Seasonal abundance of capitulum-boring insects considered for the biological control of fireweed (Senecio madagascariensis), including molecular phylogenetic analyses to reveal the field host range of lepidopteran candidate agents

Senecio madagascariensis Poiret (Asteraceae; fireweed) is a target for biological control in Australia and Hawaii, due to its severe impacts on agriculture and grazed rangelands. Prioritized candidate biocontrol agents recorded in KwaZulu-Natal Province, South Africa (accepted as the plant’s region of origin) include capitulum-boring Lepidoptera. The seasonal abundance of capitulum-boring insects was studied on S. madagascariensis populations in KwaZulu-Natal during 2017/18. The host-plant affinities of lepidopteran associates were subsequently assessed by surveying 34 non-target native Senecio species in the same region. Cytochrome c oxidase subunit 1 mitochondrial gene barcoding was used to differentiate the endophagous larvae associated with the various host plants and link them with their adult stages. Monthly sampling revealed low numbers of capitulum-boring insects in relation to available capitula, with no significant variation amongst sites or months. Larvae largely comprised Coleoptera (72.8%), with fewer Lepidoptera (15.8%) and Diptera (11.4%). Adults reared from the capitula comprised Coleoptera (Nitidulidae), Diptera (Agromyzidae, Cecidomyiidae, Tephritidae) and Lepidoptera (Pterophoridae, Pyralidae). The host-range study revealed 14 lepidopteran taxa on 13 Senecio species, with Homoeosoma stenotea Hampson (Pyralidae) and an unidentified species of Platyptilia Hübner (Pterophoridae) comprising the most abundant species and recorded mostly on S. madagascariensis. However, H. stenotea was recorded on six, and Platyptilia sp. on two, non-target Senecio species. These results suggest unacceptably broad host ranges for countries like Australia that host a diverse Senecio flora and require highly specific biocontrol agents. Both species may warrant further investigation in Hawaii, where there are no native Senecio species.