Queensland fruit fly (Qfly) is one of Australia’s most damaging horticultural pests.

Over the last decade,it has expanded its range further south and is now widespread across Victoria.

Fruit trees and vegetable plots in private gardens may provide optimal breeding environments for Qfly, particularly if they remain unmanaged. In addition to causing problems for backyard growers, urban fruit fly populations may disperse to local farms where they cause serious damage to produce and threaten sales on domestic and international markets.

Reducing Qfly populations in towns and cities is an important part of a state-wide program to tackle this pest.

Biological control using parasitoids

Parasitoid wasps are predatory insects that attack Qfly eggs and larvae developing within fruits. These beneficial insects inhabit Queensland and northern NSW, where they can significantly impact upon fruit fly populations. However, fruit fly parasitoids are largely absent from Victoria.

Agriculture Victoria (AgVic) recently led a research program that looked at the potential benefits of mass-releasing fruit fly parasitoids in Victoria.

Parasitoid releases

The research team collected two Queensland species of wild fruit fly parasitoids - both species only attack fruit flies in the same family as Qfly.

The parasitoids were mass-reared at AgVic’s Tatura SmartFarm and 95,000 were released in regional towns in Sunraysia and the Goulburn Valley in the summer and autumn of 2021 and 2022.

The AgVic research team then monitored survival, reproduction, parasitism rates, and dispersal of the parasitoids at some of the release sites. This was achieved through collecting infested fruits and then rearing out any insects in the laboratory to determine if the Qfly eggs and larvae had been parasitised.

Findings so far…

During the two years of releases, both species of parasitoid were found to parasitise Qfly eggs and larvae in the field. The released insects were observed hunting for Qfly within fruits, and small populations established locally within the season, dispersing to neighbouring properties.

One of the species (Diachasmimorpha kraussii), which is native to Queensland, appeared to perform better under Victorian conditions compared to the other species.

D. kraussii was recovered at all four of the release sites that were monitored, compared to two of the five monitored sites for the other species (Fopius arisanus). D. kraussii also had higher average parasitism rates across all sites and fruit types, at 16 per cent average parasitism (maximum 63 per cent) compared to 6 per cent for F. arisanus (maximum 58 per cent).

Both species persisted in the local environment from release dates until the cooler months (i.e., winter), with F. arisanus still being reared from infested fruits at 17 weeks and D. kraussii at six weeks (note; this latter species was released later in the season).

D. kraussii was also found to disperse between gardens in a relatively short amount of time, moving 1.5 kilometres within six weeks.

Can the parasitoids survive our chilly winter?

A crucial question is how well either species will establish in the state in the long term. The good news is, in the summer of 2023, D. kraussii has so far been found at two of the previous release sites, clearly demonstrating this species can overwinter in Victoria.

As part of a state-wide survey collecting infested fruits at the beginning of the project, the research team also discovered a small population of D. kraussii in north-eastern Victoria. This suggests that they are already making their way south but are as yet only here in low numbers—our aim is to give them a boost!

What’s next?

Research will continue monitoring the establishment and spread of Qfly parasitoids in and around release sites, evaluating the impact these natural enemies are having on Qfly populations in Victoria. This will inform future biocontrol strategies aimed at bolstering parasitoid populations through mass releases in urban areas and on farms.

This research formed part of two national fruit fly biocontrol projects funded by the Department of Agriculture Fisheries and Forestry and Hort Innovation, using the apple and pear, citrus, rubus, strawberry, summerfruit, table grape and vegetable levies, co-investment from state governments, and Australian horticultural industries.