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Biological Control Endeavours Against Australian Myrtle, Leptospermum laevigatum (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, Leptospermum laevigatum (Gaertn.) F.Muell. (Myrtaceae): a bud-galling midge, Dasineura strobila Dorchin (Diptera: Cecidomyiidae), which was inadvertently introduced, possibly in the mid-1980s, and a leaf-mining moth, Aristaea (Parectopa) thalassias (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 L. laevigatum buds on mature trees that are galled by D. strobila was monitored from 1994 until 2008. Initially the prognosis for biological control by D. strobila 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 D. strobila larvae and pupae in the galls, but their role in regulating populations of D. strobila has not been proven. A chemical exclusion experiment on seedlings showed that leaf damage by A. thalassias together with galling by D. strobila reduced the growth of young L. laevigatum 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 L. laevigatum do not appear to be good.