THE EFFECT OF WEEDS ON THE NUMBERS OF HOVERFLY (DIPTERA, SYRPHIDAE) ADULTS AND THE DISTRIBUTION AND COMPOSITION OF THEIR EGGS IN WINTER-WHEAT

The abundance and species composition of syrphid adults in herbicide-t reated and untreated headlands was recorded in a replicated within-fie ld experiment conducted over a two-year period. The highest numbers of the most abundant syrphid species, Episyrphus balteatus, were recorde d in the untreated headland strips. Analysis of the behaviour of the a dult flies showed that they were retained in the untreated strips beca use they were foraging on the flowering non-crop plants. The distribut ion of syrphid eggs between herbicide treated and untreated headland r eplicates was examined. A significant positive relationship between th e numbers of eggs per aphid and weed density was detected during June in the second year of the study. This is in part attributed to the ten dency of Melanostoma females to oviposit on non-crop plants. The impli cations of the implementation of a modified herbicide regime in the he adlands of cereal fields for the biological control of cereal aphids b y syrphid larvae is discussed.