The effect of parasitic hymenoptera on cereal aphid population development

This study examined the effect of parasitism on cereal aphid populations on farmland in Hampshire between 19:9 and 1981. Artificially established field populations of cereal aphids were parasitised by AphLdLuo nhopaeosiph.i, and A. pLc.ipe6 throughout the winter and spring in grassland, and in some autumn-drilled cereals depending on their sowing date. These crops were also shown to be potentially important sources of AphidLus parasitoids in may, at a time when parasitoid immigration was evident in the majority of wheat fields. Female AphLd.iaa emigration was related to high parasitoid density and low aphid availability. Laboratory experiments were carried out to determine development rates in relation to physiological time. Winter field trials showed that linear extrapolations to developmental threshold temperatures may underestimate rates of development when temperatures oscillate around that threshold. Aphid age at parasitism was found to have a profound effect on aphid development and reproductive potential. A simulation model was developed to quantify the impact of parasitoids on cereal aphid population development relative to other mortality factors, based on laboratory-derived parameters and data from a large-scale survey of cereal fields between March and July. While parasitoids were shown to be the major cause of aphid mortality early in the season, they were not the principal factor determining the levels of SLtob.ion avenae populations in 1981. These results are related to other years, and are discussed in the context of developing an accurate cereal aphid forecasting system.