ROLE OF THE COCOON OF ACROLEPIOPSIS-ASSECTELLA (LEP, HYPONOMEUTOIDAE)IN HOST RECOGNITION BY THE PARASITOID DIADROMUS-PULCHELLUS (HYM, ICHNEUMONIDAE)

The study of various behavioural criteria of female Diadromus pulchell us parasitoids in the presence of their Acrolepiopsis assectella hosts has shown the essentially chemical nature of the stimulant determinin g host recognition. The physical stimuli of the cocoon seem not to be implicated. Thus cocoons whose original texture has been completely al tered, either mechanically or chemically, as well as the silk excreted by the host caterpillars significantly stimulates the female parasito ids. The cocoon contact kairomones are detected in testing the aqueous extracts of A. assectella which provoke a positive behavioural respon se from the females in a threshold concentration of 1 cocoon-equivalen t. The comparison of aqueous extracts of A. assectella host cocoons, a nd of non-host species: Bombyx mori, Ephestia kuehniella and Cacoecimo rpha pronubana demonstrate the kairomone specificity of the silk, the extracts of non-host cocoons being ignored by the parasitoid, as were the silk threads left by E. kuehniella caterpillars. Finally the conta ct kairomones linked to the silk seem to be independent of the host pl ant and of the nutrient diet of the host caterpillars. The cocoons spu n by the caterpillars reared on leeks or on artificial diets with or w ithout powdered leek provoke similar responses in the parasitoids.