The alfalfa weevil invaded Japan in the early 1980s. In Southwestern Japan, the weevil infests Chinese milk vetch, which is a main source of honey products. Since apiarists avoid application of insecticides, four species of parasitoid wasps were introduced from the US into Japan for biological control of the weevil in 1988 and 1989. In 1996, one of the parasitoids, Bathyplectes anurus (Thomson) was recovered. Accordingly, we started the survey to assess the incidence and effectiveness of this parasitoid in suppressing the alfalfa weevil. B. anurus expanded its distribution during 1998-2003. In 1998 and 1999, the percentages of parasitism were mostly less than 5% but quickly increased to about 40% in 2003. The survey also showed that the extent of damage of the weevil on Chinese milk vetch decreased from 2001 to 2004; there was a negative correlation between the extent of weevil damage and the percentage parasitism one year previously. These results suggest that the parasitoid reduced damage by the alfalfa weevil.
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