Phenology of the Alfalfa Weevil (Coleoptera: Curculionidae) and its Associated Parasitoids in Minnesota

Abstract

Field surveys were conducted in Minnesota to determine the seasonal incidence of parasitism of alfalfa weevil, Hypera postica (Gyllenhal), by Bathyplectes curculionis (Thomson), B. anurus (Thomson), Oomyzus (=Tetrastichus) incertus (Ratzeburg), and Microctonus aethiopoides (Loan). During the 1991–1993 survey, alfalfa weevil population density was typically below that required to cause economic damage. In Minnesota, peak density of alfalfa weevil larvae tended to occur around 300 dd (base 9°C). Typically, first cutting of alfalfa in Minnesota occurs early enough to remove most alfalfa weevil before they have completed larval development. In our earliest collected larvae, parasitism by Bathyplectes spp. approached 30%. A low incidence of parasitism by O. incertus also was detected in these larvae. With rapidly declining numbers of host larvae available after first cutting, parasitism rates by B. curculionis and O. incertus increase greatly, with Oomyzus assuming greater importance as Bathyplectes declines. The greatest incidence of O. incertus parasitism occurred ~500 to 600 dd, but the proportion of the hosts that were parasitized peaked at ~750 to 900 dd. Insecticide sprays for potato leafhopper, Empoasca fabae (Harris), usually between 300 to 900 dd, could adversely effect B. curculionis and O. incertus.Second-generation alfalfa weevil larvae were found each year, but were most common during an unusually cool summer (1992), and in the southern half of the transect. Density of second-generation larvae peaked after mid-August (~1000 dd). Each year, peak parasitism by M. aethiopoides was observed during its first generation, around 300 dd, in overwintering alfalfa weevils. There may have been three generations of M. aethiopoides in Minnesota in 1992 and in 1993. In Minnesota, M. aethiopoides apparently overwinters as eggs, first-instar larvae, or advanced stage larvae.