Seasonal abundance of stem-boring insects associated with the invasive Senecio madagascariensis in its native range (KwaZulu-Natal, South Africa) and their potential for biological control

Abstract

ABSTRACT Fireweed, Senecio madagascariensis Poiret (Asteraceae), is a target for biological control in Australia and Hawaii. Candidate agents recorded in KwaZulu-Natal Province, South Africa, the region of origin of the invasive populations, include stem-boring insects. The seasonal abundance of stem borers associated with S. madagascariensis populations was studied at four field sites in the KwaZulu-Natal Midlands region, during 2017/18. DNA barcoding was used to differentiate and link insect taxa with their endophagous larvae. Monthly sampling revealed that stem-boring larvae comprised largely Diptera (42% of specimens) and Curculionidae (38.9%), with Lepidoptera (19.8%) the least abundant. Differences in larval herbivore loads between sites were statistically significant, with the warmer and drier sites supporting 59% higher larval numbers. DNA barcoding focused on Curculionidae and Lepidoptera, which took precedence due to restricted host ranges. Of the four lineages of weevil larvae recorded, 95% of sequenced specimens comprised two maternal lineages of Gasteroclisus tricostalis (Thunberg). Collectively, G. tricostalis was detected across nine months of the year, with peaks in abundance in autumn and summer. Similarly, of the three lineages of lepidopteran larvae recorded, 83% of sequenced specimens comprised Metamesia elegans (Walsingham) (Tortricidae). Metamesia elegans was collected across six months of the year, with an autumn peak. These results support the prioritisation of G. tricostalis and M. elegans as the most promising stem-boring agents for S. madagascariensis. The higher seasonal abundance of G. tricostalis, together with the good track record of weevils in weed biocontrol, suggest that it should take precedence in subsequent laboratory host-range assessments.