The alfalfa weevil is a destructive invasive pest of Chinese milk vetch, a main source of honey products. Because apiarists and farmers disfavor the use of insecticides, a specialist larval parasitoid wasp, Bathyplectes anurus, was introduced for biological control of the weevil. Although the introduced parasitoid initially had showed very low percentages of parasitism in early 1990’s, it gradually increased its parasitism and hence effectiveness of control. Finally, the parasitoid successfully suppresses the alfalfa weevil in 2000’s. Although this is a good example of successful biological control, one major question remains unanswered: why does the parasitoid expand and increase so slowly? Field and laboratory experiments show: (1) the parasitoid prefers alfalfa weevils on common vetch rather than those on Chinese milk vetch; (2) Chinese milk vetch fields (= rice paddies) are not the source of parasitoids, but weedy areas with common vetch plants around rice paddies are the main habitat of the parasitoid where it can successfully overwinter; (3) hunger level of adult parasitoids searching in Chinese milk vetch fields is much higher than that in common vetch fields; (4) common vetch plants have extra-floral nectary glands which are found to be a main sugar source for the introduced parasitoid; and the effectiveness of the parasitoid is rather low in areas where common vetch is rather rare. These results strongly indicate that the conservation of parasitoid habitats, in this case, weedy areas with common vetch, is a key factor affecting the success of biological control with B. anurus parasitoids. Thus, great care should be taken whether we can make the agro-environment suitable for natural enemies.