generations and one partial generation of the pest also occur in North Carolina, where the mean temperature in the period April–August is 19.5°C. Our results dealing with the influence of temperature increase on the number of Colorado potato beetle generations confirm the findings by Kocmánková et al. (2010) that Colorado potato beetle usually develops one generation in the Czech Republic but during particularly warm seasons lowland areas can experience two generations. The simulations performed by Kocmánková et al. (2010) using Climex software tool predicted an increase in the number of Colorado potato beetle generations from one to even three for 2050 as a result of expected climate change in the Czech Republic. That study also confirmed the influence of temperature increase on widening the area occupied by Colorado potato beetle. An expansion of the area occupied by this pest was also reported by Baker et al. (2000). It is known from previous research that temperature influences the speed of insect development time (Howe, 1960; Cammell & Knight, 1992), and the results from our study confirmed this for the case of development time of Colorado potato beetle, which decreased with increasing temperature. Similar results for the first generation of Colorado potato beetle were reported by Wittchen and Freier (2008), who used the SimLep 1 model to study the effect of climate change on the population dynamics of the pest. Their results showed that a temperature increase of 2 and 3.7°C decreased the developmental time from egg to appearance of adults by 7 and 15 days, respectively. However, relating the development of Colorado potato beetle solely to temperature is a major simplification of reality. According to Harrington et al. (2001), temperature does not act in isolation to influence pest status and interactions with other variables such as rainfall, humidity, irradiance and carbon dioxide concentrations should be taken into account. It is also known that insect developmental rate is a nonlinear function of temperature (Logan et al., 1976). A linear approximation for this function could result in underestimated development at low temperatures and overestimated development at high temperatures (Bergant et al., 2005). In addition, growth models of potato estimating the influence of climate change on plant ontogenesis would improve the credibility of the simulation results. However, despite the simplifications involved, the present study provides valuable information on the response of Colorado potato beetle to a temperature increase and according to some researchers, the effect of temperature on insects largely overwhelms the effects of other environmental factors (Bale et al., 2002). Faster developmental time and more generations of Colorado potato beetle as a result of expected temperature increase, as shown in this study, could lead to larger populations and consequently more damage to host plants (Bergant et al., 2005). However, such a scenario would only be possible in a situation of continuous availability of food for the insect. According to Haverkort (2008), elongation of the vegetation season in the northern hemisphere will create conditions for earlier potato crops and for obtaining higher yields. A similar view was presented by Davies et al. (1996), Peiris et al. (1996) and Carter et al. (1996), who forecast an increase in potato yields as a result of climate warming in England, Scotland and Finland, respectively. For Poland, an increase in the length of the vegetation period of between 30 and 40 days is projected for nearly all of the country as a result of temperature increase (Szwed et al., 2010). Kedziora (1995) predicted an even greater extension of the vegetation period in Poland, by 51–125 days. The longer growing season would lead to increased pest populations and associated insecticide use. Therefore one possible impact of Colorado potato beetle response to climate change would be increased cost of potato production, which would increase the market price of potatoes in the long term. Moreover, repeated exposure to pesticide is the primary driver in the development of pest resistance, and resistance can develop rapidly in pests that have many generations per year, with multiple generations exposed to pesticide. Such a situation is expected to be common for Colorado potato beetle when the temperature increases. These findings indicate a need for sustained investment in integrated pest management and in breeding for potatoes with pest resistance, in addition to increased emphasis on breeding and selection for heat tolerance.