Excluding Nontarget Species from Brown Tree Snake, Boiga irregularis (Reptilia: Colubridae), Bait Stations: Experimental Tests of Station Design and Placement

Abstract

Bait stations with toxic baits are an emerging technology for eradication of the invasive brown tree snake (Boiga irregularis) on Guam, yet potential interferences by nontarget species are largely unknown. We tested the efficacies of three bait station designs together with three commonly used station support structures to exclude nonnative rats (roof rat, Rattus rattus; Norway rat, Rattus norvegicus; Polynesian rat, Rattus exulans) and native coconut crabs (Birgus latro). When directly presented, all species readily consumed the dead neonatal mouse baits (nontoxic) including those replicating decomposing baits in the field. When bait stations were made easily accessible by placement near ground level, all rat species were able to enter all station types, but some individual roof rats and Norway rats exhibited apparent neophobia. When stations were placed up on support structures, simulating those in the field (P1 m above ground level), numbers of station accessions by roof rats and Norway rats remained essentially unchanged, but Polynesian rats then showed almost no inclination to enter stations. However, ability to access entrances of stations ( but not interiors) when on support structures was extremely high for roof rats and appreciable for the other rat species, including Polynesians. The station type currently in widest use, when placed on chain-link cyclone fence, had the highest probability of accession. Crabs readily accessed station entrances but never interiors. The two downward-angled station designs, when placed in simulated vegetation, had the lowest probabilities of accession. In areas where nontarget species are a concern, we recommend use of either of the downward-angled station designs and suspension from vegetation wherever possible.Since the realization nearly 25 years ago that the introduced brown tree snake, Boiga irregularis (Merrem), had decimated nearly all of Guam's native vertebrate species (Marshall 1985, Savidge 1986, 1987, a diversity of control methods has been investigated. The most successful of these methods are being used and refined in an integrated pest management approach (Engeman and Vice 2001) with the goals of preventing transport of snakes from Guam to other at-risk areas, detecting and eradicating potential incipient populations, and severely suppressing population levels on Guam (Colvin et al. 2005). Severe population suppression has an overarching importance because, when effectively implemented and sustained, it may be possible to reduce the scope and intensity of other containment efforts and attendant costs.No large-scale reductions have yet been conducted, but much of the basic technology

Effectiveness of bait tubes for brown treesnake control on guam

“…The horizontal tubes were easier to manufacture and easier to bait, and had no disadvantages with regard to bait removal rate by snakes. However, in sites where entry by rats or crabs is common and non‐target bait take is a problem, Mathies et al () recommended using the vertical tubes. We found no evidence that either tube type was better or worse for any particular snake size cohort, but that inference is limited to the larger snake size classes that took the bait.…”

Photographic validation of target versus nontarget take of brown treesnake baits

“…The use of dead newborn mouse baits laced with 80-mg acetaminophen tablets and applied in bait stations to exclude nontarget species (lengths of plastic tube, hereafter referred to as bait tubes) is a cost-effective alternative to trapping and poses little risk of nontarget mortality (Johnston et al 2002, Mathies et al 2011, Clark et al 2012). In addition to providing lethal control, the proportion of toxic or nontoxic baits removed, or take rates, have also become a standard metric of brown treesnake foraging activity, interpreted as an index of relative snake abundance for monitoring efforts (Savarie et al 2001, Clark et al 2012, Dorr et al 2016.…”