Summary

This project explored whether planting strips of native vegetation could support populations of the parasitoid fly, Sarcophaga villeneuveana, and strengthen biological control of conical snails in cropping systems. Eight long-term research and demonstration sites were established on Yorke Peninsula. Locally native plant species were selected to provide flowers, shelter and habitat for the fly during spring and summer.

Plants and the parasitoid were successfully established at all research sites, which will enable longer-term effects of vegetation on biocontrol of snails to be investigated. Baseline parasitism levels were low, ranging from approximately one per cent to six percent, varying between seasons and sites.

Consistent with the long-term nature of biological control programs, differences in parasitism between vegetated and non-vegetated plots were inconsistent at this early stage due to immaturity of plantings. Average parasitism levels are expected to increase in planted strips in coming years as vegetation matures and produces an abundance of flowers.

Background

Conical snails can cause blockages in headers and contaminate harvested grain, resulting in rejections or downgrades in quality. Research in Australia and overseas has shown that biological control using parasitoids can suppress snail populations, but success depends on the availability of suitable habitat and food sources for parasitoids.

The larvae of the parasitoid fly Sarcophaga villeneuveana feed on conical snails, but the adults require nectar, pollen and shelter to survive and reproduce. This project tested whether planting native vegetation could provide the resources required by adult flies and strengthen natural snail suppression at a paddock scale.

Research Aims

The core objectives of the project were to:

• Investigate whether native vegetation increases parasitism of conical snails by Sarcophaga villeneuveana
• Establish long-term research and demonstration sites using a before-after-control-impact ecological design
• Identify native plant species that provide suitable floral resources and shelter for parasitoid flies
• Measure baseline and early-stage parasitism levels in vegetated and non-vegetated areas
• Build grower awareness and confidence in biological control as part of integrated snail management.

In The Field

Eight research and demonstration sites with conical snails were established on Yorke Peninsula, including four grower paddocks, two Bunge (formerly Viterra) grain storage sites and two areas with existing remnant vegetation. The planted sites at grain storage facilities at Ardrossan and Port Giles were established to increase biosecurity awareness around potential spread of snails.

Vegetation strips measuring 150 m by 12 m were planted adjacent to cropped paddocks. A comparison plot 500 m away was left unplanted as a control. A mix of 17 locally native plant species was selected to provide near-continuous flowering from spring to summer, along with shelter for both snails and parasitoids. Parasitoid flies were mass-reared and released at all sites, with more than 26,000 flies released during the project. Snail parasitism was assessed using standardised field collection and laboratory rearing methods over multiple seasons.

Results

Across all sites and seasons, more than 42,000 snails were collected and examined for parasitism. The parasitoid fly was detected at every site, confirming that it is widespread in the region.

The fly was present at all sites, but average parasitism levels were low, ranging from less than one per cent to around six per cent depending on season and site. In the first assessment in 2022-23, parasitism averaged 0.8 per cent in non-vegetated plots and 2.7 per cent in vegetated plots.

In 2024-25, parasitism increased overall, averaging 6.2 per cent in control plots which were unplanted and 4.7 per cent in vegetated plots. Differences between vegetated and non-vegetated plots are not yet consistent, largely because plantings were still immature and flowering was still somewhat limited at the end of the project.

Individual sites recorded parasitism as high as 13 to 14 per cent, showing that meaningful suppression is possible under favourable conditions. The parasitism data establishes a baseline from which parasitism levels can be monitored over time. As vegetation matures and flowering becomes more reliable, parasitism levels in vegetated plots are expected to exceed those in non-vegetated areas.