Performance of a stem-boring weevil on Senecio madagascariensis from the native South African and invaded Australian range: differential oviposition patterns suggest agent-plant mismatch

Candidate insect biological control agents are typically assumed to be compatible with target weed populations in the invaded range. However, plants in invasive populations may diverge genetically and chemically from those in native populations, with potential mismatches in the agent-weed relationship. In this study, we tested the stem-boring Gasteroclisus tricostalis (Thunberg) (Coleoptera: Curculionidae) on Senecio madagascariensis (Poiret) (Asteraceae) plants from the native (KwaZulu-Natal, South Africa) and invaded (Australia) range. During no-choice tests, G. tricostalis females displayed significantly reduced oviposition when switched from South African to Australian plants across three series of trials, resulting in 5.8 times more eggs on South African plants. Furthermore, 94.4% of South African plants supported oviposition, compared to only 44.8% of Australian plants. Australian plants suffered substantially higher mortality (41.4%) than South African plants (5.6%), which was unrelated to larval presence. Despite this, larval recovery and survival (as a proportion of eggs laid) did not differ significantly between South African and Australian plants. Generally, plant responses to larval presence did not differ significantly between South African and Australian plants, with reductions in living shoots and capitula recorded on plants that harboured larvae. Reduced oviposition on Australian S. madagascariensis may relate to substantially higher alkaloid levels recorded earlier, but also the possibility that invasive populations originate from a different region of South Africa than originally assumed. Biological control practitioners should verify that prioritised agents from the native range are compatible with target plants in the invaded range, prior to host-range testing.