Alfalfa weevil (Coleoptera: Curculionidae) biological control: spreading the benefits

Since 1980, > 16 million parasites of Hypera postica (Gyllenhal) were distributed across the United States as part of the Alfalfa Weevil Biological Control Program. Two species of internal parasites, one of the larval stage, Bathyplectes anurus (Thomson) and one of the adult stage, Microctonus aethiopoides (Loan), were released in the largest numbers. A detection survey conducted by the USDA, Animal and Plant Health Inspection Service, found that a second larval parasite, Bathyplectes curculionis (Thomson), had already spread throughout alfalfa-growing areas in the United States. Later, an establishment survey successfully recovered parasites in nearly half of the > 700 counties where releases were made. Results from an evaluation survey indicated that the number of alfalfa weevil parasite species collected in surveyed fields in Iowa, Missouri, and Nebraska more then doubled from 1.3 to 3.0. Parasitism rates also increased in these states, whereas insecticide usage declined. However, < 3% of the 180 alfalfa fields sampled during the 8 yr evaluation survey reached the economic injury level. Both B. anurus and B. curculionis exhibited differential success when parasitizing two United States strains (eastern and western) of the alfalfa weevil. These two species parasitized an average of 21% of the larvae collected in survey samples. M. aethiopoides could possibly be an important method for keeping H. postica populations below economic injury levels. A delayed density-dependent relationship between this species and its host was demonstrated in three midwestern fields sampled for seven consecutive years. Before the first cutting of alfalfa, M. aethiopoides killed an average of 26% of adult weevils in the four areas surveyed in the eastern United States. A fungal disease, Zoopthora phytonomi (Arthur), of H. postica was detected in the majority of fields in our evaluation survey; this too may be a method for reducing weevil populations.