n African Entomology - Effect of a granulovirus larvicide, Madex®, on egg-laying of L. (Lepidoptera: Tortricidae) due to changes in chemical signalization on the apple leaf surface




Applications of Madex® (granulovirus) against the codling moth, , which targets neonate larvae before or during their initial entry into fruit, provide selective control of this key apple pest. Differences in the efficacy of Madex® treatment against larvae were observed in an experimental orchard in northern Italy on two apple tree cultivars, 'Golden Delicious' and 'Red Chief'. The intensity of egg-laying by the moth may vary from one cultivar to another. The composition of a metabolite blend on the leaf surface consisting of glucose, fructose, sucrose, sorbitol, quebrachitol and -inositol is one of the factors that could explain these variations. Our hypothesis was therefore that variations in the efficacy of Madex® could be related to variations in egg numbers, itself related to the composition of metabolites on the leaf surface of each cultivar. Differences in egg-laying (number and localization) were recorded on the two cultivars, 'Golden Delicious' and 'Red Chief', untreated or treated with the larvicide. Madex® had the same efficacy on both cultivars in terms of the reduction of fruit damage due to larval feeding. Surprisingly, the effect of Madex® on reducing damage seemed less tied to a lethal effect of the larvicide on the larvae than to egg-laying reduction, the latter being linked to changes in chemical signals of the leaf surface induced by applications of Madex®. This observation was verified by experiments on egg-laying on an artificial substrate. When the active metabolite blends mimicking the corymb leaf surface compositions of each cultivar (untreated and treated with Madex®) were applied to nylon cloth egg-laying substrates, significant reductions in acceptance (% of egg-laying females) and egg-laying stimulation (number of eggs per egg-laying female) were observed. The reduction of eggs on 'Red Chief' could be primarily explained by a drastic effect on egg-laying stimulation, whereas the reduction on 'Golden Delicious' was partly due to a lower acceptance. The number of eggs laid naturally differs from one apple tree cultivar to another. The effects of reduced egg-laying caused by applications of Madex® were due to biochemical changes in surface blends, depending on the cultivar.


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