What was it all about?

Citrus gall wasp is a native pest of citrus that causes galls that can reduce leaf and fruit production and severe dieback. The pest affects all citrus varieties. In the late 1990s, the wasp spread from Queensland and northern and central NSW into Sunraysia in far southwest NSW and the neighbouring Riverland in northeast South Australia, making further research into managing the wasp more urgent.

Methidathion is registered for control of the wasp but this broad-spectrum insecticide is not compatible with integrated pest management, now the cornerstone of citrus pest management in Australia.

This project, which ran from 2010 to 2013, studied the biology of citrus gall wasp and investigated alternative management options for its control in the Coomealla Irrigation District, in far southwest NSW.

Two alternative chemicals were identified as having potential: petroleum spray oil and imidacloprid. The spray oil deters the adults from egg laying while imidacloprid kills wasp larvae inside the galls. Both controls are less disruptive to populations of beneficial insects than the currently registered methidathion.

Timing was found to be critical for application for the two new options and the existing insecticide. Petroleum spray oil had to be applied when citrus gall wasps were most abundant in the orchard. Imidacloprid and methidathion had to be applied around the time of egg hatching.

The team also conducted biological investigations on wasp emergence and egg hatching and as a result, developed models that predict the timing of these events. This allowed the production of guidelines to help citrus growers time their sprays for best results, pending approval of the chemicals for these particular uses.

Also as part of this research, investigators confirmed that two species of native wasps, Megastigmus brevivalvus and M. trisulcus, parasitise citrus gall wasps. These predators typically emerge about two or three weeks behind the gall wasp, but they have been released in southwestern NSW to control citrus gall wasps. The researchers found that these predators became well established.