Thousands of flying exterminators released against tree pests

The South African commercial forestry sector is making strides towards controlling the costly depredations of the Blue Gum Chalcid (Leptocybe invasa) in eucalyptus plantations.

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To date, up to 4 000 females of the Blue Gum Chalcid’s nemesis, the Selitrichodes neseri parasitoid wasp, have been released at over 200 sites by the Forestry and Agricultural Biotechnology Institute of the University of Pretoria’s Tree Protection Co-operative Programme (TPCP). Dr Brett Hurley, a researcher with the TPCP, explained that the S. neseri releases had focussed mainly on the Zululand, Mpumalanga Lowveld and Tzaneen areas where highest L. invasa infestation had been recorded, but other releases had also been carried out in the KwaZulu-Natal Midlands, Piet Retief and Paulpietersburg areas.

A map indicating releases to date of S. neseri (in red). (Courtesy of the Tree Protection Co-operative Program)

“Release sites in each region are being monitored to investigate the establishment of S. neseri. Encouragingly, S. neseri has been recovered from all but one of these sites,” Hurley said. “Currently the primary goal is to distribute S. neseri as widely as possible in the infested areas, with the expectation that the releases will develop into source populations of S. neseri that will grow and spread rapidly.”

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Believed to be native to Australia, L. invasa was first discovered in SA in 2007 and has since spread rapidly through the country’s commercial eucalyptus plantations. Its females lay eggs in parts of eucalyptus trees, resulting in galls and other forms of damage. This damage can kill, or severely impair the growth and wood quality of affected trees. S. neseri is a bio-control agent against L. invasa, parasitising this pest.

“The rate of establishment of S. neseri and the impact that its establishment will have on L. invasa populations is not yet known, and we will need to monitor selected release sites carefully to better understand this,” said Hurley.