Invasive Snake Activity Before and After Automated Aerial Baiting

Abstract

Aerially delivered toxic baits have proven effective for landscape‐level control of numerous invasive vertebrate populations with major benefits for conservation and ecosystem function, but this technique has not been broadly adapted for control of invasive reptiles. Nonnative brown treesnakes (Boiga irregularis) on the Pacific island of Guam have caused severe ecological and economic damage and pose an invasion risk on other islands, making them a high‐profile candidate for application of aerial baiting methods. Although terrestrial applications of traps, toxicants, and hand‐removal are standard brown treesnake management practices, these methods are not cost‐effective for control in the island's large tracts of remote, rugged forest. In 2016, the first major in situ evaluation of a helicopter‐borne automated aerial bait delivery system applied snake‐targeted toxic baits at an effective rate of approximately 120 baits/ha over a 110‐ha forested test plot on Guam. We evaluated the extent and duration of the suppressive effect of this toxic bait application on brown treesnakes by measuring nontoxic bait take rates as a proxy index of relative snake abundance before and after toxic bait application in a treatment plot and surrounding reference area. We placed 4,420 nontoxic baits in random transects at georeferenced locations, from 1 month before until nearly 12 months after toxic bait application, allowing temporal analysis of the suppressive effect and spatial analysis of treatment plot reinvasion. Over the first 30 days after toxic bait application, average nontoxic bait take rate in the treatment plot was 41.2% lower than the pre‐application rate, and there was no immediate decrease in bait take in the reference area. Reduced snake activity was still evident nearly 12 months after bait application. Roads forming a portion of the treatment boundary appeared to slow snake movement between treated area and surrounding untreated area. Trail cameras monitoring a subset of bait tubes showed that 97.5% of baits removed were taken by snakes rather than nontarget species. We indexed rodent abundance in the treatment plot and reference area, and found no indication of a rodent population increase following toxic bait application. Our results show that automated aerial bait applications can suppress brown treesnake abundance over a large area and that reinvasion from surrounding untreated habitat occurs over several months. We anticipate that repeated bait applications could achieve and maintain greatly reduced brown treesnake abundance on a landscape scale, potentially improving biosecurity and enabling experimental reintroduction of native birds extirpated by brown treesnake predation. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. The Journal of Wildlife Management published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

Brown Treesnake Mortality After Aerial Application of Toxic Baits

“…Our findings provide direct evidence that aerial application of toxic baits increases brown treesnake mortality at the landscape‐scale, rather than indirect indices of relative abundance (Siers et al 2020 b ) and supports the findings of Nafus et al (2020). All brown treesnake mortality during the study occurred within the treatment period, indicating that deaths were likely caused by ingestion of toxic baits as opposed to other natural causes or artefacts of transmitter implantation.…”

Arboreal aperitifs: Space use and activity of a major songbird nest predator in a tropical Thailand forest

“…A further similarity with B. irregularis may suggest that the stationary periods we observed are the result of prey digestion. On Guam supplementary fed snakes showed lowered activity for 1 to 5 days post feeding (Siers et al, 2018), broadly reflecting our mean stationary period of 3.29 ± 0.66 days.…”

“…The tracked individuals often began activity just after sunset, at dusk, and ceased at dawn, just before sunrise. Similar observations exist regarding Boiga irregularis nocturnal activity on Guam, with snakes becoming active around sunset, and remaining more or less so throughout the night until ceasing their activity around sunrise (Fritts and Chiszar, 1999; Lardner et al, 2014; Siers et al, 2018). A further similarity with B. irregularis may suggest that the stationary periods we observed are the result of prey digestion.…”

Distilling Professional Opinion to Gauge Vulnerability of Guam Avifauna to Brown Treesnake Predation

“…The size distribution of BTS in an area is another crucial component in a full risk assessment. Snake size greatly affected our estimates of bird mortality due to BTS encounter, and some current suppression tactics are believed be selective for certain BTS size classes, altering size distributions of BTS Lardner et al, 2009Lardner et al, , 2013Nafus et al, 2020;Siers et al, 2020b). Ultimately, it is the difference in size between a bird and a snake that determines risk of death due to the snake's gape limiting its abilities to seize, restrain, kill, and ingest prey (BTS swallow their prey whole).…”