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Operational Control of the Brown Tree Snake on Guam

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Operational Control of the Brown Tree Snake on Guam OPERATIONAL CONTROL OF THE BROWN TREE SNAKE ON GUAM THOMAS C. HALL, USDA/APHIS/ADC, 2800 N. Lincoln Blvd., Oklahoma City, Oklahoma 73105. ABSTRACT: An operational control program for brown tree snakes (Boiga i"egularis) on Guam began in April 1993. The program focused on minimizing the dispersal of brown tree snakes to other Pacific islands and the U.S. mainland. During the first year of operation, more than 3,000 snakes were caught within a kilometer of high risk port facilities using traps, detector dogs, and spotlighting. Additionally, habitat modifications and prey-base removal were used to reduce the attractiveness of these facilities to brown tree snakes. Public awareness was also an important part of the program such as the education of cargo packers, shippers, and Customs inspectors who could further minimiu brown tree snake dispersal off-island. Initial control efforts in the program became more efficient with the recognition of brown tree snake characteristics, i.e., it was discovered that perimeter trapping a 5 ha patch of jungle was sufficient to remove most snakes instead of saturating the area with traps. KEY WORDS: brown tree snake, snake control Proc. 17th Vertebr. Pest Conf. (R.M. Timm & A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1996. INTRODUCTION methods for resolving the problem. Background Brown tree snakes have caused significant information is given here so that the complexity of this environmental and economic impacts since their problem can be understood. inadvertent introduction to the island of Guam in the late 1940s. Other islands in the Pacific and the U.S. mainland Identification have been concerned about their potential introduction The brown tree snake, kulepbla to the native people with their propensity to do damage. Hawaii was of Guam, is a member of the colubrid family. It is particularly concerned and assisted in obtaining funds for characterized by a light to dark brown coloration typically an operational brown tree snake program at commercial marked with indistinct narrow dorsal bands, a light yellow port facilities on Guam to reduce the risk of them being belly which becomes increasingly gray with age, an transported on air and surface carriers or in their cargo to extremely narrow, long body, and a distinctly wide head Hawaii. with large eyes. Most brown tree snakes are about 1 m In April 1993, the U.S. Department of Agriculture, long with some reaching lengths to over 3 m. These Animal and Plant Health Inspection Service, Animal snakes are primarily nocturnal, seeking refuge from Damage Control Program (ADC) started an operational bright light and high temperatures during the day. Unlike control program involving containment activities at most colubrids, the brown tree snake is mildly_venomous; commercial air and seaports to minimize the dispersal of toxin is contained in the Duvernoy's glands behind the brown tree snakes. The ADC program was expanded to eyes. It envenomates its victims with two upper rear include military bases on Guam in August 1993 with teeth on each side; in contrast to the hypodermic fangs of funds provided by the Department of Defense. Control vipers, the venom is channeled into the victim through has been primarily focused at the highest risk areas on grooves in the teeth as they chew. Also, unlike the Guam including Anderson Air Force Base (AAFB), Apra vipers, it uses its flexible body to constrict its prey while Harbors (AH) Naval Station and commercial port, and it injects the toxin into it; the venom helps immobilize Naval Air Station (recently changed to Tiyan Reuse prey and facilitate ingestion with digestive enzymes. The Authority after base closure), and Won Pat International brown tree snake is arboreal and has a prehensile tail Airport (NAS) where cargo and craft depart. High risk which allows it to climb remarkably well. cargo packing sites were also incorporated in the program as time allowed including sites where military personnel's Range household goods were being packed for shipment. Thus Brown tree snakes range from western Indonesia far, containment activities have appeared to be successful through Papua New Guinea to the Solomon Islands, and in minimizing brown tree snake dispersal and have the northern and eastern coasts of Australia. The provided additional insight into resolving the problem. introduced brown tree snake on Guam has characteristics that match those from Manus, an island in the BACKGROUND admiralties. It was, therefore, assumed that they arrived Since it was discovered that brown tree snakes were on Guam when military bases on Manus were closed at responsible for the decline of native bird populations the end of World War II and materials associated with (Savidge 1987), extensive research was conducted, and is these were shipped to Guam (Rodda et al. 1992). They ongoing, on the brown tree snake in hopes of eliminating spread relatively fast after their appearance in the late them and their continued threat; research was conducted 1940s and by the early 1980s, they were found island­ on Guam, in its native range, and elsewhere to provide wide (Fritts 1987, 1988). They have since been found on information on the natural history of the species, several other Pacific islands including Oahu, Saipan, determine the extent of the problem, and develop potential Tinian, Rota, Pohnpei, and Kwajilein and Diego Garcia 234 (Fritts 1987, 1988) and the U.S. mainland in Texas; these food, and eggs. The primary diet of brown tree snakes have all been associated with cargo or carriers from less than 60 cm usually consists of ectothermic prey such Guam. Saipan is of particular concern because it has had as lizards-geckoes, skinks, and anoles; warm-blooded several reported sightings over the past ten years and a prey such as rodents and birds are included thereafter; live snake was found along a fence line at their airport in and at lengths greater than 110 cm, their diets are shifted 1994 indicating that they may currently have a breeding almost exclusively to endothermic prey (Fritts 1988). population; no snakes, though, have been trapped in extensive efforts by biologists in the Commonwealth of General Biology. Reproduction and Behavior the Northern Marianas where sightings have occurred. Brown tree snakes have been uninhibited on Guam and have reached densities of up to 58 snakes/ha (about Geography 15,000 snakes per square mile) in unfragmented jungle Guam is the southernmost island in the Marianas areas during the early 1990s (Rodda et al. 1992). These archipelago. It is approximately 212 square miles with densities are much greater than any other snake in the mountains reaching 300 m. The temperature rarely goes world. Recent density data in 1995 found a significant below 75°F or above 95°F and it has an average annual drop in the population to 11-20 snakes/ha (about 3,000- rainfall of nearly 100 inches. The native people of Guam, 5,000/mi21 (G. Rodda, pers. comm.). In urban habitats Chammorros, have inhabited the island for more than a with fragmented stands of tangentangen or other forest thousand years. The current population including military plants near port facilities, ADC personnel trapped personnel is about 140,000 people with more than a approximately 7 snakes/ha (1800/mi2) during 1993-1994. million annual visitors. Guam has several surface Brown tree snakes reach sexual maturity when they shipping ports, airports, and marinas. Apra harbor, are about l m in length. Hatchlings are about 35 cm, located on the west, central side, has a commercial females rarely exceed 2 m, and males reach the greatest shipping facility and military harbor. The commercial length at over 3 m. Males are distinguished from females airport and recently closed Naval Air Station is in central only by their hemipenes located just below the vent on Guam. Anderson Air Force Base is located on the both sides and those of large size. northeast side of Guam and currently services most Brown tree snakes are oviparous (egg-laying). Little military aircraft. These facilities all transport cargo via is known about their eggs and hatcbling development even surface or air to many of the other Pacific Islands as well with the densities found on Guam. Few clutches of eggs as the U.S. mainland and the Orient. Cargo is packed have ever been found and documented. ADC personnel island-wide prior to staging at the port facilities, ma.king have recently found and hatched two clutches of eggs, and control of the brown tree snake a daunting task. Guam are gathering data about the eggs and hatchlings (M. also has three marinas where boats from them travel Linnell, pers. comm.). Gravid females and clutches that regularly to other islands, especially Rota, Tinian, and are found are typically 5 to 8 and do not exceed 12. It is Saipan in the Northern Marianas. believed that females can store sperm for several years after copulation, giving them the most potential for Habitat colonizing other islands. Most of northern Guam consisted of limestone forests; Brown tree snakes are aggressive and display threats currently, the only large tracts remaining are found near if cornered. They often strike continuously at intruders the rugged cliff line on military lands. The remainder of when cornered or grabbed, more often than most other the north bas been developed. The southern half of Guam snakes. However, most threats are bannless and typically consists primarily of savannabs, wetlands, and mountains only serve to warn the intruder; they usually quit when with scattered stands of limestone forests and urban areas. the intruder retreats or lets go. Much of the native forests in the central part of the island The population of brown tree snakes on Guam has have been replaced by introduced tangentangen scrub mostly been uninhibited.
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