Classical biological control of the alfalfa weevil in Japan.

The alfalfa weevil was accidentally introduced into Japan in early 1980’s and has been heavily infesting the Chinese milk vetch in paddy fields ever since. Chinese milk vetch is main source of honey products for apiarists in spring. Thus, the alfalfa weevil is not a pasture pest but a pest of apiculture. Four species of parasitoids were introduced into Japan as biological control agents for this pest from U.S.A. in 1988 and 1989. One of the parasitoids, Bathyplectes anurus was recovered in 1997. The percentage parasitism by B. anurus is higher on Narrowleafed vetch in surrounding grasslands than on the Chinese milk vetch in paddy fields, suggesting that the conservation of these grasslands is important for the biological control of the alfalfa weevil. INTRODUCTION The alfalfa weevil, Hypera postica (Gyllenhal) (Coleoptera: Curculionidae) is the most serious pest of alfalfa in the United States (USDA 1991). This pest is of Eurasian origin and was accidentally introduced into North America. Classical biological control of this pasture pest has a had almost 100 years of history in the U.S.A. and this has been summarized by Radcaliffe et al. (1998). In all, 12 species of natural enemies were involved in the combined USDA biological control program against the alfalfa weevil (Bryan et al. 1993). In most major American alfalfa production areas, several natural enemies of the alfalfa weevil are now well established and alfalfa growers benefit from their effects (Radcaliffe et al. 1998). The alfalfa weevil was also accidentally introduced into Japan during the early 1980’s (Okumura 2002), however, its impact on alfalfa has thus far been limited because it has yet to reach the commercial alfalfa production areas located near Hokkaido. The crop heavily in_________________________________________ Alfalfa Weevil in Japan and Classical Biological Control Second International Symposium on Biological Control of Arthropods 15 fested by the weevil is Chinese milk vetch (Astragalus sinicus L.), grown in paddy fields during the early spring. Japanese rice farmers keep Chinese milk vetch as green manure in winter and the flowers of the plant provide the main source of honey products for apiarists during spring. Thus, the alfalfa weevil is not a pasture pest but more a pest of the paddy field agroecosystem during the non-crop season in Japan. Moji Plant Protection Station introduced four species of parasitoids from U.S.A. into Japan for the biological control of this pest, (Kimura and Kaku 1991). One of the parasitoids, Bathyplectes anurus (Thomson) (Hymenoptera: Ichneumonidae) was recently recovered and is expanding its distribution. In this paper, we summarize the invasion biology of the alfalfa weevil in Japan and efforts towards its classical biological control. We also discuss the importance of natural enemy conservation and the future prospects of biological control of the alfalfa weevil. INVASION BIOLOGY OF THE ALFALFA WEEVIL IN JAPAN The alfalfa weevil was first discovered near Fukuoka airport in 1982 (Kimura et al. 1988). Subsequent surveys in the same year revealed that it was present in an area close to Fukuoka airport and widely distributed in Okinawa Island (Fig. 1). After that, the alfalfa weevil expanded its distribution towards eastern and northern Japan and has now reached Tokyo. The alfalfa weevil infests leguminous plants such as Astragalus sinicus (Chinese milk vetch), Medicago lupulina (black medick), M.hispida (burr medic), M. sativa (alfalfa), Melitotus officinalis (yellow sweet clover), Trifolium pratense (red clover), T. repens (white clover), and Vicia sativa (narrow-leaved vetch) in Japan (Yoshida et al. 1987). All of these leguminous plants in Japan are wild except alfalfa in Hokkaido, a northern island of Japan, and Chinese milk vetch in paddy fields of south and western Japan. The larvae of the alfalfa weevil infest and seriously damage Chinese milk vetch flower buds and flowers in early spring. The production of honey from this source is greatly decreased as a consequence. When the farmers begin to flood the fields in May, the new adult weevils are forced to emigrate from paddy fields to aestivate following their emergence. These flooding events also displace natural enemies. After harvesting rice in the autumn, farmers seed with Chinese milk vetch to fertilize the fallow paddy fields and adult weevils are able to return to the fields. Adult female weevils oviposit on alfalfa during the winter and early spring, and larvae hatch and start damaging alfalfa from February to May. INTRODUCTION EFFORTS OF NATURAL ENEMIES Some indigenous parasitoids attack the alfalfa weevil but the percentage parasitism is very low and ineffective in suppressing populations of the alfalfa weevil (Okumura 1987). In order to conserve the paddy field agro-ecosystem during winter and spring, four species of parasitoids, Bathyplectes anurus, B. curculionis (Thomson), Microctonus aethiopoides Loan (Hymenoptera: Braconidae) and M. colesi Drea, were introduced as biological control agents of the alfalfa weevil from U.S.A. into Japan during 1988 and 1989 (Kimura and Kaku 1991).