The diamondback moth,Plutella xylostellaLinnaeus (Yponomeutidae), remains one of the most serious pests of crucifers in many parts of the world, particularly in South East Asia. This paper reviews the wide array of strategies which have been used in attempts to controlP. xylostellaover the last 40 years and consolidates the fragmented and sometimes hard-to-access literature on two and three level trophic interactions (e.g. host plant—herbivore, herbivore—natural enemy and host plant-herbivore—natural enemy interactions) involving the pest. Special reference is made to studies and examples from South East Asia, and in particular Malaysia, whereP. xylostellaand its primary parasitoids have been well studied owing to the great economic significance of the pest in this region. It is suggested that a multitrophic approach to research may assist in the development of more sustainable methods for the management ofP. xylostellaand overcome some of the problems inherent with insecticide-intensive methods.
The methods used to evaluate and categorise the effects of pesticides on beneficial arthropods are reviewed, including the potential significance of indirect, 'sub-lethal' activity and the importance of bioavailability of pesticides under semi-field or field conditions. Interspecific and intraspecific differences in the susceptibility of natural enemies (parasitoids and predators) to pesticides are considered, including the use of resistant strains in Integrated Pest Management (IPM) systems. The potential impact of pesticide resistance in the target pest on its natural enemies is also discussed. The need to assess the influence of the host plant/cultivar on the efficacy of pesticides for use in integrated control programmes (ditrophic effects) and of the possible effects of such chemicals on host plant/cultivar-pest-parasitoid systems (tritrophic effects) are then described with specific reference to the diamondback moth and key endolarval parasitoids. Finally, the potential for manipulation of tritrophic interactions to enhance parasitism by application of low doses of toxicants is demonstrated.
This review identifies key ways in which manipulations of the crop environment based on detailed understanding of tritrophic interactions can contribute to improvements in the control of insect pests. Such approaches are likely to be of particular benefit against those pests, notably certain species of Lepidoptera and aphid, which are difficult to control with insecticides because of insecticide resistance or suppression of natural enemies. Particular attention is given to the compatibility (or otherwise) of partial plant resistance and biological control, citing examples which support contrasting tritrophic theories. Other areas considered and supported with examples include the use or effects of allelochemicals, refugia, intercropping, crop backgrounds, fertilization regimes, parasitoid conditioning (by host plants) and transgenic crops. Examples of manipulations involving use of selective insecticides which show compatibility with biological methods are also included owing to their possible suitability in integrated crop management programmes.
BBSRC IACR Rothamsted, Harpenden, Hertfordshire AL5 254, UK Abstract: The efficacy of Bacillus thuringiensis (Bt) subsp. kurstaki HD-1 ('Dipel'@; Btk; CryIA & CryII) and Bt. subsp. aizawai('Florbac'@; Bta; CryIA & CryIC) was assessed against larvae from various field populations of Plutellu xylostella (F2 generation) collected in the Cameron Highlands, Malaysia in April 1994 and a lowland population (SERD 2; F10 generation) collected in December 1993. Evidence of resistance to Btk and to a lesser extent Bta is reported in these populations (LC,, Toxicity Ratios [TR] = 3-14 and 2-8 respectively), most notably in SERD 2. The first recorded evidence of resistance to abamectin (TR = 17-195-fold) in field populations of P. xylostellu is also reported. In an unselected sub-population of SERD 2, the TR values for Btk, Bta and abamectin declined 2-to 3-fold ( P < 0.01) over six generations in the laboratory (F10-F16) while in sub-populations of SERD 2 selected with these products there was a significant ( P < 0.01) increase in the TR (15-, 3-and 2.5-fold respectively) when compared with the F10 generation. This suggests the presence of marked resistance to Btk and some resistance to Bta and abamectin. There is also evidence of slight cross-resistance to Btk in the Bta-selected sub-population but no evidence for the reverse selection of resistance or for cross-resistance between Btk and abamectin. Concurrent selection studies (Fll-Fl5) with another sub-population of SERD 2 demonstrated resistance to the acylurea insect growth regulator, teflubenzuron ('Nomolt'@) (29-fold increase in TR). Based on the selection experiments with SERD 2, estimates of realised heritability (h2) of resistance gave very high values for teflubenzuron and Btk (c.0.7) and moderate values for abamectin and Bta (c.0.3). The results are discussed in relation to integrated pest management (IPM) and insecticide resistance management (IRM) strategies for P. xylostella.
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