Longevity of rodenticide bait pellets in a tropical environment following a rat eradication program

Invasive rodents (primarily Rattus spp.) are responsible for loss of biodiversity in island ecosystems worldwide. Large-scale rodenticide applications are typically used to eradicate rats and restore ecological communities. In tropical ecosystems, environmental conditions rapidly degrade baits and competition for baits by non-target animals can result in eradication failure. Our objective was to evaluate persistence of rodenticide baits during a rat eradication program on Palmyra Atoll; a remote tropical atoll with intense competition for resources by land crabs. Following aerial application, bait condition was monitored in four terrestrial environments and in the canopy foliage of coconut palms. Ten circular PVC hoops were fixed in place in each of Palmyra’s four primary terrestrial habitats and five rodenticide pellets were placed in each hoop. Five coconut palms were selected in three distinct regions of the atoll. One rodenticide pellet was placed on each of five palm fronds in each coconut palm. Fresh baits were placed in all monitoring locations after each broadcast bait application. Bait condition and survival was monitored for 7 days after the first bait application and 6 days after second application. Bait survival curves differed between applications at most monitoring sites, suggesting a decrease in overall rat activity as a result of rodenticide treatment. One terrestrial site showed near 100 % bait survival after both applications, likely due to low localized rat and crab densities. Median days to pellet disappearance were one and two days for the first and second application, respectively. Differences in survival curves were not detected in canopy sites between bait applications. Median days to pellet disappearance in canopy sites were 2 and 4 days for the first and second application, respectively. Frequent rainfall likely contributed to rapid degradation of bait pellets in coconut palm fronds.     

Taking the bait: species taking oral rabies vaccine baits intended for raccoons

Raccoon rabies in eastern USA is managed by strategically distributing oral rabies vaccine (ORV) baits. The attractiveness, palativity, density, and non-target species bait take affect ORV effectiveness. We examined raccoon and non-target species differences in investigating/removing fish-meal polymer and coated sachet baits applied to simulate two aerial bait distribution densities. Bait densities of 150 baits/km² and 75 baits/km² were evaluated, respectively, in zones expected to have high and low raccoon densities. Three primary non-target species visited baits: coyotes, white-tailed deer, and feral swine. The proportion of bait stations visited by raccoons during 1 week observation periods ranged from 50 to 70%, exceeding non-target species visitation. Raccoon take rates for visited baits averaged from 59 to 100%. Raccoon visitation was similar for both bait densities, indicating a proportionally greater quantity of baits were taken in the higher bait density zone. Coyote visitation rates ranged from 16 to 26%, with take rates for visited baits between 46 and 100%. Coyotes were expected to take baits intended for raccoons, because similar baits are applied to vaccinate coyotes. Deer regularly investigated but rarely took baits. Feral swine were in low abundance in the high bait density zone (higher human density) and visited ≤?1% of baits there but visited baits at frequencies similar to coyotes and deer in the low-density zone and were likely to take encountered baits (63–100%). Non-target bait consumption could be a concern in some circumstances for achieving sufficient raccoon sero-conversion rates.

     

Assessing spatial variation and overall density of aerially broadcast toxic bait during a rat eradication on Palmyra Atoll

Baits containing brodifacoum rodenticide were aerially applied to eradicate invasive black rats from Palmyra Atoll, an important biodiversity center. Bait application must be sufficient to be effective, while minimizing environmental hazards by not exceeding designated label rates, prompting our bait density assessments for two aerial drops. With few physical or human resources on this remote, uninhabited atoll, assessments were particularly challenging, requiring observations within 30 min of aerial application to avoid bait loss to rats, crabs, or elements. We estimated bait density using quadrat sampling within 13 terrestrial sampling areas. We also sampled 10 tidal flat areas to assess inadvertent bait scatter into marine aquatic environments. Of particular value for challenging sampling circumstances, our quadrats had to be lightweight and durable, which we addressed by using widely available PVC hoops (“Hula Hoops”), the size of which was ideal for sampling purposes. At 77.5 and 78.7 kg/ha, overall bait densities were very near to the target densities of 80 and 75 kg/ha, respectively. However, considerable variability in bait densities existed among sampled areas, 8.6–178.2 and 31.4–129.5 kg/ha for the respective drops, respectively. Environmental, human, and equipment factors likely accounted for this variability. Tidal flat sampling revealed variable bait scatter into aquatic environments, from 0–46.3 kg/ha across the two drops. No differences were found in average bait densities among 1-, 4-, and 7-m distances from high tide lines. Our methods might broadly assist bait density (and other) surveys under challenging circumstances.