Potential for competition among natural enemies of larvae of Hypera postica (Coleoptera: Curculionidae) in the southern Plains

The effectiveness of natural enemies for regulating populations of the alfalfa weevil, Hypera postica (Gyllenhal), in Oklahoma was evaluated over a 20-yr period from 1975 to 1994. A consistent sampling protocol was followed at research sites in Grady and Payne counties for assessment of larval population densities of H. postica along with dissection and rearing of larvae to determine prevalence of parasitism by Bathyplectes spp. and infection by the fungal pathogen Zoophthora phytonomi (Arthur). Statewide surveys were conducted with dissection of weevil larvae to estimate the prevalence of parasitism by Bathyplectes spp. throughout Oklahoma. Prevalence of parasitism by B. curculionis (Thomson) was much higher from 1975 to 1982 than from 1983 to 1994. In contrast, the incidence of parasitism by B. anurus (Thomson) increased greatly after 1983. There was little evidence that the decline in B. curculionis resulted from competition with B. anurus. However, it may have been caused by mortality of immatures of B. curculionis in weevils infected by Z. phytonomi, resulting in reduced numbers of parasites entering aestivation surviving to the next year. Much less mortality of B. anurus was caused by fungal epizootics. It appeared that increased parasitism by B. anurus has compensated to some extent for the reduced prevalence of B. curculionis in regulating populations of H. postica. However, the total mortality resulting from the 3 species of natural enemies has averaged 30% of the H. postica larval population annully