Host-plant affinities of stem-boring weevils and moths associated with fireweed (Senecio madagascariensis) in South Africa: evaluating native host ranges of candidate biocontrol agents using DNA barcoding

Abstract

ABSTRACT Native to South Africa and Madagascar, the invasive fireweed, Senecio madagascariensis Poiret (Asteraceae), is a target for biological control in Australia and Hawaii, where substantial infestations persist. Earlier studies in the weed’s region of origin, namely KwaZulu-Natal Province in South Africa, assessed the seasonal abundance of stem-boring insect herbivores in S. madagascariensis populations. The weevil Gasteroclisus tricostalis (Thunberg) (Curculionidae) and moth Metamesia elegans (Walsingham) (Tortricidae) were prioritised as candidate agents for Australia due to their persistence across seasons. In this study, the host ranges of these two candidate agents were assessed by comparing the stem-boring curculionids and lepidopterans that are associated with S. madagascariensis with those associated with non-target Senecio species in the KwaZulu-Natal Midlands region. DNA barcoding of the adults and endophagous larvae recovered during field sampling of S. madagascariensis and 17 non-target Senecio species in 2017/18, as well as those from earlier field surveys, was used to differentiate the insect species and elucidate their host-plant affinities. Although weevil larvae were recovered from the stems of seven non-target Senecio species, G. tricostalis was associated only with Senecio inaequidens DC. and Senecio skirrhodon DC., both in the S. madagascariensis species complex. Lepidopteran larvae were associated with three non-target Senecio species, all in the S. madagascariensis species complex, with M. elegans associated only with Senecio harveianus MacOwan and S. inaequidens. These results suggest that G. tricostalis and M. elegans have restricted host ranges and that further host-specificity studies in quarantine are warranted to demonstrate their suitability for release.