Parasitism rate of bird cherry-oat aphid and cabbage aphid (Hemiptera: Aphididae) by Diaeretiella rapae (M’Intosh) (Hymenoptera:Braconidae) in an experienced versus a novel environment

The aphid parasitoid, Diaeretiella rapae (M’Intosh), is considered a specialist on Cruciferous plants, but is also capable of using aphid hosts in some grasses. In Oklahoma where canola, Brassica napus, is rotated with wheat, Triticum aestivum, there is opportunity for movement of D. rapae between crops and hosts. Laboratory colonies of D. rapae were maintained in both winter wheat infested with bird cherry-oat aphids, Rhopalosiphum padi, and canola infested with cabbage aphids, Brevicoryne brassicae. Diaeretiella rapae were collected as mummies from the two emergence environments and stored in individual containers at 22±1°C and 16:8 L/D until the adult wasp emerged. Adult females that were unmated were collected and a single wasp was introduced into cone-tainers containing either wheat infested with bird cherry-oat aphids or canola infested with cabbage aphids. After 24hr the D. rapae female was removed and the aphids and plants were maintained in the cone-tainers for an additional 48hr before being dissected in 70% EtOH. Aphids were divided into either those that were alive when collected and those that were dead upon collection to allow the division of parasitism rate into total, incomplete, and complete parasitism. For all three parasitism types D. rapae that were introduced into canola, regardless of emergence environment, had higher rates than those emerging from wheat. Diaeretiella rapae also experienced a decrease in parasitism rate when the introduced environment was different from the emergence environment.