Winning the shell game: environmental DNA (eDNA) confirms local control of the invasive apple snail, Pomacea maculata

In 2019, the South American apple snail Pomacea maculata established a reproductive population in a six-acre retention pond, Bear Lake, in south Austin, Texas, USA.Community-led removal efforts provide a case study to examine how environmental DNA (eDNA) monitoring may play a role in management of biological invasions.Since 2020, concerned neighbors have removed 4,942 snails by hand and destroyed 1,734 egg clutches.In October 2021, we began collecting surface water samples for eDNA analysis with quantitative PCR as visual surveys still occasionally found snails.Without intervention, the high reproductive capacity of P. maculata could cause the population to rebound.However, multiple factors, including homeowner hand removal and cold winter freezes, have kept population numbers low.A few monthly nearshore eDNA samples indicated the ongoing presence of apple snails, and we hypothesized that deeper areas of Bear Lake might provide refuge.To test our hypothesis, we used a Van Dorn sampler to collect water at five mid-lake locations from surface, 1 m, 2 m, and (variable) bottom depths.We detected no eDNA to indicate snail presence in deeper water.Particularly in the colder months (November-January), detection of small amounts of apple snail eDNA in surface water samples occurred without coincident collection of snails or egg clutches, suggesting either a residual source of eDNA or a few individuals remaining.Throughout 2023, we continued to encounter fewer than 5 snails per month, which did not correspond to the monthly eDNA sampling results, indicating standing eDNA concentrations below the limit of detection or a need for more frequent sampling.Overall, due to intense efforts, especially in 2020 (with an estimated 99% of snails removed), visual and eDNA monitoring efforts suggest that the snail population may be close to local extirpation.We recommend cautious optimism but continued vigilance to assure that the population does not recover.