Native range studies on insect herbivores associated with fireweed (Senecio madagascariensis) in KwaZulu-Natal, South Africa, with prospects for biological control in invaded countries.
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Senecio madagascariensis Poir. (fireweed; Asteraceae), native to southern Africa and Madagascar, has invaded Australia, Hawaii, South America and Japan, reducing pastoral productivity and poisoning livestock. Interest in biological control by Australia and Hawaii led to initial surveys for potential insect agents in Madagascar. However, molecular evidence revealed that both the Australian and Hawaiian populations originated from KwaZulu-Natal Province, South Africa. Efforts to find suitable biocontrol agents have since been focussed within this region. Studies on potential biocontrol agents in the weed’s native range, which consider its centre of origin, closely related plants and seasonal variation in the abundance of its natural enemies, can provide valuable information for selecting host specific and effective agents. This study aimed to prioritise potential insect agents for Australia and determine their response to possible changes in alkaloid concentrations in plants from within the invaded Australian range. The insect herbivore fauna associated with S. madagascariensis was quantitatively surveyed across 21 sites in KwaZulu-Natal to provide a comprehensive list of herbivores and identify potential agents. A total of 64 herbivorous taxa were recorded. Many of these were recorded rarely, but at least 17 taxa were considered as potential agents having been successful in previous biological control programs. Of these, the most promising were a capitulum feeder (Lepidoptera: Pyralidae), three stem borers (Coleoptera: Curculionidae; Diptera: Tephritidae; Lepidoptera: Tortricidae) and a root-feeding flea beetle (Coleoptera: Chrysomelidae: Alticinae). Biological control programs are more successful when agents attack the target plants throughout the year. The abundance of the insects associated with S. madagascariensis was sampled once per season at two sites in Pietermaritzburg to determine which are present throughout the year and are thus capable of inflicting sustained damage. There was significant seasonal variation in the abundances of the main insect taxa. Of the most promising potential agents, four were recovered all year round, two during three seasons, two during two seasons and one during summer only. Releasing a complement of natural enemies that attack the plant at different times may thus be required to ensure that S. madagascariensis is attacked throughout the year. A number of invasive Asteraceae have been targeted for biological control. An evaluation of the successes and failures of different insect taxa and feeding guilds used in previous programs was carried out to prioritise agents that are most likely to be successful on fireweed. The most effective insect taxa for the biocontrol of species of Asteraceae were Coleoptera (Curculionidae and Chrysomelidae) and Lepidoptera (Pterophoridae and Tortricidae), while root-feeding and stem-feeding species were the most effective guilds. This verified that the root-feeding flea beetle (Chrysomelidae), stem-boring moth (Tortricidae) and stem-boring weevil (Curculionidae) should be prioritized as candidate agents for S. madagascariensis. Agent host specificity is particularly important for Australia which has 87 native Senecio species (Hawaii has none). The field host range of endophagous Coleoptera, Lepidoptera and Diptera associated with fireweed was assessed by comparisons of these taxa across 18 Senecio species native to South Africa. Ten plants of each Senecio species were collected from each of three sites. The COI gene of insect larvae recorded within the tissues of the various Senecio species was sequenced to assess their host specificity. Stem-boring Curculionidae, capitulum-feeding Diptera and stem-boring and capitulum-feeding Lepidoptera that were recorded on S. madagascariensis were restricted to the Senecio madagascariensis species complex, and could thus be suitable for release in Australia and almost certainly in Hawaii. Laboratory host-range tests in Australia are needed to confirm the specificity of these insects. Invasive species have a large adaptive capacity to establish successfully in new environments and may evolve in response to the new range. It is imperative to understand whether any adaptive or evolutionary response can influence a weed’s interaction with natural enemies from the native range. In particular, increased alkaloid concentrations in Australian fireweed populations may affect the efficacy of insect biocontrol agents. A field experiment compared the biomass, insect assemblages recruited in the field, and alkaloid profiles between invasive Australian and native South African populations of S. madagascariensis. Minimal variation in plant biomass and insect community composition was detected, despite some variation in alkaloid composition and concentrations, between the regions and countries. There was no relationship between alkaloid concentrations and insect communities indicating that potential insect biocontrol agents are unlikely to be affected by increased plant defences in Australian fireweed populations. Several insect agents were prioritised for further consideration in Australia based on the different criteria examined in this thesis. The root-feeding flea beetle (Longitarsus basutoensis Bechnyé), stem-boring weevil (probably Gasteroclisus tricostalis (Thunberg)) and stem-boring tortricid moth (unidentified species) will be the focus of additional field and laboratory studies that examine their life cycles and host range, prior to their introduction into quarantine in Australia.