APHID-PARASITOID INTERACTIONS IN A WINTER CEREAL CROP - FIELD TRIALS INVOLVING INSECTICIDE APPLICATION

Large scale field trials were conducted to investigate aphid-parasitoi d interactions in winter wheat following spring applications of contac t or systemic insecticides. Percentage parasitism was also investigate d in the dominant aphid species using acrylamide-gel electrophoretic m ethods. Sitobion avenae (F.) Metopolophium dirhodum (Walk.) and Rhopal osiphum padi (L.) were the commonest aphids and showed typical pattern s of abundance through the growing season. Six parasitoid species were found: Aphidius rhopalosiphi de St.-Per., A. ervi Hal., A. picipes (N ees) and Praon volucre (Hal.) being the most common primary parasitoid s that attack the three main aphid species. The present work demonstra ted a degree of temporal separation of these dominant cereal aphid par asitoid species which has rarely been documented. There was also evide nce of earlier female activity in the crop in four of the six species. Parasitoid populations increased into June and peaked in July, coinci ding with and continuing after peak aphid populations, before rapidly declining into August. Both contact and systemic insecticides sprayed in spring produced only a slight initial trend of suppression of aphid populations compared to control treatments; no influence on late summ er populations was found. However, significant between-field differenc es occurred. Neither contact nor systemic insecticides had any effect on population sizes, species composition or life history of aphid para sitoids (Braconidae; Aphidiinae, Aphelinidae) compared to control plot s in the present study. Percentage parasitism was likewise unaffected by insecticides and reached a peak, based on electrophoretic methods, of 40% at peak aphid densities at the end of June. Estimates of parasi tism based on presence of mummies were up to 10 times lower. Possible reasons for the lack of effect of insecticide application in the prese nt study include timing of application (prior to main immigration peri od of aphids and parasitoids), mobility of populations and insecticide -induced stimulation of reproduction in aphids.