Biocontrol of Invasive Conical Snails by the Parasitoid Fly Sarcophaga villeneuveana in South Australia 20 Years after Release

SIMPLE SUMMARY: Two invasive conical snail species are major pests of pastures and grain crops in Australia. In 2000, a parasitoid fly, Sarcophaga villeneuveana, was sourced from the snails’ native Mediterranean range and introduced on the Yorke Peninsula, South Australia, for biological control of the conical snail Cochlicella acuta. The fly successfully established in the region but assessments of the fly’s impact in different snail habitats were limited. Twenty years on, four field surveys were performed over two years to measure its geographic spread and parasitism rates on C. acuta and the small pointed snail, C. barbara. In total, >88,000 snails were collected using standardised sampling methods to investigate the parasitism of host snails in different habitats. The fly was found at 13 of 19 sampled sites up to 34 km from nursery release sites. Total parasitism rates of suitably sized snails (≥5 mm shell height) were ≈3% for both C. acuta and C. barbara. Rates were higher in C. acuta (5.4%) and C. barbara (15.2%) in exposed habitats above ground level. Parasitism rates up to 48% in C. acuta and 29% in C. barbara at sites near flowering vegetation suggested that the fly benefits from floral resources. ABSTRACT: Two conical snail species introduced to Australia from the Mediterranean region during the 20th century are major pests of pastures and grain crops. In 2000, a parasitoid fly, Sarcophaga villeneuveana, was introduced into South Australia for biocontrol of the conical snail, Cochlicella acuta. The fly successfully established in the region but assessments of its impact in different snail aestivation microhabitats were limited. Twenty years on, field surveys were conducted to assess the geographic distribution and parasitism rates of S. villeneuveana on conical snails in the Yorke Peninsula region. Nineteen sites were sampled on four occasions in January and April of both 2019 and 2020. In total, >85,600 C. acuta and >2400 C. barbara were collected from cryptic (ground or plant refuge) and exposed (open ground or elevated substrate) aestivation habitats and assessed for parasitism. The fly was detected at 13 of 19 sampled sites up to 34 km from nursery release sites. Total parasitism rates of suitably sized snails (≥5 mm shell height) were 2.9% for C. acuta and 3.4% for C. barbara. Maximum parasitism rates of 48% for C. acuta and 29% for C. barbara were found at sites adjacent to spring- and summer-flowering native vegetation. Across 13 sites, parasitism rates were higher for C. acuta (5.4%) and C. barbara (15.2%) in exposed habitats above ground level. However, only 34% of C. acuta and 14% of C. barbara were found in elevated habitats as most snails were found in cryptic refuges. There was a seasonal decline in abundance of C. acuta (66%) and C. barbara (45%) between January and April, suggesting natural mortality. Although the overall impact of the fly is limited, high parasitism rates in local environments with flowering resources indicates the potential to enhance biocontrol of both invasive conical snail species.