Predator water balance alters intraguild predation in a streamsidefood web

Previous work suggests that animal water balance can influence trophic interactions, with predators increasing their consumption of water-laden prey to meet water demands.But it is unclear how the need for water interacts with the need for energy to drive trophic interactions under shifting conditions. Using manipulative field experiments, we show that water balance influences the effects of top predators on prey with contrasting ratios of water and energy, altering the frequency of intraguild predation. Water-stressed top predators (large spiders) negatively affect water-laden basal prey (crickets), especially male prey with higher water content, whereas alleviation of water limitation causes top predators to switch to negatively affecting energy-rich midlevel predators (small spiders). Thus, the relative water and energy content of multiple prey, combined with the water demand of the top predator, influences trophic interactions in ways that can alter the strength of intraguild predation. These findings underscore the need for integration of multi resource approaches for understanding implications of global change for food webs

The Path to U.S. National Registration of a Toxic Bait for the Control of the Small Indian Mongoose

The small Indian mongoose (Urva auropunctata [syn. Herpestes auropunctatus]; mongoose) is a highly invasive species in its introduced range that negatively impacts ecosystems. Mongooses depredate native species, serve as a vector of disease posing a risk to human health, and cause sanitation issues in food processing facilities and public areas. Introduced for biocontrol in the late 1800s in Hawaiʻi and the Caribbean, mongooses currently have well-established populations across multiple islands in both island archipelagos and have invaded numerous other locations throughout the world. The concern of accidental introduction to mongoose-free islands, the difficulty in species detection, and the high cost and labor demand of trapping present the need for a novel control method. A target-specific and efficacious toxic bait can provide an additional tool to reduce mongoose abundance, to eradicate incipient populations, and for biocontrol at ports of entry. In this paper, we document the pathway to registration for a toxic bait for mongoose control with the U.S. Environmental Protection Agency. A registered product must demonstrate a low risk to nontarget species, meet standards for human health and safety, and show no unreasonable adverse effects to the environment. There are no other comparable invasive small mammalian carnivores for which toxic baits have been developed and registered for bait station deployment in the United States

Relative palatability and efficacy of brodifacoum-25D conservation rodenticide pellets for mouse eradication on Midway Atoll

Invasive mice (Mus spp.) can negatively impact island species and ecosystems. Because fewer island rodent eradications have been attempted for mice compared to rats (Rattus spp.), less is known about efficacy and palatability of rodenticide baits for mouse eradications. We performed a series of bait acceptance and efficacy cage trials using a standard formulation of brodifacoum-based rodenticide on wild-caught mice from Sand Island, Midway Atoll, to help inform a proposed eradication there. Mice were offered ad libitum brodifacoum pellets along with various alternative food sources, and a “no choice” treatment group received only bait pellets. Mortality in the no choice trial was 100%; however, when offered alternative foods, mice preferred the alternative diets to the bait, leading to low mortality (40%). Because there was concern that the bittering agent Bitrex® in the formulation may have reduced palatability, we conducted a subsequent trial comparing brodifacoum bait with and without Bitrex. Mortality in the with-Bitrex treatment group was slightly higher, indicating that the bittering agent was not likely responsible for low efficacy. Laboratory trials cannot account for the numerous environmental and behavioral factors that influence bait acceptance nor replicate the true availability of alternative food sources in the environment, so low efficacy results from these trials should be interpreted cautiously and not necessarily as a measure of the likelihood of success or failure of a proposed eradication

Validation of a death assay for Angiostrongylus cantonensis larvae (L3) using propidium iodide in a rat model (Rattus norvegicus)

Angiostrongylus cantonensis is a pathogenic nematode and the cause of neuroangiostrongyliasis, an eosinophilic meningitis more commonly known as rat lungworm disease. Transmission is thought to be primarily due to ingestion of infective third stage larvae (L3) in gastropods, on produce, or in contaminated water. The gold standard to determine the effects of physical and chemical treatments on the infectivity of A. cantonensis L3 larvae is to infect rodents with treated L3 larvae and monitor for infection, but animal studies are laborious and expensive and also raise ethical concerns. This study demonstrates propidium iodide (PI) to be a reliable marker of parasite death and loss of infective potential without adversely affecting the development and future reproduction of live A. cantonensis larvae. PI staining allows evaluation of the efficacy of test substances in vitro, an improvement upon the use of lack of motility as an indicator of death. Some potential applications of this assay include determining the effectiveness of various anthelmintics, vegetable washes, electromagnetic radiation and other treatments intended to kill larvae in the prevention and treatment of neuroangiostrongyliasis

Laboratory Evaluation of the Effectiveness of the Fertility Control Bait ContraPest® on Wild-Captured Black Rats (Rattus rattus)

A non-toxic liquid fertility control bait for rats has recently become commercially available (ContraPest® from SenesTech, Inc.). This product contains two chemicals, both of which impair spermatogenesis in male and reduce ovulations in female rats. We tested the efficacy of this bait in wild-caught adult black rats from the island of Hawai’i in a short-term laboratory trial. A control group (n = 25) was offered placebo bait and the treatment group (n = 25) was offered fertility control bait, both ad libitum, during a 15-day introduction period and during the first of four breeding rounds, for a total of 58 days of exposure. After treatment, all rats were provided placebo bait for the remainder of the study and randomly paired with mates from within their treatment groups for two additional breeding cycles. Treatment and control groups comprised 10 breeding pairs each, with random re-pairings between breeding rounds. The treatment group produced no litters during the first and second breeding rounds, while 70% of the control females produced litters. In the third breeding round, 70 days after stopping treatment, the treatment group produced three litters (six pups) compared to seven litters (24 pups) in the control group. During a fourth and final breeding round, control rats were crossed with treated rats, producing six litters (27 pups) from treated dams and nine litters (40 pups) from control dams, indicating no apparent infertility effect 99 days after cessation of treatment. This study demonstrates that the reproduction rate of wild-caught black rats can be chemically suppressed if provided ad libitum access to the fertility control bait under laboratory conditions

Development and Evaluation of Prototype Toxicant-Delivery Bait Stations for the Control of the Small Indian Mongoose

We conducted research to develop a safe and effective toxic bait to control the small Indian mongoose (Urva auropunctata), an invasive vertebrate predator impacting the survival of native species in Hawai‘i (United States) and in other parts of the world. A preserved fish-based bait product was found to be highly palatable to mongooses in cage trials and subsequent formulations with diphacinone (0.005%) showed promise as an efficacious toxic bait for mongooses. This product is intended for future use to control mongooses in conservation and urban areas, and as a biosecurity tool at ports of entry to address accidental introductions into mongoose-free areas. Anticipated delivery of this toxic bait is in tamper-proof bait stations. We designed three prototype bait stations constructed with polyvinyl chloride (PVC) drainage pipes and evaluated their performance in enclosure trials with wild-caught mongooses and in field trials with free-ranging mongooses. A commercially available tamper-resistant rodenticide bait station was also used for comparison to the prototypes in our trials. The goal was to develop a bait station that is readily used by mongooses, allows for bait consumption in place, prevents removal of bait, and restricts access to non-target animals. We used a non-toxic formulation of the fish-based bait product and monitored bait station use, visitation rates, bait uptake, and spillage. All four bait station types were used by mongooses in the laboratory, and one PVC bait station design and the commercial bait station exhibited multiple mongoose visitations with minimal bait spillage in the field. We did not record any significant non-target species interactions with the bait during the field trials. The PVC bait station design and commercial bait station are approved methods of bait delivery in the subsequent field efficacy trials under an Experimental Use Permit for the upcoming registration of “Fish-based Bait for Mongooses” with the Environmental Protection Agency

Development of a Novel Vertebrate Pesticide for the Invasive Small Indian Mongoose

Small Indian mongooses are detrimental introduced predators in the United States, where they depredate native species, serve as vector of disease, and threaten public safety. Due to the risk of accidental introduction to mongoose-free islands, high cost and limitations to trapping, and no national (Section 3) Environmental Protection Agency (EPA)-registered toxicants for mongoose control, there is a need for an efficacious toxic bait for mongooses for use in conservation areas and at points of entry in the United States. Over the last five years, the National Wildlife Research Center (NWRC) worked to develop a toxic bait for mongooses for registration with the EPA. This paper outlines the development pathway to registration of a toxic bait for mongooses in the United States

Placebo Bait Uptake Trial to Test Feasibility of Polynesian Rat (Rattus exulans) Eradication on Wake Atoll

Rodent eradications have contributed to the recovery of many threatened species, but challenges often exist for campaigns that occur on tropical islands when compared to more temperate regions. A post-operational review of a rat eradication operation on Wake Atoll indicated that certain areas, such as those with high alternative food abundance, may have contributed to the failure to remove all Polynesian rats. We conducted a nontoxic bait uptake trial to evaluate whether the maximum prescribed bait application rate for Brodifacoum-25W rodenticide pellets was sufficient to expose all rats to a lethal dose at three sites on Wake Atoll, including around a solid waste aggregation area (SWAA), which was previously identified as “high risk.” We monitored bait persistence and condition throughout the treatment period as well as rat movement via radio tracking. Bait uptake by rats was also assessed by trapping and examination of rat orifices and gastrointestinal contents for pyranine biomarker incorporated into the bait pellets. The rate of bait disappearance differed by site, with bait disappearing the fastest in vicinity of the SWAA. Rat movement also varied by site, with rats observed traveling greater distances around the SWAA, sometimes exceeding 300 m. The SWAA was the only site at which we observed rats negative for biomarker exposure. We suggest that these negative observations resulted from lack of bait availability or movement of rats into the core trapping area from outside the treatment area. However, we cannot rule out preferential selection of alternative food sources over bait pellets and suggest that this possibility should receive further attention. Based on our results, we conclude that, of the three sites, the maximum bait application rate prescribed on the product label was not high enough to provide every rat an opportunity to encounter bait at and around the SWAA. Given the rapid disappearance of bait and the regular immigration of rats from distant habitat, we recommend that an even greater application rate be prescribed and that the heavier treatment be extended over a much larger area surrounding the SWAA

Placebo Bait Uptake Trial to Test Feasibility of Polynesian Rat (Rattus exulans) Eradication on Wake Atoll

Rodent eradications have contributed to the recovery of many threatened species, but challenges often exist for campaigns that occur on tropical islands when compared to more temperate regions. A post-operational review of a rat eradication operation on Wake Atoll indicated that certain areas, such as those with high alternative food abundance, may have contributed to the failure to remove all Polynesian rats. We conducted a nontoxic bait uptake trial to evaluate whether the maximum prescribed bait application rate for Brodifacoum-25W rodenticide pellets was sufficient to expose all rats to a lethal dose at three sites on Wake Atoll, including around a solid waste aggregation area (SWAA), which was previously identified as “high risk.” We monitored bait persistence and condition throughout the treatment period as well as rat movement via radio tracking. Bait uptake by rats was also assessed by trapping and examination of rat orifices and gastrointestinal contents for pyranine biomarker incorporated into the bait pellets. The rate of bait disappearance differed by site, with bait disappearing the fastest in vicinity of the SWAA. Rat movement also varied by site, with rats observed traveling greater distances around the SWAA, sometimes exceeding 300 m. The SWAA was the only site at which we observed rats negative for biomarker exposure. We suggest that these negative observations resulted from lack of bait availability or movement of rats into the core trapping area from outside the treatment area. However, we cannot rule out preferential selection of alternative food sources over bait pellets and suggest that this possibility should receive further attention. Based on our results, we conclude that, of the three sites, the maximum bait application rate prescribed on the product label was not high enough to provide every rat an opportunity to encounter bait at and around the SWAA. Given the rapid disappearance of bait and the regular immigration of rats from distant habitat, we recommend that an even greater application rate be prescribed and that the heavier treatment be extended over a much larger area surrounding the SWAA