Response of the invasive fireweed (Senecio madagascariensis) to variable levels of larval stem-boring by Gasteroclisus tricostalis (Coleoptera: Curculionidae), a potential biological control agent

Abstract

The stem-boring weevil Gasteroclisus tricostalis (Thunberg) (Curculionidae) is under investigation as a biological control agent for Senecio madagascariensis Poiret (Asteraceae) in Australia and Hawaii. Besides mandatory host-specificity testing, pre-release studies on the weevil’s impact can be a useful predictor of its efficacy when released in the invaded range. In this study, we investigated the response (i.e., growth, floral production, and biomass accumulation) of mature flowering S. madagascariensis plants to varying densities of G. tricostalis larvae in their stems. Although there were weak negative relationships between plant response variables and larval densities, these were only significant in the production of side branches (shoots) and floral components (capitula). Regression analyses estimated that six and 12 larvae per mature plant could prevent the production of new capitula and new shoots, respectively. A significant negative relationship between the percentage of eggs that resulted in larval recoveries and the number of eggs deposited per plant may suggest aggressive interactions between the larvae. Given precedents from other biocontrol programmes against asteraceous weeds, G. tricostalis may exhibit higher impacts on young pre-flowering plants than on mature plants, a possibility that requires confirmation. Because G. tricostalis is present is the field throughout most of the year, additional testing involving multiple generations of the weevil can assess the ultimate impact of sustained damage on plants. Since high floral production is a major driver of invasions by S. madagascariensis, substantial reductions in flowering by G. tricostalis would likely make a significant contribution to the weed’s management.