Bottom-up effects of crop seeding methods on densities of the alfalfa weevil hypera postica and its introduced parasitoid bathyplectes anurus

The alfalfa weevil Hypera postica (Gyllenhal) is the most destructive pest of leguminous crops such as alfalfa, Chinese milk vetch, and hairy vetch throughout the world, including Japan. To control H. postica, the solitary endoparasitoid Bathyplectes anurus (Thomson) was released in Japan as a classical biological control agent. In our study, we investigated the bottom-up effects of hairy vetch seeding methods (i.e., control of seeding timing and density) on the B. anurus density to develop a field production method for B. anurus. The average densities of H. postica and B. anurus were significantly higher with early timing and a higher seeding density compared with early timing and a lower seeding density, late timing and a higher seeding density and late timing, and a lower seeding density. This confirmed that the densities of H. postica and B. anurus could be adjusted by the bottom-up effect of hairy vetch seeding methods. During the tri-trophic interaction among hairy vetch, H. postica, and B. anurus, the interaction between the hairy vetch weight and H. postica densities was high, whereas the interaction between the H. postica and B. anurus densities was very high. These results suggested that early timing and high density seeding indirectly enhanced the density of B. anurus by strong direct effects of the H. postica density on the B. anurus during the tri-trophic interaction. Overall, we concluded that a combination of early timing and high density hairy vetch seeding may facilitate efficient field productions of B. anurus.