The fungal pathogen Erynia phytonomi, which attacks the alfalfa weevil, is widely distributed in the Midwest. In Oklahoma it was first discovered in 1983, and is now widely distributed throughtout the state. Seasonal occurrence of the pathogen and alfalfa weevil mortality were studied at two Oklahoma locations during 1983-1987. The first infections were usually detected in weevils after 200-250 degree days (10øC base) accumulated from Jan. 1, which corresponded with peak occurrence of weevil larvae. In every year except 1987 (a dry year) rainfall and temperature conditions were favorable for the fungus, and 60% or more of the larvae were infected on one or more sampling dates in each year. However, peak infection of larvae usually occurred after an economic threshold had been exceeded. Infection of prepupae frequently exceeded 80% and up to 52% of pupae were infected. The value of the pathogen is in reducing later generations of alfalfa weevils. The pathogen appears to be an important natural control agent and often reduced the number of insecticide applications needed for alfalfa weevil in Oklahoma.
In a companion study, fungus-infected weevil larvae were dissected to determine the proportion of weevils containing eggs or larvae of the imported ichneumonid wasp, Bathyplectes curculionis. High levels of the fungus corresponded with high levels of wasp parasitism by the second generation of B. curculionis, and reduced wasp survival by over 90%. Although there is competition between the fungus and wasp, long term reductions in parasite populations have not been confirmed.
Sources:
Goh, K. S., R. C. Berberet, L. J. Young & K. E. Conway. 1989. Mortality of Hypera postica in Oklahoma caused by Erynia phytonomi. Environ. Entomol. 18: 964-969
Goh, K. S., R. C. Berberet, L. J. Young & K. E. Conway. 1989. Mortality of the parasite, Bathyplectes curculionis during epizootics of Erynia phytonomi in the alfalfa weevil. Environ. Entomol. 18: 1131-1135.
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