Tackling citrus gall wasp

Researcher – Dr Jianhua Mo, NSW DPI

As citrus gall wasps impact crops in key growing regions across the country, NSW Department of Primary Industries (NSW DPI) entomologist Dr Jianhua Mo is working with industry to deliver a sustainable solution.

They are around 3mm in size, black and can live less than a week during spring. Despite their short lifespan, the citrus gall wasp can do significant damage to trees. Females lay up to 100 eggs under new bark growth, and once the larvae hatch, they can feed inside stems for 10 months before adults emerge. The end result can be swollen bulbs or ‘galls’ on tree limbs, which can impact tree health and cripple production.

However, the citrus gall wasp may have met its match through a project Hort Innovation is co-investing in with the NSW DPI, using citrus levy funds and contributions from the Australian Government. Headed up by Dr Mo, the project team has made a landmark discovery.

“One of the exciting parts of this project is that we have found the two main native parasitic wasp species – Megastigmus brevivalvusand M. trisulcus – in the new incursions areas in the south. With the continued effort to release the parasitic wasps and good integrated pest management practices, populations of the parasitic wasps are expected to rise up gradually and eventually bring down gall wasp numbers,” Dr Mo said.

The finding, combined with other management strategies being developed as part of this project, mean the pest’s days are numbered.

How parasitic wasps work

It’s been said nature has a way of sorting itself out and researchers have found this is very much the case in Queensland. In its traditional habitats in the north, citrus gall wasp populations are, in most years, kept below damaging levels by the two parasitic wasp species. Their attack is simple. The parasitic wasps insert their eggs into citrus gall wasp eggs, and after they hatch, the larvae feed on the citrus gall wasp larvae, eventually killing them.

Bringing solutions south

The citrus gall wasp is a relatively new pest in the southern Australian growing regions. It was first detected in the Sunraysia in the early 2000s and has since spread to all corners of Australia’s citrus growing regions.

Dr Mo said that managing the citrus gall wasp in southern Australia has been no easy task, with less prevalence of the parasitic wasps and limited chemical control options.

“The most challenging aspect of this project has been to quickly find management strategies for a pest that is rapidly expanding in distribution and is only visible for a few weeks in a whole year,” he said.

Parasitic wasp releases are taking place each year in new gall wasp incursion areas in the southern Australian citrus growing regions where the parasitic wasps have not yet established.

Chemical control findings to date

The team is in the process of trialing new chemical options on farms in the Sunraysia and Riverland. Findings to date have been positive. For example, Surround – a clay-based product – has been found to be highly repellent to the adult citrus gall wasps, thereby reducing egg lay and gall numbers. On the other hand, several systemic insecticides have shown excellent control of gall wasp larvae inside the galls, resulting in reduced adult wasps coming out of the galls the following season.

Pruning as a control method

Dr Mo’s team are also investigating optimal gall pruning times to limit damage to trees and production, which Dr Mo said presents a unique set of challenges.

“If pruning is done sometime before the wasp emerges, removed galls will wither and wasps inside won’t emerge. However, if pruning is done too close to wasp emergence, the pruned galls have to be mulched or burned to prevent the wasps from coming out,” Dr Mo said.

“On the other hand, pruning also encourages new flush, making the trees more attractive to the pest. And pruning too late may reduce yield – so it’s vital we find that perfect balance.”

Where to from here

On top of a series of field days and workshops hosted by the researchers, Dr Mo and his team will continue to seek grower input throughout the life of the project, which will be complete in 2018.

The team is exploring the prospect of mass rearing of parasitic wasps using potted trees, mapping hotspots of parasitic wasps in key growing regions, investigating side effects of insecticides on beneficial insects in citrus, and developing integrated pest management strategies for the gall wasp.

The team will share its findings and recommendations with industry through future field days, industry representative body publications and Hort Innovation channels.

Read more about the citrus gall wasp project in this fact sheet from NSW DPI.