Cat's claw creeper, Macfadyena unguis-cati (L.) Gentry (Bignoniaceae), introduced as an ornamental plant, has invaded several ecologically sensitive habitats in the higher rainfall regions of South Africa. Biological control was initiated with the release of the leaf-feeding tortoise beetle Charidotis auroguttata (Boheman) (Coleoptera: Chrysomelidae) in 1999. Although established at several field sites, population densities of the beetle have remained low with no assessment of impact to date. However, simulated herbivory studies in Australia revealed that severe and repeated defoliations could slow the weed's growth rate and its accumulation of biomass in the subterranean tubers. The aim of this study was to quantify the impact of medium (50 %) and high-intensity (100 %) defoliations by C. auroguttata, in relation to equivalent simulated damage, on the growth and biomass accumulation of M. unguis-cati. The trials were monitored for 16 weeks with repeated defoliations at two-week intervals. Despite considerable variation in the response variables, plants that were defoliated by C. auroguttata, but not those defoliated mechanically, displayed reduced growth rates. However, significant decreases in stem length and numbers of nodes were demonstrated only after high-intensity beetle defoliations. Despite reductions in tuber biomass in the beetle-defoliated treatments, the differences were not significant, while only high-intensity beetle defoliations caused a significantly lower accumulation of total plant biomass. Simulated herbivory was a poor predictor of insect damage in this study which suggested that C. auroguttata could prove damaging to M. unguis-cati, but only at high population densities. However, in field situations, synergism with a range of additional environmental stressors may increase the negative impact of feeding damage.
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Vol. 20 • No. 1