Black wattle (Acacia mearnsii) is rated as one of South Africa’s worst invasive weeds, having colonized almost every province within the country, where it has the potential to dominate the landscape and have negative impacts on biodiversity, agriculture and water resources. However, the sight of black wattle trees heavily laden with seed pods could well be a thing of the past if a tiny insect has its way.
This insect, Dasineura rubiformis by name, is a tiny fly (midge) native to Australia, which was introduced into South Africa as a biological control agent of black wattle a number of years back. A small population was established in Stellenbosch by 2006, and it subsequently became abundant at this locality and also colonized several additional sites in the vicinity. Natural dispersal is relatively slow, but since 2010 biological control practitioners have distributed the midge to numerous sites in Mpumalanga, Kwa-Zulu Natal and the Western and Eastern Cape provinces.
The midge itself is less than 2mm long, the adults do not feed and only live about 5 days. Females lay their eggs within the open flowers of black wattle during late spring, and once hatched the larvae feed on the plant ovaries causing the formation of gall tissue which prevents pod development. Each larva develops within a cavity in the gall tissue and a single galled flower can contain up to five such larval chambers. Since most of the flowers in a flower head become galls, characteristic clusters which resemble the young fruit of a blackberry result. By the start of winter, the galls are fully developed and the larvae leave the galls and drop to the ground. Here they pupate within silken cocoons in the leaf litter or at shallow depths in the soil. Adult midges emerge several months later which coincides with the flowering season of black wattle, when they are able to initiate the cycle again.
At sites where the midges are well established, pod production has declined dramatically (by as much as 92% within a 3-year period) and in some cases has virtually ceased, with only large bunches of galls evident on the trees, where pods once hung in abundance. Galling by the midge has no negative impact on the vegetative growth of black wattle, so the trees themselves continue to grow unaffected. For many this is seen as a positive feature, since the benefits of the plant (i.e., its uses as a source of firewood, and also tannin and pulp) will still remain.
However, the soil-stored seed banks remain a problem in the management of this plant. The seed banks are long-lived and generally huge, making clearing efforts futile unless ongoing and regular follow up treatments are adhered to. However a reduction in seed production should ultimately reduce the aggressiveness and spread of the plants, so the potential of this tiny midge holds much promise. After all, dynamite comes in small packages!