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.
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Date
2021
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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.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.