Indicator blocks, also called “chew-tag-cards,” made from a small square of corrugated plastic partially filled with an attractant, are an established rodent detection tool used in many different settings. Here, we discuss the utility of indicator blocks in detecting the presence of rats at Palmyra Atoll National Wildlife Refuge (Palmyra) in the Northern Line Islands. The detection of invasive rodents, in this case black rats, can be challenging in the presence of non-target species that may interfere with detection devices. Palmyra supports a robust community of land crabs. Five of the 7 species of land crabs found at Palmyra routinely interfere with commonly-used rodent detection devices: snap traps, live-capture traps, tracking tunnels, motion-sensing cameras, and gnaw sticks. Interference by crabs renders some of the detection methods useless (gnaw sticks) and reduces the sensitivity of others through false triggering (traps and cameras). Coconut crabs, which can exceed 7 kg and are found throughout Palmyra Atoll, can easily destroy tracking tunnels, traps, and even ruggedized motion-sensing cameras. Prior to the successful eradication of rats from the atoll in 2011, we compared the rate of detecting rats using indicator blocks with that of live-capture traps and tracking tunnels in paired, independent samples (90 chew block versus trap samples, 20 chew block versus tracking tunnel samples). Sampling occurred in September and October 2010 and measures were collected consecutively for 19 days. The frequency at which the indicator blocks detected rats was significantly higher (P < 0.001) than that for traps or tracking tunnels, even when interference by land-crabs was minimized by placing the devices on overturned 5-gallon buckets. The results from this study suggest that indicator blocks are an effective and efficient tool for detecting rodents in the presence of non-target species that interfere with rodent detection devices.

Rodent eradication campaigns on tropical islands face challenges not experienced in temperate climates, such as competition for bait by land crabs and extensive use of the forest canopy by rats. In 2011, black rats were successfully eradicated from Palmyra Atoll with a campaign built on lessons learned from a prior, unsuccessful ground-based eradiation attempt. The 2011 eradication team, at considerable cost and effort, ensured that bait (brodifacoum, 0.0025%) was placed in every potential rat territory, including the atoll’s coconut palm canopy. Coconut palm is the dominant canopy tree at Palmyra, covering 45% of the 235 ha land area. Bait applied to palm crowns is not available to land crabs and persists longer than bait placed on the ground. Rats were frequently observed traveling to and from the crowns of palms overhanging the water, yet this important habitat could not be treated by aerial broadcast (the primary bait application method) as this would place bait into the marine environment. To deliver bait to overhanging palms lining Palmyra’s shoreline, we developed a “bola” canopy bait (BCB) ‒ two cotton-gauze sacks connected by 20 cm of twine, each containing 12.5g of bait. A manufacturing system was created to produce the quantities of BCBs needed to treat Palmyra’s overhanging palm habitat. BCBs were slung by hand or with a sling-shot, or dropped into palm crowns by a team member suspended beneath a helicopter. Handheld GPS units were used to record the location of each palm crown treated with a BCB so that the bait application could be tracked. We believe that the successful outcome of the 2011 eradication was due, in part, to the comprehensive treatment of Palmyra’s palm canopy.