Beginning in 2009, the Army’s Natural Resource Program on O’ahu implemented the first of three ecosystem-scale rat trapping grids of traditional snap traps in the Waianae Mountains using the model outlined in The New Zealand Department of Conservation’s current best practices for kill-trapping rats. Traps were generally checked every two weeks, but bait often remained palatable for just a few nights due to slug interference. Because of the amount of labor required for single set traps, trials with Goodnature A24 self-resetting traps were conducted from 2014-2016. Early findings showed that traps were malfunctioning at a rate of ~25% and there were major deficiencies with the bait and bait-delivery system. In 2016 the bait system was improved when Goodnature developed the automatic lure pump that continually releases fresh bait for ~4-6 months. Other improvements were also made to the A24 trap to decrease the malfunction rate. In 2017, we replaced more than 1,300 snap traps at all ecosystem-scale grids with 1,000 A24s. Tracking tunnels were used as an independent monitoring system to determine rat control effectiveness. At all sites, rat activity measured in the tracking tunnels has been low (less than 15%) for over 18 months. In this paper we discuss the results of the transition from single- to multi-set rat traps, highlights some successes and obstacles, and describes grid spacing and applicability to other sites.

Three rat species, the Norway rat, black rat or ship rat, Pacific or Polynesian rat, and the house mouse are among the most widespread and destructive invasive mammals affecting islands. Through mostly unintentional introductions by humans, these rodents occupy >80% of the major islands worldwide. As a consequence of their omnivorous diet and large incisor teeth, introduced rats are probably the invasive animals responsible for the greatest number of plant and animal extinctions on islands. The effects of house mice on island ecosystems are less well known when compared with rats. We have reviewed past diet studies of these 4 omnivorous rodent species. Our review suggests that due to the high variability in diet, as well as direct and indirect effects of predation, all 4 species pose potential threats to many plants and animals on Pacific islands. Although rodent diets greatly differ among sites, diets appear to roughly follow this pattern: Norway rats and house mice generally consume proportionally more animal than plant (Norway rats consume many vertebrates and house mice mostly consumes arthropods), whereas black and Pacific rats generally consume proportionally more plant than animal. Studies specifically linking rodent diets with the population status of surrounding biota are needed in order to clarify the effective impacts of these rodent species. Much could be learned from rodent removal experiments, which could further expand our knowledge of invasive rodent species effects, and native species conservation, on islands.

Roof rats (Rattus rattus) are an invasive rodent that can cause substantial damage in citrus orchards. Their populations appear to be expanding throughout California, yet little is known about efficacious, cost-effective strategies to manage this invasive pest while minimizing pesticide use. Therefore, we developed two Integrated Pest Management (IPM) programs based on results of recent studies that incorporated observations of roof rat movement patterns, compared monitoring tools, and tested trapping and baiting options. Both IPM programs utilized elevated bait stations containing 0.005% diphacinone-treated oats and trapping, and we compared those programs to a bait-station only approach to determine which strategies were most practical. Initial IPM plots included a combination of elevated bait stations followed by a brief snap-trapping program and a longer-duration trapping program with Goodnature® A24 traps to hopefully keep rat numbers at low levels (Trial 1). Although initial bait applications were effective at reducing rat numbers, populations quickly rebounded within both the bait station only and IPM treatment areas within two-months following the completion of the baiting programs. Additionally, costs for this initial IPM approach were almost five times as much as a bait station approach. Our second IPM strategy (Trial 2) again incorporated an initial bait application period to knock down roof rat populations, followed by the use of trapping tunnels that contained two snap traps to further reduce/maintain rat numbers longer-term. We again observed effective knockdown with bait applications. However, in contrast to Trial 1, we observed substantial success with trapping tunnels at maintaining, and even increasing, overall efficacy within IPM plots, and IPM plots were always more efficacious than bait station only plots. Although the bait station only approach was less costly than the IPM approach used in Trial 2, the cost disparity was substantially less than that for Trial 1, and the cost difference disappeared during subsequent years, indicating long-term cost-effectiveness of this IPM approach. Collectively, the relatively low cost and high efficacy of a management program that incorporates initial bait applications to knock down roof rat populations, followed by a long-term snap-trapping program to maintain low densities, should provide an effective strategy for managing roof rats in citrus orchards.

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.

Roof rats cause extensive damage in orchards throughout the world. Integrated Pest Management (IPM) systems are the best option for managing rodents, yet few management systems have been developed and tested to control roof rats in agricultural settings. We initiated a study in 2020 to provide the foundation for an IPM program to manage roof rats in California citrus orchards. Our initial efforts centered on developing effective monitoring strategies for roof rats to determine when management actions are needed, assessing rat movement patterns to determine proper placement of management tools, and conducting initial tests of Goodnature A24 self-resetting traps and elevated bait stations containing 0.005% diphacinone-treated oats. We determined that the use of both tracking tunnels and remote-triggered cameras served as effective monitoring tools for roof rats in citrus orchards, and a smaller 3 × 3 grid placement of these monitoring tools was as effective as a 5 × 5 grid, indicating that substantial material and labor costs could be saved by using the smaller grid size. Placement of the monitoring tools on the ground or up in trees did not influence the effectiveness of this approach. We determined that roof rats exclusively used orchard habitats rather than surrounding fields, so control efforts should be focused in these areas. Roof rats moved substantial distances daily (~170-190 m), but results from bait station trials suggest that spacing of bait stations and traps should be closer together to increase roof rat encounter rates of these devices. Our trial with A24 traps elevated in orchard canopies suggest that there is an advantage to including a platform underneath the trap to increase trap activation. Collectively, this information provides a baseline for future research targeted at developing an effective IPM program for managing roof rats in citrus orchards.

Since 1995, the Army’s Natural Resources Program on O‘ahu has been controlling rodents in O‘ahu’s forests to protect native plants, invertebrates, and birds. Bait longevity and attractiveness are keys to successful rodent trapping. Success is impeded when slugs interfere with bait intended for rodents. Slugs can consume all or a portion of the bait, make it less attractive to rodents via their slime, and large slugs can trigger the traps. The goal of this study was to determine whether food grade citric acid (up to 5% concentration) added to common rodent bait (i.e., commercial peanut butter and Goodnature lure) would repel slugs. We conducted several trials including where captive slugs were offered both a test (0.5-5% citric acid added) and control bait. We found slugs significantly preferred the control bait (i.e., bait without any citric acid) in the two-choice feeding experiment when the bait was Goodnature rat + mouse lure. Goodnature rat + mouse lure is the standard bait used in A24 self-resetting rat traps. The addition of citric acid may improve the longevity and attractiveness of bait to rodents in slug-inhabited environments, thereby having the potential to aid in rodent control programs.

Prairie dogs are an accessible and enjoyed wildlife species in Colorado that require occasional surveys because populations can change abruptly due to plague outbreaks or human-induced control. We evaluated the use of small copter drones at four prairie dog colonies on Open Space and Mountain Parks lands, City of Boulder, to determine if this methodology improves efficiency over ground-based survey methods. We counted prairie dogs and burrows using two types of drones (DJI Matric 210 and Autel Evo II) at altitudes 100', 150', and 400' (burrows only). We recorded video and merged still images into orthomosaics prior to having USDA staff analyze this imagery. We then compared the drone imagery counts to those of our simultaneous ground-based counts of prairie dogs. We determined that 100' altitude mosaics produced using DJI Matric 210 drone were most accurate (closest to true, ground-based counts) for burrow abundance. We were not able to identify the best drone and altitude combination for drone-based prairie dog abundance. Overall video and mosaics both had similar accuracy in most prairie dog counts, however 150' video was more accurate than 100' video. One staff member counted burrows more closely to true than did the other. Both staff members required about the same amount of time to count and analyze imagery; videos could be evaluated slightly faster than mosaics (average of 3.8 hours vs. 5.5 hours per imagery) when counting prairie dogs, and burrow counts (of mosaics) generally took 2-3 times longer to analyze (averaging 8.1 hours per imagery; range: 3-13 hours) than did prairie dog counts. The labor requirement of using drones for burrow and prairie dog counts is far more time consuming (3-4 times longer per hectare) than having field staff conduct the traditional on-the-ground counts that include repeated prairie dog counts in a day. Until drone technology improves to allow targeting larger colonies (>2 km2) and automated detection and counting of wildlife become more commonplace, drone surveys are unlikely to be a more efficient technique than ground-based surveys for evaluating prairie dog abundances.

Use of a contraceptive (i.e., fertility control) is attractive for rodent management where lethal control is unwanted. Although population reduction is generally unachievable with small-scale, short-term contraceptive use, reduced juvenile recruitment is achievable. The injectable immunocontraceptive vaccine GonaCon (active ingredient: gonadotropin releasing hormone [GnRH]) was registered in 2022 by the U.S. EPA for controlling fertility of female prairie dogs (Cynomys, a type of ground squirrel) in urban/suburban settings. Here we: 1) describe past research, including a replicated field study in Colorado (GonaCon treatment vs. control sites) testing efficacy of GonaCon in prairie dogs, which gave rise to the EPA registration of this product, 2) outline future research needs for prairie dog population management with GonaCon–Prairie Dogs, 3) describe the steps required to possibly register GonaCon for additional rodent species, and 4) describe the other fertility control pesticide products that are currently registered for use against rodents in the U.S. During the replicated field study in Colorado, prairie dogs were live-trapped in a portion (avg: 18.5%, range: 7-37%) of each colony’s total area. In treatment plots, every female captured ≥ 660 g was injected with 0.4 ml of GonaCon. GonaCon was highly effective in controlling female fertility during the first year (2019), as juvenile density was reduced 3×, but not in the second year (2020) following treatment. Treating whole colonies of prairie dogs is favored, yet if small or partial colony treatment is desired then annual GonaCon treatment may be needed. An amendment to the EPA label is in progress to allow treatment of both male and female prairie dogs. Due to recent interest from land and pest managers, other fertility control products and the steps required to possibly get GonaCon registered for additional rodent species are also outlined.

The eradication of black rats from Dry Tortugas National Park (DRTO; 42 ha land mass) was completed in January 2022 through collaboration between USDA APHIS Wildlife Services and the National Park Service. Black rats had been documented throughout DRTO since the park was established in 1935. Rat predation of ground nesting birds at DRTO has included brown noddies and sooty terns. Additionally, black rats have damaged equipment and infrastructure throughout the park, and caused human health and safety issues in staff residences. For this eradication, WS chose to use a diphacinone-based rodenticide: Diphacinone-50 Conservation. Diphacinone was chosen instead of brodifacoum because it poses lower risks to non-target species (e.g., 100× less toxic to birds) and has less environmental accumulation over time. Diphacinone-50 Conservation was deployed throughout the park in bait stations placed on a 30m × 30m grid. Bait stations were baited and monitored daily for 14 days. Additionally, three hand broadcast applications were utilized during the eradication in areas of the park closed to visitors. Daily monitoring of the bait stations showed a high initial uptake of the toxicant. Also, monitoring using thermal imaging optics was used throughout the eradication to surveil bait uptake and rat activity. No rats were detected during the last four days of the eradication. Post-eradication monitoring was conducted quarterly for the first year following the eradication, and twice during the second year. Post eradication monitoring consisted of 120 snap traps and chew sachets placed throughout the park for four nights. In addition, track surveys were completed along the dune lines and thermal monitoring was conducted after dark during each monitoring period. The eradication was deemed a success in January 2024. National Park Service currently maintains a strict biosecurity plan for DRTO. This plan includes permanent bait stations and cameras placed in areas of likely reintroductions, monitoring of docking vessels, and checking cargo before distribution. Two weeks post eradication, NPS personnel observed a rat jump off a vessel onto DRTO. This rat was later found dead after consuming toxicant from the permanent bait stations. No rats have been detected at DRTO since.

House mice and rats have been introduced to most countries and islands worldwide and represent serious threats to biodiversity, economic enterprise, and human health. Genome editing and gene drives are being explored as new genetic biocontrol methods to effectively suppress rodent pests. An essential step in the translation of this technology to field-ready tools is to administer breeding and behavioral trials with freely-interacting genetically modified (GM) rodents. Due to the potential impacts of unintended release of GM organisms, these trials will require biosecure animal facilities that are rigorously tested to ensure physical containment. This study was conducted to develop and evaluate the biosecurity of a physical containment facility for house mouse trials at USDA’s National Wildlife Research Center. First, we conducted >20 trials with 75 wild-caught (non-GM) house mice to test their ability to escape from small containment units (0.35 m2). During these trials that lasted >160 days, mouse behaviors and escape attempts were documented following exposure to attractants (high value foods and potential mates) as motivators for escape. Just two mice success­fully breached containment during early trials, and both were from chewing small holes in plastic walls that allowed escape to the other side of the containment unit. In a second series of trials, we assessed containment efficacy in a large (24 m2) arena intended to more closely replicate conditions of free-breeding wild mice. In these trials, mice were held in groups of six to 26 for up to 6 months. Across trials, only one mouse escaped the arena, an incident most likely attributable to human error during routine animal husbandry activities. This mouse was captured in the secondary containment (live-trap) within hours of breaching the primary containment. Overall, the containment strategy utilized here presents a robust design, with redundant containment mechanisms that should serve as a model for future behavioral trials using GM rodents. Additionally, our study highlights the need for rigorous staff training, careful attention to construction methods and materials, and adhering to biosecurity protocols to ensure the highest levels of containment that will be essential for testing efficacy of genetic biocontrols.