Aerially applied citric acid reduces the density of an invasive frog in Hawaii, USA

Abstract

The Puerto Rican frog (Eleutherodactylus coqui) is an invasive pest in Hawaii. Citric acid is approved for controlling these frogs, but has been limited to terrain accessible by foot or vehicle. We determined the effectiveness of helicopter applications of 16% citric acid and repeated 11% citric acid treatments for eradicating and/or reducing densities of E. coqui by monitoring populations before and after treatment using mark–recapture. We also evaluated the effects of the marking technique, toe-clipping, and weather changes on population parameter estimates. We found that a 16% citric acid treatment appears to have reduced adult E. coqui density 3-fold in a plot, T1, completely covered with citric acid, but did not reduce adult density in a plot, T2, where 6% of the plot was unintentionally not treated. Preadults were reduced 3- to 5-fold in treated plots. The apparent reduction in adults in T1 lasted at least 5 months. Repeated treatments of 11% citric acid were studied in T2 and likely reduced adults 440-fold while preadults were reduced 9-fold. E. coqui that had fewer toe-clips had greater recapture probability and survival estimates, while weather had no effect on parameter estimates. In summary, we found that 16% and 11% citric acid treatments can reduce E. coqui density, treatment effects can last 5 months for adults, and repeated treatments appear more effective for reducing density than single applications.