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Crop Protection 30 (2011) 1103e1105

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Crop Protection

journal homepage: www.elsevier .com/locate/cropro

Short communication

Could movement of neonates from non-host plants affect the potentialof polyculture to reduce crop colonisation by pest insects?

D.R. George a,*, R. Collier b, C. Whitfield a, G. Port a

a School of Biology, Newcastle University, Newcastle upon Tyne, NE1 7RU, UKbWarwick Crop Centre, School of Life Sciences, University of Warwick, Wellesbourne, Warwick, CV35 9EF, UK

a r t i c l e i n f o

Article history:Received 25 February 2011Received in revised form18 April 2011Accepted 22 April 2011

Keywords:PolycultureNon-host ovipositionLarval movementPlutella sp

* Corresponding author. Lancaster Environment CLancaster, LA1 4YQ, UK.

E-mail address: d.george@lancaster.ac.uk (D.R. Geo

0261-2194/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.cropro.2011.04.009

a b s t r a c t

Existing and impending constraints to the use of synthetic pesticides, along with changing legislation infavour of an integrated approach to pest management, are driving uptake of ‘alternative’ approaches topest control. Polyculture is one such approach, often relying upon the presence of non-host plants withinthe crop to reduce colonisation by pest insects. The diamondback moth, Plutella xylostella (L.), is known tooviposit on non-host plants and the present study showed that neonates were able to move relativelylarge distances, enabling them to relocate from non-host natal plants to nearby hosts. Although fewerlarvae were found on host plants when they hatched on non-hosts, it is concluded that even a low levelof re-location of neonates could potentially reduce the potential for polyculture to control this pest. It isproposed that non-host oviposition and neonate movement could explain why polyculture sometimesfails in controlling pest insects.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The agricultural industry faces a range of issues linked toconventional crop protection using synthetic pesticides. Theseinclude pesticide resistance, product withdrawal, retailer andconsumer demand for ‘environmentally friendly’ crop productiontechniques and future legislation that will require growers to adoptan integrated approach to pest control. These pressures haveresulted in the need for a more considered approach to pesticideuse and for greater exploitation of alternative methods for main-taining pest populations below economic damage thresholds.

Companion planting, intercropping and undersowing are allforms of polyculture that have been shown to reduce pest insectnumbers in crops, with increased within-crop diversity disruptinghost location in several pest insect species (Finch and Collier, 2000).However, in a number of studies where diversity of plants withinthe crop was increased, pest numbers were not reduced, the effectswere variable, or pest numbers increased (Andow, 1991).

It is possible that in some cases pest insects could benefit fromthe presence of non-host plants because they provide alternativeoviposition sites. Oviposition on non-host surfaces is not uncommonamongst Lepidoptera and has been observed in the brassica pest,

entre, Lancaster University,

rge).

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Plutella xylostella (L.) (Charleston and Kfir, 2000; Badenes-Perez et al.,2004; George et al., 2007). Neonate larvae, particularly in the Lepi-doptera, may migrate in response to the low quality of a natal plant(Zalucki et al., 2002) or display intra-plant movement in response tosub-optimal eclosion sites (Perkins et al., 2010). It follows thatneonates hatching upon non-hosts could display similar behaviour,where the chance of successfully locating a suitable host would beincreased if these were in close proximity, as with undersowing,intercropping or companion planting schemes.

Oviposition on non-host plants and host-switching by neonatesare behaviours that could occur together, and their joint influencecould explain why the responses of pest insects to polyculture mayvary. The aim of the current study was to investigate whetherneonate P. xylostella hatching on non-host plants would move tonearby hosts. Movement of larvae between host plants was alsoassessed and an attempt was made to quantify the potential ofneonate P. xylostella to move. P. xylostella was selected as the testspecies due to its propensity for non-host oviposition (see above)and its status as a globally important brassica pest (Sarfraz et al.,2006; Grzywacz et al., 2010).

2. Materials and methods

Two treatments were used, each replicated eight times ina single experimental run. The first (control) comprised six cauli-flower host plants (Brassica oleracea var. botrytis ’Lateman’ (L.))placed in three rows within a mesh cage (60 cm2), with each row

D.R. George et al. / Crop Protection 30 (2011) 1103e11051104

containing two plants. The second treatment was arranged in thesame way, but the middle row of two cauliflower plants wasreplaced by two non-host tomato plants (cv The Amateur (Lyco-persicon esculentum (Mill.)). Within and between adjacent rows,plants were in contact with their nearest neighbour, but not thecage. All plants were greenhouse grown, potted and 5e6 weeks oldwhen used. Cauliflower plants were at the 5-leaf stage andapproximately 20 cm tall when used, with tomato plants beingslightly larger at the start of the experiment (c. 6-leaf stage and25 cm tall).

P. xylostella eggs (1e3 days old) were obtained from cultures atNewcastle University (UK) and 20 eggs placed on each plant in themiddle row in each treatment; 20 on the first true leaf of one plantand 20 on the stem of the other. Eight days later all plants werechecked for larvae. In control treatments, a distinction was madebetween the larvae found on natal and non-natal cauliflower.

Differences between treatments in the total numbers of larvaerecovered from each cage, the numbers of larvae that had movedfrom natal plants, and the number of larvae remaining on theirnatal plants as compared with those that had left them in thecontrol treatment, were analysed by a Generalised Linear Modelwith a log link function for Poisson distributed (count) data. Bothcage aspect (either East or West facing) and treatment wereconsidered as main factors. All analyses were run using SPSS v.17.

The potential of neonate P. xylostella to move was assessed usinga radial-trapping technique. Four circles with radii of 7.5, 15, 25, and50 cm where drawn onto separate sheets of white card and thecircumference of each coated with Tangle-Trap�. Approximatelytwenty 2 day old P. xylostella eggs were placed into the centre ofeach circle and left for 72 h, after which the number of neonatestrapped was counted (T), as was the number that had emerged, buthad not been trapped (N). The percentage radial movement over72 hwas then calculated by ‘T/(TþN)� 100’ and regression used toestimate radial movement potential. The experiment was con-ducted at room temperature (c. 20 �C) under natural laboratorylighting (c. 16:8 Light:Dark). Each treatment was replicated once.

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Fig. 1. Mean Plutella xylostella larvae (from 40 initial eggs) recovered from a). All plants, b).control cages. Eggs were initially placed on middle rows of cauliflower or tomato plants. Ersignificantly different.

3. Results and discussion

Significantly fewer (60%) P. xylostella larvae were able to surviveif placed on a non-host plant in the vicinity of a host plant, than ifplaced on a host plant in the same position (c2

(1) ¼ 43.50, P < 0.01,cage aspect was not significant) (Fig. 1a). Nevertheless, neonateswere capable of moving from non-host to host plants, albeit ata cost to their overall survival. As female P. xylostellamay lay a largepercentage of their eggs on non-host plants in polycultures (Georgeet al., 2007, though see also Sarfraz et al., 2006), even relatively lowlevels of larval movement from these plants could present signifi-cant pest pressure to a nearby crop.

Larval movement to the non-natal cauliflower plants was morethan two and a half times greater when the eggs were placed ontomato, suggesting that larval movement increased in response tohatching on a non-host vs a host plant (c2

(1)¼ 18.88, P< 0.001, cageaspect was not significant) (Fig. 1b). In support, significantly morelarvae were recovered from their natal plant than from adjacentplants in the control cages (c2

(1) ¼ 69.23, P< 0.001, cage aspect wasnot significant) (Fig. 1c), suggesting that neonates do not readilymove from suitable natal plants, at least not in large numbers.Nevertheless, some movement between host plants did occur with16% of larvae recovered fromnon-natal cauliflower in control cages.This figure would have been higher still if movement between natalcauliflowers could have been considered. Though beyond the scopeof the current paper, it is worthy of note that any inherent inter-host movement of this type could affect the efficacy of trap crop-ping for P. xylostella control. Such movement could theoreticallyincrease over-spill of larvae from trap to main crop plants andpossibly explain the unreliable nature of trap cropping forP. xylostella control (Charleston and Kfir, 2000; Sarfraz et al., 2006).

More than 50% of neonate P. xylostella hatching on a benignsurface (card) were capable of radial movement of at least 50 cmover 72 h, with radial movement potential explained by theequation: ‘% neonates trapped ¼ �0.7399 � radius þ 88.797’(where R2¼ 0.7485) (Fig. 2). Based on this equation, a projected 15%

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Outer rows of cauliflower plants, and c). Middle and outer rows of cauliflower plants inror bars show � SEs. n ¼ 8 for all means. Treatments not sharing a common letter are

Fig. 2. Percentage radial movement of Plutella xylostella neonates over 72 h. Equationof linear relationship: y ¼ �0.7399x þ 88.797. R2 ¼ 0.7485. Dashed line showsextrapolation of the linear relationship beyond the experimental data.

D.R. George et al. / Crop Protection 30 (2011) 1103e1105 1105

of neonates could be able to travel a radial distance of 1 m overa 72 h period (Fig. 2). However, as the methodology used under-estimates actual distances travelled (by not accounting forturning), it is likely than in reality neonates could be expected totravel much greater distances. Though results can only be consid-ered preliminary in nature (due to a lack of replication andextrapolations that await confirmation), this supports findingselsewhere that P. xylostella neonates may be relatively mobile for atleast short periods if not on a suitable host (Eigenbrode and Pillai,1998).

Previous studies have indicated that undersowing, intercrop-ping and companion planting may have a smaller effect on hostcolonisation by P. xylostella compared with other brassica pests.Oviposition by P. xylostella on brassica host plants was, for example,considerably less affected by a clover background (40% reduction)than oviposition by several other species of Lepidoptera; 50%, 70%,70% and 90% reductions, respectively, for Pieris rapae (L.), Evergestisforficalis (L.), Pieris brassicae (L.) and Mamestra brassicae (L.)(Kienegger and Finch, 1995). Assuming oviposition on non-hostplants occurs in wild P. xylostella, as suggested by evidence ofhost range expansion (Löhr and Gathu, 2002; Henniges-Janssenet al., 2011), then the efficacy of many polycultures forP. xylostella control could be further reduced by non-host oviposi-tion and neonate movement.

Oviposition on non-host plants is not restricted to P. xylostella, orindeed to the Lepidoptera. When accompanied by neonate move-ment, such behaviour could reduce the effectiveness of companionplanting, intercropping and undersowing in pest management fora number of species, thus also explaining inconsistencies in theeffect of polyculture upon pest insects.

Acknowledgements

This work was funded by the Horticultural DevelopmentCompany (UK) and supported by the Yorkshire Agricultural Society(UK).

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