Reservoirs forAphidius erviHaliday (Hymenoptera: Aphidiidae), a polyphagous parasitoid of cereal aphids (Hemiptera: Aphididae)

Aphidius ervi Hal. parasitised natural populations of Acyrthosiphon pisum (Harris), Sitobion avenae (F.) and Microlophium carnosum (Buckt.) in Britain but parasitised few examples of S. avenae placed on trap plants in winter wheat, nettle-grass edge areas or lucerne-wheat strip crops. In laboratory experiments, Aphidius ervi reared from Acyrthosiphon pisum did not parasitise M. carnosum , and significantly fewer mummies were formed on S. avenae than on A. pisum . After rearing Aphidius ervi for 4–5 generations on S. avenae , similar numbers of mummies were formed on both hosts. The switch of hosts from Acyrthosiphon pisum to S. avenae was associated with the loss of certain parasitoid esterase enzyme bands revealed following electrophoretic separation of parasitoid proteins. Slow adaptation to new aphid hosts by polyphagous parasitoids such as Aphidius ervi may reduce the usefulness of reservoirs as a source of beneficial species.