Comparison of Diadegma insulare (Hymenoptera : Ichneumonidae) and Microplitis plutellae (Hymenoptera : Braconidae) as biological control agents of Plutella xylostella (Leptidoptera : Plutellidae): Field parasitism, insecticide susceptibility, and host-searching

Parasitism of Plutella xylostella (L.) third and fourth instars was evaluat ed in a cabbage field in Geneva, NY, in 1999. Over the entire season, avera ge parasitism was 33.6% for third instars and 53.6% for fourth instars, and the main parasitoids were Diadegma insulare (Cresson) and Microplitis plut ellae Muesbeck. In the early season, total parasitism was low and mainly ca used by D. insulare. However, later in the season, parasitism reached >80% for the forth instars and 50% for the third instars. Our survey indicated t hat M. plutellae heavily parasitized P. xylostella and provided higher para sitism rates than D. insulare in the late season. Comparison of these two s pecies in laboratory bioassays indicated there were no significant differen ces in susceptibility to four insecticides commonly used in crucifer fields . For both parasitoids, an experience with P. xylostella on a damaged leaf increased their host-searching efficacy. Compared with M. plutellae, D. ins ulare was a better host-searcher both for the naive and the experienced adu lts. Although both parasitoids can cause high mortality rates of P. xyloste lla, D, insulare may be more suitable to be released in fields to enhance n atural control against P. xylostella.