Despite a general perception that there is an abundance of nonlethal control technologies, the fact remains that there are fewer registered products and active ingredients for repellents in the U.S. than there were 10 and 20 years ago. This review discusses the technical issues relating to the discovery, formulation, and delivery of chemical repellents, and suggests future avenues of research that would improve our ability to develop effective chemical repellents.

Repellents substances and devices cause pest species to avoid otherwise attractive or palatable materials. For birds, repellents can be visual, auditory, pyrotechnic, tactile, chemosensory, physiologic, or physical. Here, we consider chemical agents only. Few substances are registered with the U.S. Environmental Protection Agency (EPA), and thus legally available for use. This lack of available bird repellent technology reflects the small demonstrable economic impact of many agricultural bird damage problems. Accurate information about damage and market size is virtually nonexistent, and private companies are reluctant to invest resources in the unknown. To successfully commercialize new repellents, clearly lucrative markets must be identified. Efforts must be made to empirically quantify damage and to estimate whether control methods are economical relative to the protection that they confer. We intend the present manuscript as a first step in these directions.

Brown tree snake introduction to the snake-free island of Guam has severely impacted its native terrestrial vertebrates and economy. These nocturnal predators have extirpated forest birds, caused drastic reductions of lizards, and there is a major concern that snakes could be dispersed to other areas through Guam’s cargo transportation systems. Techniques to deter the spread of snakes include cargo inspection with detector dogs and live trapping using live mice as lures to reduce snake densities in cargo handling areas. Maintaining mice in traps is expensive, and an inanimate lure would be highly desirable. Oral baiting with dead neonatal mice (DNM) treated with acetaminophen is another technique being implemented for reducing snake populations. An objection to DNM is that they decompose and become putrid after 3-4 days. Consumption of 21 candidate bait matrices was compared to DNM in free-choice tests in individually caged snakes under laboratory conditions. Consumption of dead quail chicks and geckos was equivalent to DNM. Processed canned meat products, stew beef, and chicken were 30-50% as effective as DNM. Baits not consumed included dog blood, beef liver, and quail eggs. Under field conditions with live trapped snakes, consumption of 7 bait matrices was compared to DNM in 1-choice tests. DNM were preferred (55% consumption) over all other baits. Consumption of plastic lizards, chicken fat, and mealworms was 33%, 14%, and 13%, respectively. Odoriferous chemicals (n = 23), characteristic of decomposing animal flesh, were evaluated for their ability to attract snakes in the field. Chemicals were individually evaluated by placing them on tofu baits in PVC tubes (bait stations). Snake activity around the bait stations was monitored with infrared videography. Some of the 23 chemicals (i.e., L-methionine, 3-methyl-1-butanethiol) appeared to attract snakes to the tubes. However, no tofu baits treated with chemicals were consumed, whereas 100% of the snakes that visited tubes baited with DNM consumed the dead mice. These results, along with analysis of videos, indicate that chemicals may act as long distance lures but are insufficient for stimulating and initiating consumptive or appetitive search behaviors at close range.

Traditional protective measures to keep wildlife away from areas include exclusion by use of netting, hazing, and chemical repellents. The primary problem with most hazing systems is that wildlife quickly habituate to the devices if their use falls into a predictable pattern. Repellent substances cause wildlife species to avoid otherwise attractive or palatable resources by creating a disincentive to visit a specific area or consume a particular resource. Chemical repellents, both lethal and non-lethal, are typically used for agricultural and horticultural purposes, but in addition may provide a strategy to deter wildlife in other contexts. Aerosol delivery of chemical repellents might work to effectively target birds in the air prior to landing in a hazardous area (i.e., a toxic waste water impoundment). In theory, aerosol delivery of a known avian irritant could be used as an ancillary tool in bird hazing systems, to complement more traditional auditory and visual scare tactics.

The economically and ecologically catastrophic introduction of invasive brown treesnakes to the Pacific Island of Guam has long served as a cautionary tale about the dangers of invasive species and the seeming impossibility of their management on a landscape scale. USDA Wildlife Services and federal and private partners have engineered a system for the automated manufacture and aerial delivery of toxic baits for landscape-scale suppression of brown treesnakes in large and remote forest plots. The helicopter-borne dispensing module can launch four bait cartridges per second, and a single payload of 3,600 cartridges can treat 30 ha of forest at 120 baits/ha in 15 to 30 minutes depending on flightpath efficiency. In this paper we recap the research, development, testing, and implementation of the system, including the procedures for monitoring biological responses to bait applications during an experimental suppression within a 55-ha forest plot surrounded by a snake-proof barrier.