Enhancing parasitism of wheat aphids through apparent competition: a tool for biological control

As cereal aphids are only present during 2-3 months per year in cereal crops, their parasitoids rely on alternative hosts to complete their life cycle. These parasitoids often have the same aphid hosts, which leads to apparent competition. Development on alternative host may increase parasitoid abundance and consequently increase their pressure on cereal aphids. Intensive agriculture frequently reduces the availability of non-crop habitats where alternative aphids may be present. The apparent competition hypothesis was examined in two traditional wheat plots compared to two other plots, with strips of alternative host aphids on leguminosae (red clover and alfalfa) and graminae (ryegrass and meadow fescue). In June/July, parasitism on wheat aphids (Metopolophium dirhodum, Sitobion avenae, and Rhopalosiphum padi) was higher in 1998 than in 1997, in all the plots studied. During summer 1998, significantly more parasitised wheat aphids were found in strip-designed plots than in the controls, and this higher parasitism rate was positively correlated to a higher parasitoid activity during May, due to the presence of alternative hosts. The interaction between wheat aphids in summer and aphids in the strips in spring is an example of apparent competition, mediated by shared parasitoids. Alternative hosts allow parasitoids to increase their numbers and consequently to apply a higher pressure on aphids in wheat crops. (C) 2003 Elsevier B.V. All rights reserved.