This species expanded gradually from the invaded place of Fukuoka Prefecture. Until 8th year (1988) after the first findings, the distribution areas of this weevil were restricted in Kyusyu and Okinawa areas and Hyogo Pref. in Kinki area. Until 1999, the distribution areas expanded in western Japan and the most eastern area where this species was found was Gifu Prefecture (Hayashikawa, 1999). Until 2003, this species expanded to all prefectures in Kanto area. It was speculated that the population of the alfalfa weevil in Hokkaido found in 2000 might immigrate by another route from foreign country (Kanda et al., 2004).

It was supposed that the alfalfa weevil expanded the distribution area by transportation on traffic vehicles and flight of newly emerged adults (Suenaga, 2002). The flight behavior of the adults was observed using sticky traps during two periods, from May to June and from middle of November to middle of December (Hashimoto et al., 1987).

The pinkish flowers of Chinese milk vetch are enjoyed by people as a springlike farmland landscape. If the alfalfa weevil infests heavily on Chinese milk vetch, the weevil eats up the flowers and robs people of such useful functions brought by the plant.

The weevil distributed in Kagoshima Prefecture for the first time in 1988. After then the weevil increased extremely and the flowers of Chinese milk vetch decreased remarkably. And 10,000 or more larvae of the alfalfa weevil were sometimes collected by net sweeping of 20 times in 1991 (Hayashikawa, 1999). The amount of honey production decreased greatly during the period. So a project for controlling alfalfa weevil had been conducted by the researchers of Kagoshima Prefecture supported by the National Government.

Recently, the damage of Chinese milk vetch in a part of Kitakyushu City in Fukuoka Prefecture, where the weevil was firstly found and the parasitoid wasps, Bathyplectes anurus, were released, has been decreasing remarkably (Ueno, 2007).

In the case of heavy infestation on host plants, the alfalfa weevil rarely injures cucumber and melon. But the damages were slight.

An adult of the alfalfa weevil lays about 115 egg-masses (10.5 eggs per egg-mass) and totally 1200 eggs per female (Okumura and Sadoshima, 1986).

The domestic natural enemies were examined and 11 species of parasitic wasps were found in Japan, but they were not effective (within only 2% of parasitism). The microbes such as Entomophaga, Beauveria, and Metarhizium are effective to the larvae of the weevil (Hayashikawa, 1999). The ant, Lasius japonicus, worked with efficiency to exclude larvae of the alfalfa weevil on the common vetch (Katayama and Suzuki, 2005).

Four species of parasitic wasps were introduced from U.S.A. from 1988 to 1989 by Moji Plant Protection Station. Within these species, only Bathyplectes anurus has established and expanded gradually in Kitakyusyu City and Yamaguchi Prefecture (Okumura and Shiraishi, 2002). The reproduction of the alfalfa weevil has been suppressed well around the places where B. anurus has release and established in Kitakyusyu City (Ueno, 2007).

Okumura et al. (1990) and Yamada et al. (1996) studied on the threshold temperature and effective accumulative temperature of the alfalfa weevil reared on alfalfa. The threshold temperatures were 11.5 C and 9.7 C, and the effective accumulative temperatures were 340.7 day-degrees and 393.7 day-degrees, respectively (both data from egg to adult).

Yamada et al. (1996) studied on the growth of the alfalfa weevil on several Leguminorous plants and resulted that the emergence rate of the weevil reared on Vicia angustifolia, Medicago sativa, M. hispida, Astragalus sinicus and V. villosa were ranged from 70% to 78%.

As cultural control methods against the alfalfa weevil, the infestation of the weevil and the damage will be decreased by late seeding with Chinese milk vetch from middle November to early December in Fukuoka Prefecture (Takemoto, 1993) or from late October to early November in Kagoshima Prefecture (Nishioka et al., 2003) or during November in Gifu Prefecture (Suzuki et al., 2004).

But if Chinese milk vetch is seeded in late season, the percentage of the germination will decrease. So it is necessary to increase the volume of the seeds more than 50%. Furthermore, the competition with weeds will occur. It is necessary to cultivate before seeding and suppress the soil after seeding. Though the peak of the flowering of Chinese milk vetch delayed about 15 days, the number of flowers and honey bees visited didn't decreased (Takemoto, 1993).

Natural enemies, four species of parasitoid wasps: Bathyplectes curculionis, Bathyplectes anurus, Microctonus aethiopoides and Microctonus colesi against the alfalfa weevil were introduced from USA in 1987-1989 (Kimura and Kaku, 1991). Without M. colesi, three species of the wasps were mass-reared and released by Moji Plant Protection Station. But the released wasps had not established in the fields in Kagoshima Prefecture when the examination was conducted from 1989 to 1996 (Hayashikawa, 1999).

But it was recognized that only Bathyplectes anurus established in the fields around the releasing sites in 1996 for the first time. The wasp was released in 1991-1992 there. After then, B. anurus has been gradually expanding the distribution areas (Okumura and Shiraishi, 2002).

Ueno (2007) reported that B. anurus established well in Kitakyusyu City and the wasps could decreased the infestation caused by the alfalfa weevil below the level of economic damage around the releasing sites where the percentage of parasitism increased and reached nearly 50%.