Intraguild predators and the spatial distribution of a parasitoid

An experimental plot of the aphid Aphis fabae on various host plant species was colonized by natural populations of the aphidiine parasitoid Lysiphleb us fabarum and insect predators, especially coccinellids. Parasitism of A. fabae by L. fabarum was significantly depressed on plants bearing coccinell ids. The number of parasitized aphids increased with aphid abundance on thr ee plant species (Papaver dubium, Rumex obtusifolius, Vicia faba), but not on the plant species (Chenopodium album) which bore very high numbers of co ccinellids. In complementary laboratory experiments, L. fabarum offered a c hoice between odours of plants infested with A. fabae and/or coccinellids s elected the odour fields from coccinellid treatments at significantly lower frequency than the odour fields of other treatments. It is concluded that avoidance of coccinellids by L. fabarum contributes to the negative associa tion between the abundance of coccinellids and parasitoids in the field.