Effectiveness of Snap-trapping, Goodnature A24 Automated Traps, and Hand-broadcast of Diphacinone Anticoagulant Baits to Suppress Invasive Rats (Rattus spp.) and Mice (Mus musculus) in Hawaiian Forest

Abstract

Invasive rodents (rats and mice) commonly occur on islands and often damage natural resources largely by predation of native species. Suppressing invasive rodent populations and their damages is therefore a common practice in many parts of the Hawaiian Islands, and land managers such as the Army Natural Resources Program on Oahu often control rodent populations by using large-scale rat snap-trapping and Goodnature A24 automated rat traps (henceforth A24s). While rat traps can be effective at suppressing rat populations, mouse populations are not generally suppressed and may expand greatly. In an effort to reduce rodent populations to levels below that accomplished with rat traps alone at a 5-ha mesic forest on Oahu (Ohikilolo), we assessed the effectiveness of a one-time (two application) hand-broadcast of anticoagulant (Diphacinone-50) bait pellets applied at 13.8 kg/ha per application while A24s and rat snap-traps were active. We monitored rat and mouse activity during trapping and before, during, and after the bait applications using tracking tunnels, which are baited ink cards placed in tunnels so that foot prints of animal visitors can be identified. We found that rat trapping alone was effective at reducing rat populations but not the mouse population, and that the one-time hand-broadcast of diphacinone bait reduced both rat and mouse activity to 0% tracking for about 1 month. However, rat and mouse populations rebounded 2 months later to 15% rat tracking and 41% mouse tracking, which were roughly pre-treatment levels. Rat suppression using A24s at Ohikilolo appeared much more effective year-round than at a nearby 26 ha site (Kahanahaiki), though mouse suppression was poor at Ohikilolo relative to Kahanahaiki. The hand-broadcast of diphacinone bait at both Ohikilolo and Kahanahaiki was effective but short-lived, so repeated baiting during the seasonal peaks in rodent abundance and increasing the size of the buffer area would more likely protect target natural resources from rats and mice.