Wasp vs wasp: new citrus pest control strategy pits predator against prey

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Citrus gall wasp (CGW) damage on dwarf lime tree.
Citrus gall wasp (CGW) damage on dwarf lime tree.
Suburban Tomato, suburbantomato.com

A national project helmed by NSW Department of Primary Industries (DPI) entomologist Jianhua Mo is trialling a new integrated pest management (IPM) strategy aimed at curbing the costly citrus gall wasp (CGW).

Dr Mo, who is based at Yanco Agricultural Institute in the new CGW zone, says the gall-wasp management project – a research collaboration between DPI, South Australia’s Fruit Doctors and Department of Agriculture, and Food Western Australia, funded by DPI and Horticulture Innovation Australia (Hort Innovation) – will trial several CGW management strategies identified in a previous local study.

“We’re building on results from a pilot study in the Sunraysia, which developed a timing guide to target vulnerable stages of the pest, identified an alternative chemical treatment using paraffinic oil and confirmed the establishment of two native parasitic wasps which attack CGW,” she says.

NSW Department of Primary Industries' Scott Munro inspects for citrus gall wasp.
NSW Department of Primary Industries' Scott Munro inspects for citrus gall wasp.
NSW Department of Primary Industries (DPI)

A new IPM strategy for citrus gall wasp

The new project aims to develop national integrated pest management (IPM) strategies for the citrus gall wasp based around four sustainable methods that can be deployed concurrently:

  • enhanced biological control, achieved by releasing native parasitic wasps into areas recently infested by CGW;
  • natural, paraffin-oil-based repellent sprays;
  • new chemical control options compatible with IPM; and
  • a “robust forecasting tool” to guide the timing of spraying and releases of the parasitic wasps.
Citrus gall wasp (CGW) damage: the galls can be seen on the stalks.
Citrus gall wasp (CGW) damage: the galls can be seen on the stalks.
NSW Department of Primary Industries (DPI)

What’s so bad about citrus gall wasps?

The natural host of the citrus gall wasp, also known as Bruchophagus fellis is the Australian finger lime (Citrus australasica) but this indigenous insect species will happily infest exotic lime, lemon, grapefruit and orange trees if there are no native limes around.

In late spring, the wasp burrows into the trunk or branches of its prospective host plant to lay its eggs, with the next generation spending most of their lives developing inside the resulting galls (which look like lumpy protrusions), emerging the following spring to lay eggs and start the cycle anew.

CGW has long been a pest to citrus crops in Queensland and mid-to-north NSW but until recently, the species has rarely ventured further south.

Over the past few years, however, it has emerged as a major pest in Australia’s southern citrus-growing regions, infesting orchards in the Sunraysia, Riverland and Riverina districts (where production of the nation’s oranges is concentrated) and negatively impacting plant vigour, fruit size and yields.

The key to controlling CGW: killer native wasps

As do most creatures, the citrus gall wasp has natural predators – and fortunately, some particularly nasty ones live in Australia. They’re wasps that feed on the larvae of other wasp species, and when they’re around, CGW populations are literally decimated, says Dr Mo.

“Studies in Queensland, where CGW and their natural enemies originated, showed that when parasitic wasp populations are high, up to 90 percent of gall wasp larvae can be parasitised,” she says.

The Sunraysia pilot study identified two native parasitic wasps – Megastigmus brevivalvus and M. trisulcus – that could become key weapons in the battle to control southern Australia’s citrus gall wasp population.

“With both species now established, we are identifying parasitic wasp hot spots to conserve their populations in southern citrus regions, and releasing parasitic wasps in new CGW incursion areas to boost their effects in managing the pest,’ says Dr Mo.

Dr Mo said studies are underway to develop best-management practices for parasitic wasp releases.

Diagram showing the life cycle of a citrus gall wasp.
Diagram showing the life cycle of a citrus gall wasp.
Illustration by EH Zeck, courtesy of NSW Department of Primary Industries (DPI)

Oil-based repellents and improved chemical control

The project team is also working on improved spray control and repellent options.

Existing chemical insecticides that target adult CGW, says DPI, are “largely ineffective”, and are incompatible with integrated pest management (IPM) strategies.

Previous studies have found that 0.5% oil repellent sprays, which deter the CGW from laying eggs, offer “reasonable control” provided they’re applied thoroughly at 10- to 14-day intervals throughout the gall-wasp season, starting in early spring when adult wasps first emerge from the galls.

Dr Mo says the first application should take place as soon as the adult gall wasps are readi visible, noting that “monitoring is critical”. It’s also important not to spray oil repellents during flowering.

No chemical sprays suitable for controlling CGW are currently available to Australian growers, and one of the goals of the new study is to find an effective insecticide that won’t harm citrus crops, humans or the environment.

“It’s important to manage the pest in an environmentally-friendly way that allows growers to produce healthy citrus crops and meet consumer demand,” Dr Mo asserts.

Hort Innovation CEO John Lloyd says the project, which is due for completion in 2018, will have far-reaching benefits.

“Using grower levies, the national program is working with citrus growers to raise awareness of CGW biology, damage and management, and deliver the tools industry needs to better manage the pest,” he says.

For more information, visit the NSW DPI website.

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