Differences in host plant selection among three Athalia sawflies feeding on crucifers in Japan

Abstract 1. The ways of using host plants were compared among the three Athalia sawflies [ A. japonica (Klug), A. rosae ruficornis Jakovlev, and A. infumata (Marlatt)] feeding on crucifers in Japan to determine whether host specialisation can explain the difference in their life-history traits. The occurrence of their larvae was examined on each crucifer species in the field, and the suitability of each crucifer species for the three successive steps of host use by the sawflies was evaluated: microhabitat selection by adult females, female oviposition, and larval growth.
2. There were 11 species of crucifer in the study area, and A. japonica , A. rosae , and A. infumata used nine, seven, and eight species respectively. Thus, sawfly host ranges overlapped.
3. Adult females of A. japonica , A. rosae , and A. infumata preferred shady clumps of crucifers, sunny clumps of crucifers, and disturbed areas respectively.
4. Unsuitable hosts for larval performance such as Brassica oleracea and Arabis plants were eliminated from the host ranges of the three sawflies.
5. Once they chose microhabitats, the suitability of each host plant for female oviposition and larval growth was similar.
6. Because of the divergent preferences for microhabitats, the host plants that were suitable for all the three steps were restricted to different sets of plants among the sawflies: Cardamine for A. japonica , cultivated crucifers ( Raphanus and Brassica ) for A. rosae , and Rorippa for A. infumata . These plants could be recognised as the respective primary host plants.
7. The spatio-temporal distributions of primary hosts were consistent with and explained the pattern of diapause and migration of each sawfly, suggesting that host specialisation caused their life-history traits to differentiate.