Senecio madagascariensis Poir. (Asteraceae), which is native to southern Africa, has invaded agricultural lands in several countries, reducing pastoral productivity and poisoning livestock. Severe infestations in Australia and Hawaii have prompted investigations into the feasibility of biological control. Surveys in KwaZulu-Natal, South Africa, considered to be the origin of the plants that have invaded Australia and Hawaii, revealed several insect herbivore taxa that attack the plant's floral and structural tissues. However, their potential as candidate agentsmaybe influenced by the extent of their association with the plant throughout the year. Populations of S. madagascariensis around Pietermaritzburg were sampled four times over one year (i.e. across seasons) to determine the presence and abundance of the major herbivore taxa relative to the plant's phenology. Similar amounts of foliar and floral material were available to the various insect herbivore guilds throughout the year. The sampled plant populations supported 84 % of the insect herbivore taxa known to be associated with S. madagascariensis in KwaZulu-Natal. Nine of the 10 taxa that were deemed promising as candidate agents were recovered and included three capitulum feeders, four stem borers and two foliage feeders. Of these nine taxa, four were recovered during all four sampling occasions, while two were recovered on three occasions, two on two occasions and one on one occasion only. There were significant differences across seasons in the abundance of these candidate agents. The release of combinations of agents, that attack the same or different tissues, may be required to compensate for differences in abundance and ensure that herbivore pressure is sustained throughout the year.
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Vol. 23 • No. 1