Evaluating lethal toxicant doses for the largest individuals of an invasive vertebrate predator with indeterminate growth

The brown treesnake ( Boiga irregularis ) was accidentally introduced to Guam and caused severe ecological and economic damages. Acetaminophen is an effective, low-risk oral toxicant for invasive brown treesnakes, and an automated aerial delivery system (ADS) has been developed for landscape-scale toxic bait distribution. A fixed dose of 80 mg of acetaminophen within a tablet inserted into a dead neonatal mouse (DNM) was lethal for all brown treesnakes in previous trials; however, these trials did not include very large individuals which are difficult to acquire for testing. Because most reptiles continue to grow throughout their lifespan, a small number reach much greater than average body sizes. Here, we tested effectiveness of 80 mg acetaminophen DNM baits for unusually large brown treesnakes as they became available. Our results confirmed that an 80 mg dose is lethal for the vast majority of snakes on Guam, but efficacy starts to diminish around 200 g of body mass. We also tested an alternative mouse bait configuration with 160 mg of acetaminophen that could be incorporated into the ADS to improve control of unusually large snakes. The 160 mg dose is expected to be effective for nearly all female snakes; males grow much larger and additional methods will be needed for extraordinarily large individuals. We describe a full dose-response curve for brown treesnakes to acetaminophen tablets and estimate the LD 90 at 299 mg/kg and the LD 99 at 578 mg/kg. To our knowledge, this is the first published dose-response curve for an invasive vertebrate with indeterminate growth.