Aphid‐parasitoid interactions in a winter cereal crop: field trials involving insecticide application

Abstract

Large scale field trials were conducted to investigate aphid-parasitoid interactions in winter wheat following spring applications of contact or systemic insecticides. Percentage parasitism was also investigated in the dominant aphid species using acrylamide-gel electrophoretic methods. Sitohion uwnue (F.) Metopoloplziuni dirhodunz (Walk.) and Rhoyulosiphunz pa& (L.) were the commonest aphids and showed typical patterns of abundance through the growing season. Six parasitoid species were found: Aphidius rhopalo.siplii de %.-Per., A . ewi Hal.. A . picipcs (Nees) and Pruon zducre (Hal.) being the most common primary parasitoids that attack the three main aphid species. The present work demonstrated a degree of temporal separation of these dominant cereal aphid parasitoid species which has rarely been documented. There was also evidence of earlier female activity in the crop in four of the six species. Parasitoid populations increased into June and peaked in July, coinciding 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 summer populations was found. However, significant between-field differences occurred. Neither contact nor systemic insecticides had any effect on population sizes, species composition or life history of aphid parasitoids (Braconidae; Aphidiinae, Aphelinidae) compared to control plots 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 parasitism based on presence of mummies were up to 10 times lower.Possible reasons for the lack of effect of insecticide application in the present study include timing of application (prior to main immigration period of aphids and parasitoids), mobility of populations and insecticide-induced stimulation of reproduction in aphids. Appl. Ecol. 22,[825][826][827][828][829][830][831][832][833][834][835][836][837][838]

Farming practices change food web structures in cereal aphid–parasitoid–hyperparasitoid communities

“…Parasitism rates were below values which have been suggested to be effective for biological aphid control by parasitoids (Hawkins and Cornell 1994; Giller et al 1995) during aphid population development at the flowering and milk-ripening stages. Despite the lower food web complexity, parasitism rates were higher in the organically managed than conventional fields later in the season (at peak ripening).…”

The relationship between agricultural intensification and biological control: experimental tests across Europe

“…Aphid densities averaged 126 6 123 individuals per 100 shoots (n ¼ 160 plots) in the flowering period and 143 6 16 individuals per 100 shoots in the milk-ripening period in five European regions, thereby reaching the threshold level of economic damage (3-5 per shoot; Giller et al 1995) in 17% (27 of 160 plots) and 18% of experimental plots (29 of 160 plots), respectively, in the two sampling periods.…”

Relative importance of predators and parasitoids for cereal aphid control

“…Aphid infestations averaged 73.6 individuals per 100 shoots, well below the threshold level of economic damage (five per shoot; Giller et al 1995). The open-field population densities for aphids and their natural enemies are summarized in table 1.…”