The role of natural enemies in cereal aphid population dynamics.
Cereal aphids have achieved high densities (over 40 aphids/tiller) in 4 of the the last 6 years in the Norwich area. In 1976, 1977 and 1980 the English grain aphid, Sitobion avenae (F.), was the commonest species but in 1979 the rose grain aphid, Metopolophium dirhodum (Wlk.), reached outbreak levels. S. avenae, in particular, regularly causes yield losses exceeding 10% (George & Gair, 1979) and can also reduce grain quality (Lee, Stevens, Stokes & Wratten. 1981).
As many species of natural enemy attack cereal aphids it is difficult to evaluate their individual effects. In an attempt to resolve this problem a simulation approach was used in conjunction with laboratory and field studies of three groups of natural enemies: parasites (in particular Aphidius spp.), polyphagous (linyphiid spiders) and aphid-specific (coccinellids) predators. Field data for the spiders and coccinellids were collected at North Farm, West Sussex, and for the parasites at Norwich.