Modifications to prevent non-target lethality of Goodnature A24 rat traps – effects on rodent kill rates

Abstract

On Pacific islands, introduced rats (Rattus spp.) frequently drive extinction of endemic species. Since 2014, managers in Hawaii have frequently relied on Goodnature™ A24 self-resetting traps for rat control, but these traps can incur non-target mortality. After A24 traps killed several passerines in 2018–2019, we field tested trap modifications (lure type, lure flavor, blockers, and trap height) that might deter birds, but not rats. In 2019–2020, we assessed rat kill rates using digital counters on traps and counts of carcasses below modified and reference traps. We conducted trials at two sites with historic rat control on Kauai Island and two sites with no recent rat control on Oahu Island. At both Kauai sites, space and time variables more strongly affected kill rates than did experimental treatments; on Oahu, site and treatment both affected kill rates. Traps near streams had higher kill rates than upland areas or plateaus. Kill rates were highest in fall (Mohihi) and spring (Halepaakai), possibly reflecting scarce food resources that rendered lure more attractive to rodents or larger post-breeding rodent populations. Our results will improve configuration of trap grids and allocation of staff and hardware resources. On Kauai, there was no effect of trap height (12 cm vs 50 cm) on kill rates. Both black blockers and cinnamon lure (presumed to be distasteful to birds) slightly depressed kill rates versus unobstructed traps and chocolate lure. Automatic lure pumps had similar kill rates static lure, but sample sizes were small. On Oahu, traps with metal blockers had lower kill rates than traps with black or no blockers. We conclude that all tested lures except cinnamon, trap heights of 12 cm and 50 cm, and adding black blockers would not significantly affect rat control, so our next question becomes which modifications best deter birds. Additionally, future A24 trap deployments will focus on the seasonal timing and landscape positions to maximize rat kill rates.