Pore size criteria of mesh crop covers for the exclusion of tomato-potato psyllid (Bactericera cockerelli)

Abstract

Control methods for tomato-potato psyllid (TPP; Bactericera cockerelli) are currently dominated by the use of synthetic biocides. A non-chemical alternative for TPP management is crop mesh, which forms a physical barrier between crop and pest. This study examined the ability of TPP adults to penetrate, or lay eggs through, 22 commercially available crop meshes using a laboratory bioassay. Adult TPP were prevented from moving through the mesh if the mesh complied with any one of the following criteria: shortest pore length <0.55 mm; longest pore length <0.60 mm; diagonal length <0.85 mm; area <0.3 mm². Eggs were found on foliage only when TPP adults had penetrated the mesh, suggesting that eggs could not be laid through the mesh. The results indicate that crop meshes may provide non-chemical control for TPP that can be used by organic growers, and producers attempting to reduce chemical inputs as part of integrated pest management.

Keywords:

• barrier controls
• Hemiptera
• non-chemical
• organic farming
• pest control

Introduction

The tomato-potato psyllid, (TPP) Bactericera cockerelli (Šulc) (Homoptera: Psyllidae), was confirmed as present in New Zealand in 2006 (Teulon et al. 2009). TPP attacks a wide range of fruit, vegetables and non-crop species within the Solanaceae, and causes plant damage due to the removal of photosynthates, introducing phytotoxic compounds in the saliva and the transmission of the bacterial pathogen Candidatus Liberibacter solanacearum, the cause of zebra chip disease in potatoes (Liefting et al. 2008; Thomas et al. 2011; Butler & Trumble 2012).

Research on direct management of TPP has primarily been based on synthetic insecticides, with some investigation of the potential of botanical pesticides and of biological control (Berry et al. 2009; Page-Weir et al. 2011; O’Connell et al. 2012; Jorgensen et al. 2013). While there has been some success achieved in the use of existing or newly registered synthetic insecticides, they are not considered an ideal (or permanent) solution in New Zealand because many growers of solanaceous field crops now utilise regimes of integrated pest management that aim to reduce the use of such chemicals (e.g. Cameron et al. 2009). Consequently, organic and other producers who do not wish to use, or are prohibited from using, synthetic insecticides currently have limited TPP management options.

Physical barriers to insects locating or settling on crop plants, such as mulches and kaolin dusts, provide a viable option for growers seeking an alternative to chemical pest control (Collier 2001, 2002; Peng et al. 2011). Mesh covers form a barrier between insect and crop, and are being developed in various regions of the world for the management of a wide range of invertebrate pests on a variety of vegetable crops, including Solanaceae (Keun et al. 2013; Saidi et al. 2013). Crop mesh is usually woven from monofilament materials such as nylon and polypropylene and is produced in a range of pore sizes depending on the target pest. Covers with smaller pores are heavier (per m²), more expensive and generally require more effort to handle. Therefore, in practice, mesh with the largest pore size that prevents entry of the target pest is often used, as this tends to be most economical, makes handling easier and minimises the effect of the cover on the crop microclimate.

Prior to full-scale field trials being performed on the efficacy of crop meshes to exclude TPP, it was desirable to establish which crop mesh covers, if any, could actually prevent movement of adult TPP on to foliage. The aim of this investigation was to assess the ability of a number of commercially available crop meshes to form a barrier to TPP, ascertain the maximum allowable pore dimensions (e.g. length, area) that still prevent movement of TPP, and determine the ability of TPP females to lay eggs through mesh that is in contact with leaf surfaces.

Materials and methods

Estimation of pore dimensions in cover meshes

The measurements of mesh pore size provided by suppliers vary in the type of measurement (e.g. thread gauge vs pore size) and we therefore assessed pore size independently. Four replicate pieces from 22 meshes (from 12 manufacturers) were examined microscopically (see Table 1 for details). Cosio, Empak, Redpath and QuantumGrow meshes were sourced from New Zealand; CropSolutions, Gromax and Wondermesh were sourced from the UK; and LS Econet was sourced from the Netherlands. In each piece of cloth, measurements were taken of the shortest and longest side of four pores selected on an ad hoc basis. These values were averaged for each piece of cloth, and then an overall mean calculated from the four replicate pieces of each type of cloth. The pores were assumed to be rectangular or square so a simplistic measure of pore area was calculated as Area = (longest side × shortest side), and length of the diagonal estimated by Diagonal = √(longest² + shortest²).

Table 1 Pore dimensions of commercial crop meshes and the number of TPP adults (x/5) and eggs found on the surface of a Capsicum leaf after 7 days’ exposure in a laboratory trial (see Methods for details) (mean ± sem; n = 4).

	Mesh		Pore size				Psyllids
ID	Description	Brand	Length (mm)	Width (mm)	Diagonal (mm)	Area (mm^2)	Adults (x/5)	Eggs
1	BioMesh	Cosio	0.691 (0.010)	0.228 (0.005)	0.708 (0.010)	0.158 (0.005)	0.00 (0.00)	0.00 (0.00)
2	125 gsm	Cosio	0.778 (0.012)	0.481 (0.007)	0.864 (0.014)	0.374 (0.007)	0.00 (0.00)	0.00 (0.00)
3	90 gsm	Cosio	0.831 (0.004)	0.831 (0.004)	1.082 (0.007)	0.692 (0.004)	0.13 (0.25)	0.25 (0.22)
4	45 gsm	Cosio	1.841 (0.119)	1.027 (0.038)	2.644 (0.202)	1.895 (0.038)	1.50 (0.41)	8.75 (4.41)
5	BioMesh	Redpath	0.686 (0.019)	0.230 (0.011)	0.704 (0.020)	0.158 (0.011)	0.00 (0.00)	0.00 (0.00)
6	0.3 mm thrip	CropSolutions	0.813 (0.009)	0.342 (0.008)	0.859 (0.010)	0.278 (0.008)	0.00 (0.00)	0.00 (0.00)
7	0.8 mm flea beetle	CropSolutions	0.828 (0.032)	0.828 (0.032)	1.083 (0.060)	0.695 (0.032)	0.25 (0.29)	0.00 (0.00)
8	1.3 mm cabbage fly	CropSolutions	1.420 (0.006)	1.420 ).006)	2.468 (0.019)	2.018 (0.006)	2.00 (0.00)	2.25 (1.24)
9	0.6 mm aphid^a	CropSolutions	0.573 (0.020)	0.570 (0.020)	0.663 (0.035)	0.332 (0.023)	0.00 (0.00)	0.00 (0.00)
10	Biomesh	QuantumGrow	0.806 (0.011)	0.266 (0.007)	0.835 (0.011)	0.214 (0.007)	0.00 (0.00)	0.00 (0.00)
11	WM16	Wondermesh	1.327 (0.043)	1.327 (0.043)	2.216 (0.117)	1.772 (0.043)	2.25 (0.29)	33.00 (26.05)
12	WM24	Wondermesh	0.770 (0.005)	0.770 (0.005)	0.974 (0.007)	0.596 (0.005)	0.38 (0.48)	0.00 (0.00)
13	WM32	Wondermesh	0.639 (0.023)	0.639 (0.023)	0.762 (0.036)	0.414 (0.023)	0.00 (0.00)	0.00 (0.00)
14	WM50^a	Wondermesh	0.800 (–)	0.231 (–)	0.821 (–)	0.185 (–)	0.00 (0.00)	0.00 (0.00)
15	wnm-110-16/10	Empak	0.800 (0.007)	0.363 (0.007)	0.851 (0.006)	0.290 (0.007)	0.00 (0.00)	0.00 (0.00)
16	wnm-130-20/10	Empak	0.797 (0.012)	0.267 (0.011)	0.825 (0.014)	0.213 (0.011)	0.00 (0.00)	0.00 (0.00)
17	0.8 × 0.25 mm	Gromax	0.773 (0.002)	0.253 (0.002)	0.798 (0.001)	0.196 (0.002)	0.00 (0.00)	0.00 (0.00)
18	1515	LS Econet	0.141 (0.005)	0.141 (0.005)	0.142 (0.006)	0.020 (0.005)	0.00 (0.00)	0.00 (0.00)
19	1535	LS Econet	0.345 (0.005)	0.161 (0.005)	0.350 (0.005)	0.056 (0.005)	0.00 (0.00)	0.00 (0.00)
20	2777	LS Econet	0.792 (0.030)	0.377 (0.007)	0.847 (0.033)	0.299 (0.007)	0.00 (0.00)	0.00 (0.00)
21	3977	LS Econet	0.802 (0.009)	0.258 (0.005)	0.828 (0.010)	0.207 (0.005)	0.00 (0.00)	0.00 (0.00)
22	4045	LS Econet	0.398 (0.029)	0.398 (0.029)	0.438 (0.043)	0.173 (0.029	0.00 (0.00)	0.00 (0.00)

^a Replicates lost due to eathquake damage.

Efficacy of TPP exclusion by different meshes

A laboratory experiment was conducted to determine the effect of mesh pore dimensions on penetration by adult TPP, and also examine whether psyllids were able to lay eggs on foliage through the mesh. Five adult TPP were placed in a lightproof plastic cup (70 mm diameter, 90 mm high). The cup was placed over a piece of mesh held between two metal plates with 40 mm diameter holes in their centre. Below the mesh, a second plastic container, of the same size, but transparent, housed a piece of phenolic floral foam (approx. 40 × 40 × 50 mm) saturated with water into which was inserted a single, freshly detached leaf from a bell pepper (Capsicum annuum L.) of approx. 400–900 mm² surface area, placed so the leaf pressed against the underside of the mesh. No other food or water was present in either container. This setup was repeated four times for each of the 22 types of mesh. The containers were housed in a controlled environment room at 25 ± 1 °C, 50%–60% RH, and a photoperiod of 16:8 hours L:D, for a period of 7 days, after which the number of TPP that had penetrated through the mesh, both alive and dead, and the number of eggs (on the leaf, mesh, cup sides etc.) were counted.

Statistical analysis

All statistical analyses were performed using Genstat (v16, VSN International, Hemel Hempstead, UK). The numbers of TPP passing through the mesh in each replicate chamber were analysed by logistic regression (of binomial proportions). The length of the shortest pore side, longest side, diagonal and area were each used as explanatory variables, and in each case a probit transformation was applied to the response variable and the dispersion parameter of each generalised linear model was estimated as part of the analysis. From the models produced, estimates were made of the dimensions that would exclude 50%, 95% and 99% of TPP, along with their 95% confidence intervals.

Results

From the original 88 containers, four replicates were lost (due to the lightproof cups being dislodged due to earthquakes). Of the remaining 84 chambers, TPP eggs were found on seven leaves, all of which also had TPP adults present. Thus there were no occasions when eggs were discovered in the absence of TPP adults, indicating TPP cannot, or preferred not to, lay eggs through the mesh (see Table 1). Eggs were also not found on any other substrate in the experimental arenas (e.g. mesh, walls of the container or floral foam).

TPP adults passed through the mesh cover on 17 occasions but for only six types of mesh (Table 1). The number of TPP passing through the mesh exhibited fairly clear logistic relationships with all mesh dimensions measured (Fig. 1). Estimates of the maximum allowable pore dimensions to exclude 99% of TPP are given in Table 2, and can be rounded to approximate values of: short length <0.55 mm; long length <0.60 mm; diagonal <0.85 mm; area <0.3 mm².

Table 2 Maximum pore dimensions (±95% CI) estimated by probit regression that would allow a given proportion of TPP to penetrate a mesh crop cover.

	Critical pore dimensions (mm [95% CI])
Pore dimension	50% exclusion	95% exclusion	99% exclusion
Short length (mm)	1.092 (1.054–1.133)	0.707 (0.651–0.753)	0.547 (0.472–0.608)
Long length (mm)	1.380 (1.293–1.493)	0.832 (0.741–0.907)	0.605 (0.475–0.701)
Diagonal (mm)	1.734 (1.666–1.809)	1.118 (1.032–1.192)	0.862 (0.750–0.955)
Area (mm^2)	1.507 (1.420–1.603)	0.663 (0.554–0.759)	0.313 (0.172–0.432)

Figure 1 Logistic regression curves displaying the proportion (%) of adult TPP able to pass through crop mesh (over 7 days) related to various mesh pore dimensions (short length, long length, diagonal length and area). The mean dimensions for each of 22 meshes were used as the explanatory variable. There were four replicates for each mesh (although many of these points overlap, and are thus not visible on the graphs, and four replicates were lost).

Only three of the meshes tested complied with the maximum allowable dimensions for all four parameters, which were all produced by LS Econet and all demonstrated 100% exclusion (Table 1). The six meshes that allowed TPP to penetrate complied with none of the maximum allowable dimensions, suggesting that if a mesh meets any of the necessary criteria it may be capable of excluding TPP.

Another mesh, Wondermesh WM32, also met none of the maximum allowable dimension criteria but had no TPP penetration, suggesting some other factor(s) we have not measured may have a role in TPP exclusion.

Discussion

The results of the study indicate that crop meshes with certain pore size dimensions can be a highly effective barrier to TPP adults and that eggs were only laid directly on to leaves and not through (or on) the mesh, even though the leaf was in contact with the mesh. Extrapolated to a field situation, this suggests that once in place and the edges ‘sealed’ by burying in soil, meshes could provide a complete barrier to TPP accessing crops and provide 100% exclusion until the crop is harvested. Other control methods, such as use of chemical sprays and the use of biocontrol organisms (e.g. entomopathic fungi and insect predators such as Coccinellidae; Lacey et al. 2009; O’Connell et al. 2012) are mostly curative, i.e. they kill TPP once it has infested a crop, and therefore after crop damage has occurred. In comparison, crop meshes prevent TPP accessing the crop, thereby preventing any damage occurring in the first place. In addition, once the mesh is in position there should be no need for resetting or ‘reapplication’ as is often the case with chemical insecticides.

Currently, purpose-designed woven crop mesh covers retail between NZ$6000–9000 per ha depending on the amounts purchased and mesh size (Wondermesh, pers. comm.). Spread over the expected 10 year working life of the mesh, this represents a $600–$900 annual cost.

These costs are not dissimilar to the costs of agrichemical control, with growers spending an additional NZ$300–1200 ha⁻¹ ($700/ha on average) on additional agrichemicals (Ogden 2011). Initial field trials at the Biological Husbandry Unit, Canterbury, indicate that the covers can increase the yield of marketable potatoes (compared with uncovered control plots) by approximately 9 tonne ha⁻¹ which, at current prices of NZ$800 tonne ex. field (Scott Lawson, Lawson’s Organic Farms Ltd, pers. comm.), equals NZ$7200 to the grower, giving a considerable return on investment. In addition, in warmer cropping areas, such as Hawke’s Bay on the North Island of New Zealand, untreated potato crops can yield zero marketable tubers (Scott Lawson, Lawson’s Organic Farms Ltd, pers. comm.), so the expected economic return could be higher still in such locations.

Although these results indicate that mesh crop covers show promise for controlling TPP, there are a number of issues not addressed by these laboratory assays. For example, although the mesh may deter TPP and other insects from accessing the crop, they may not bar access by micropathogens such as fungal spores. Similarly, if meshes alter aspects of the microclimate in the crop, such as temperature and relative humidity, this may influence the development of fungal diseases of solanaceous crops such as Phytophthora infestans and Alternaria solani (although a reduction in the incidence of disease is also possible; see Diez et al. 1999). As yet we have no information on the durability and longevity of meshes under New Zealand field conditions, or if the efficacy of meshes change as they age. Future work will examine the ability of meshes tested in this study to exclude TPP (from potato crops) under field conditions, and monitor any side effects of the mesh cover on crop microclimate and the development of crop diseases.