Rodent pest species threaten many of the world’s resources, including those in agriculture, urban, and natural environments. Goodnature® A24 rat+stoat self-resetting traps (A24s) are used extensively for invasive rat control in several countries and environments, especially in island ecosystems. Unlike traditional single-set kill traps, A24s fire and reset up to 24 times per CO₂ cartridge, and represent a nontoxic device for rodent pest control. Given the rapid rise in use of A24s as a management tool, our goal was to understand why A24s work in some cases and not others. To address this goal, we briefly review A24 literature, and then we: 1) introduce a special issue of A24 effectiveness and hazard studies, and 2) propose future research needs and recommended uses for A24s including a list of potentially susceptible rodent species for which further A24 testing is required. Most papers in the special issue focus on Rattus rattus control on islands in Hawaii and New Zealand. Additionally, A24 effectiveness was compared to toxic bait use against rats in California agricultural landscapes, and snap-traps against house mice (Mus musculus) in Colorado pens. Behavioral studies in this special issue demonstrated that both rats and mice repeatedly visit and pass by A24s much more frequently than they trigger A24s. Some large-scale trapping grids in Hawaiian forests reduced rat populations, but a few endangered birds were unintentionally killed by A24s, which stimulated research on bird excluder attachments for A24s. Understanding the duration of carcass persistence of rats and birds killed by A24s has helped trap users account for unobserved target and non-target kills. Future research may investigate A24 uses on rodent species other than Rattus, as the types of species potentially vulnerable to A24s range in size from mice (e.g., Apodemus, Mus, Peromyscus, Reithrodontomys; ≥ 14 g) to squirrels (e.g., Otospermophilus, Sciurus; 353–527 g). A24s can be used successfully to control rodent populations of some species in particular environments. Studies that compare A24s with alternative and synergistic rodent management methods will help determine the most effective and efficient rodent control methods that minimize non-target risks.

Invasive ship rats (Rattus rattus) are a major threat to the native species and ecosystems of islands. We used 10 self-resetting traps (A24 rat and stoat traps, Goodnature Ltd., Wellington, NZ), along with existing single kill DOC200 traps at two devices per hectare on a 9.3 hectare island in New Zealand to reduce rat numbers and ideally achieve eradication. Each self-resetting trap was monitored with motion-activated cameras to analyse rat behaviour and A24 kill numbers were documented using Goodnature digital strike counters. The traps were checked on 10 occasions from August 2016 to October 2017. The videos documented initial high rat activity on the island, which reduced over time following initial trapping success. An immediately obvious neophobic response towards the A24 traps was not observed. Rats interacted with the A24 traps within hours after initial deployment and 60% of the traps were triggered in the first night. After three nights, all traps were triggered at least once. While rats interacted with the traps at all times of the year the number of observed trap-triggers was relatively low. High number of interactions resulted in high kill numbers in late spring when population size was increasing and seasonal food abundance had not yet reached its peak. A second peak was observed in late autumn when rat abundance was presumably high. Recruitment of naïve individuals was a probable cause for high kill numbers during the breeding season. In winter, when rat abundance was presumably low, a few individuals were the likely cause for a high number of interactions while kill numbers were low. A knock-down (i.e. suppression from high to low abundance) of rats using both trap types was achieved in the first 100 days. However, kill numbers of A24s declined over time. After the initial suppression, the number of rats killed was insufficient to offset intrinsic population growth and reinvasion from the adjacent coast, thereby preventing eradication.

Lethal trapping of island invasive rodents is a critical practice used by management organizations to protect native biota. Carcass detection from lethal trapping is dependent on the trapping method, carcass palatability, and scavengers present. Goodnature A24 self-resetting rat traps are an effective tool in remote areas and complex terrain because traps can be visited at 4–6 month intervals and produce multiple carcasses during that interval. The goal of this study was to determine whether we are a) underestimating target mortality with carcass counts and b) failing to detect non-target mortality between A24 trap checks at two field sites on the island of Kauai. Both sites have established Goodnature A24 trap grids with 300 A24s deployed by the Kauai Forest Bird Recovery Project (KFBRP), and one site is fenced to exclude invasive ungulates. KFBRP conducts routine trap checks every four months, finding 0–3 rat or mouse carcasses at each trap. We assumed that traps kill more animals than indicated by carcass counts because 75% of traps have counters to record when traps fire, and counter tallies usually exceed carcass counts. In 2018 and 2019, several bird carcasses were found under traps; therefore, we needed to investigate the likelihood that non-target mortality went undetected. In May 2019, we placed 60 carcasses (30 non-native birds and 30 rats) at a fenced site, and 60 carcasses (30 bird, 30 rat) at an unfenced site, in both gulches and uplands. Carcasses were periodically surveyed for 120 days after deployment. The unfenced site had greater removal rates, notably in gulches; 33 of 60 carcasses remained detectable, compared to 52 of 60 carcasses at the fenced site. Bird and rat carcasses did not differ in persistence, and taxon did not affect scavenger preference. These results suggest that significant non-target mortality has not gone undetected in our A24 trap grid because we are likely to detect most target and non-target carcasses after four months in fenced areas, and especially upland unfenced areas. However, we are less likely to detect carcasses in the unfenced gulches where ungulate scavengers are prevalent, and increased monitoring may be needed in such gulches.

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.

Birds can be unintentionally injured or killed by mammal traps. Unfortunately, some birds in Hawai‘i, including puaiohi (Myadestes palmeri), and in New Zealand, have been killed by the widely used Goodnature® A24 rat-stoat self-resetting trap (A24s) during rodent control. To help address this problem, we conducted two sets of aviary trials using (1) barriers and (2) existing A24 excluders with red-winged blackbirds (Agelaius phoeniceus), European starlings (Sturnus vulgaris), and a single puaiohi. We conducted barrier trials to inform future bird excluder designs by establishing the minimum gap-height beneath a barrier that blackbirds and starlings successfully overcome. During barrier trials, starlings defeated barriers significantly lower (≥ 1.9 cm) than blackbirds (≥ 2.9 cm). We then presented disarmed A24s with no excluder or with one of two excluders (plastic Goodnature, 11 cm length; and metal mesh, 10 cm, or 15–17 cm length for puaiohi) to each bird species. We placed the A24s and excluders low (11–25 cm trap height, 0–10 cm excluder height) and high (50–83 cm trap height, 37–72 cm excluder height) above ground to test bird entry abilities at trap heights used by land managers to control rodents. During the A24 excluder trials, all three bird species entered the trap at low or high heights with no excluder, confirming bird risk of injury when excluders aren’t used. Excluders greatly decreased entry into A24s by blackbirds and puaiohi, but not by starlings. Plastic and metal excluders prevented puaiohi entry at low height only. Based on our trials and recent field uses, the plastic excluder performed best for excluding all three bird species if placed so the lower edge of the excluder is 0–2 cm above ground. However, plastic and metal excluders can clog with dead rodents when positioned low. Future trials aiming to exclude all small birds from A24s while maintaining trap efficacy against target rodents should consider new excluder designs incorporating the 1.9–2.9 cm gap-height thresholds. When deciding whether to use excluders and appropriate trap heights, managers need to consider both the risk of non-target injuries or deaths and the impact of potentially decreasing trap efficacy.

House mice (Mus musculus) are worldwide pests in urban, agricultural, and natural settings. Goodnature® A24 rat+stoat self-resetting traps (A24s) are used globally for invasive rat control, yet adequate efficacy testing has not occurred against house mice. Our objective was to test efficacy of A24s against wild house mice. We first used cage/pen trials to determine whether the time from A24 impact to death was short and met international animal welfare standards (New Zealand National Animal Welfare Advisory Committee [NAWAC]). We also varied lure type (peanut butter vs. Goodnature chocolate lure) and trap configuration (vertical vs. angled in a trap stand) to assess trap attractiveness. Of the 80 mice tested, 67 triggered A24s, and just three required our intervention and euthanasia because they were still alive 2 minutes after being struck by the A24. Time to death for the remaining 64 mice averaged (± SE) 50.9 ± 2.6 seconds (median: 46.8 seconds, range: 19.8–120 seconds). Thus, A24s passed NAWAC standards of Class B for kill-traps against house mice. Although mice frequently contacted A24s during the 3-day trials, average time to trigger was 4.6 ± 0.6 hours and 13 mice never triggered A24s. Baiting A24s with peanut butter resulted in significantly greater mortality (98%) than by using Goodnature chocolate lure (70%). Mice triggered A24s baited with peanut butter 2.3 times faster than Goodnature chocolate lure, and 2.7 times faster if A24s were angled in trap stands rather than positioned vertically. In further trials, we released groups of up to five mice into a 24 m² arena containing either two A24s or two snap-traps+two A24s. Two A24s in the arena resulted in all 25 mice triggering A24s and dying; mice were undeterred from triggering A24s when mouse carcasses were near A24s. When snap-traps and A24s were present, one of 38 mice survived the 9-day trials. If snap-traps were reset each 24 hours they killed significantly more mice than A24s, yet if they were not reset during trials the A24s killed significantly more mice than snap-traps. A24s appear adequate for use against house mice, especially if baited with peanut butter and angled in trap stands.

Invasive black rats (Rattus rattus) are one of the most damaging vertebrate species to agriculture globally. In citrus orchards, rat damage includes fruit consumption and contamination, girdling of branches, and gnawing irrigation equipment. Managing black rats in citrus is challenging given the abundance of food and cover provided by the trees year-round. Anticoagulant rodenticide applications, such as diphacinone-laced baits applied via bait stations, are sometimes used to manage black rats in orchards, but have not been tested in an evergreen crop like citrus. Goodnature A24 self-resetting rat traps are increasingly used to manage black rats for conservation purposes, but have not been tested in agricultural settings. Therefore, we used a replicated treatment-control randomized block design to test the efficacy of: 1) 0.005% diphacinone-treated oats applied via elevated bait stations and 2) A24 traps that were elevated to approximately 1 m height to match the bait station heights. This study was conducted across four citrus orchards in the southern San Joaquin Valley, California, to better identify how to implement these tools to manage invasive black rats in this economically important crop. Although neither trapping nor rodenticide baiting yielded the desired reduction in black rats across all sites, we identified strategies that hold promise for future testing. For rodenticides, reducing the spacing between bait stations may increase efficacy by increasing encounter rates by rats. For trapping, the use of a platform under the A24 traps appeared to increase its effectiveness by allowing easier access to the trap trigger. Furthermore, reducing spacing between traps may also yield better results. Ultimately, a management plan that combines trapping and rodenticide baiting may prove more efficacious than our initial study design and should be investigated further.