Biology, seasonal abundance and host range of capitulum-feeding insects associated with the invasive weed Senecio madagascariensis (Asteraceae) in its native range in KwaZulu-Natal, South Africa.

Abstract

Native to KwaZulu-Natal, South Africa, Senecio madagascariensis (fireweed) is a herbaceous plant that has become highly invasive in many countries where it was accidentally introduced in contaminated fodder. Rapid growth rates, high fecundity and toxic secondary compounds that poison livestock have caused severe economic impacts in infested pastures and rangelands. Biological control, using imported natural enemies from South Africa, is being pursued as a long-term control option for invaded countries, particularly Australia where invasions are most severe. This study forms part of a collaboration with the CSIRO in Australia to source and assess potential insect biocontrol agents that could be imported into Australia. The aims of this study were to: (i) investigate and identify species of capitulum-feeding insects on fireweed populations; (ii) determine the seasonal abundance of capitulum-feeding insects on fireweed populations in the field; (iii) differentiate between the different lepidopteran and dipteran species associated with fireweed by means of DNA barcoding; and (iv) verify the host range of these insects by surveying related Senecio species in the field and comparing the associated insects using DNA barcoding. Insects with capitulum-feeding larvae included Coleoptera, Lepidoptera and Diptera and were most abundant during late summer and autumn. The most important potential biocontrol agents were the lepidopterans Homoeosoma stenotea (Pyralidae) and an unidentified species of Platyptilia (Pterophoridae), while the flies, which included two unidentified species of Trupanea (Tephritidae), were less abundant. DNA barcoding of the COI gene revealed distinct genetic lineages (possible species) of lepidopterans that were recorded on eight of the 36 surveyed Senecio species, with most specimens conforming to H. stenotea and Platyptilia sp. Homoeosoma stenotea was recorded on three, and Platyptilia sp. on one, non-target Senecio species, respectively. The species of Trupanea were restricted to S. madagascariensis, but since they were collected only during seasonal surveys, studies of their host specificity were not concluded. Since the two lepidopteran species do not appear to be strictly host specific, they may not be suitable biocontrol agents for countries like Australia that have a diverse native Senecio flora. However, countries that lack native or economically important Senecio species may choose to further consider these potential agents.