| Summary: | 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, <i>Sarcophaga villeneuveana</i>, was introduced into South Australia for biocontrol of the conical snail, <i>Cochlicella acuta</i>. 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 <i>S. villeneuveana</i> 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 <i>C. acuta</i> and >2400 <i>C. barbara</i> 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 <i>C. acuta</i> and 3.4% for <i>C. barbara</i>. Maximum parasitism rates of 48% for <i>C. acuta</i> and 29% for <i>C. barbara</i> were found at sites adjacent to spring- and summer-flowering native vegetation. Across 13 sites, parasitism rates were higher for <i>C. acuta</i> (5.4%) and <i>C. barbara</i> (15.2%) in exposed habitats above ground level. However, only 34% of <i>C. acuta</i> and 14% of <i>C. barbara</i> were found in elevated habitats as most snails were found in cryptic refuges. There was a seasonal decline in abundance of <i>C. acuta</i> (66%) and <i>C. barbara</i> (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.
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