Frequency Occurrence of Bathyplectes curculionis1 within Instars of Hypera postica2 Larvae in Illinois 3

In an attempt to better understand the host-parasite relationship of the alfalfa weevil, Hypera postica (Gyllenhal) (Coleoptera:Curculionidae) and Bathyplectes curculionis (Thomson) (Hymenoptera:Ichneumonidae), all life stages of the parasite were monitored throughout the spring. Sampling was conducted on a degree day basis from early March–late May in an alfalfa field in Mason County, Illinois. Dissections of host alfalfa weevil larvae revealed 2nd and 3rd host instars were preferred larvae for oviposition. Peak parasite densities were found to precede peak total host larval densities, but coincided with peak densities of the preferred 2nd and 3rd instars. Larger host instars were superparasitized at a higher rate than smaller instars. The standard procedure for determining percent parasitism in an alfalfa field by rearing large larvae was found to give an artificially high estimate, compared to dissection. Insecticide treatments in central Illinois coincide with peak spring emergence of the parasite. A method for predicting spring emergence is suggested that may enable insecticide applications to be adjusted around the parasite population.