Interrelationship of Hypera postica (Coleoptera: Curculionidae) and Bathyplectes curculionis (Hymenoptera: Ichneumonidae) in the Eastern United States with Particular Reference to Encapsulation of the Parasite Eggs by the Weevil Larvae

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Abstract
In the Eastern United States, larvae of the alfalfa weevil, Hypera postica (Gyllenhal), encapsulate eggs of the ichneumonid Bathyplectes curculionis (Thomson). Therefore, a high percentage of weevil larvae are immune to the parasite. Under laboratory conditions, encapsulation was observed in all larval instars parasitized but was more pronounced in second through fourth instars containing single parasite eggs. However, in superparasitized larvae, immunity usually broke down because of the inability of host larvae to encapsulate supernumerary eggs. The degree of immunity under field conditions was apparently related to the density of both host and parasite and to their seasonal development. Encapsulation in the H. postica–B. curculionis relationship in the East, therefore, appears to reduce the effectiveness of the parasite as a biological control agent. The host-parasite relationship in California differs, since there encapsulation occurs rarely. The eastern H. postica may be a different biological strain from that in the West.