n African Entomology - Biological control endeavours against Australian myrtle, (Gaertn.) F. Muell. (Myrtaceae), in South Africa




In South Africa, two imported insect species have been used in attempts to control invasive Australian myrtle trees, (Gaertn.) F. Muell. (Myrtaceae): a bud-galling midge, Dorchin (Diptera: Cecidomyiidae), which was inadvertently introduced, possibly in the mid-1980s, and a leaf-mining moth, (Meyrick) (Lepidoptera: Gracillariidae), which was released in 1996. The latter agent attacks young leaves only and has no discernible impact on mature trees. The number of buds on mature trees that are galled by was monitored from 1994 until 2008. Initially the prognosis for biological control by was extremely promising. However, the numbers of galls then declined sharply at most of the sites, on average to less than half of their previous peak levels. Gall-midge mortality, induced by native parasitoids, was very low initially, and, some years later, peaked at an average of only about 8%. In 2004, predatory mites, mostly Pyemotes species (Trombidiformes: Pyemotidae), were discovered, killing an average of 27% (9.8-61.3 %) of the larvae and pupae in the galls, but their role in regulating populations of has not been proven. A chemical exclusion experiment on seedlings showed that leaf damage by together with galling by reduced the growth of young plants by nearly 50 %, but, again, the impact of the two agents in aggregate, on mature plants, is negligible. A gall-inducing scale insect is presently under consideration as a potential agent, and there are some other possible agents that might be useful, but, overall, the prospects for biological control of do not appear to be good.


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