Control and Containment Strategies on Guam: Trapping

Photograph taken from inside a snake trap showing entry of a brown tree snake. Trap is wire mesh. Of particular interest is the wire mesh screening of the entry port which the snake can push in while entering, but which cannot be pushed out for exit. USGS photo by T.H. Fritts.

Trap capture is currently the most effective way of capturing snakes in high-density areas on Guam. Years of research and trial and error in testing over 49 different trap designs and conducting more than 24,000 trap-nights of tests produced a trap that consistently attracts and captures brown Treesnakes. The resulting design is a modified funnel or minnow trap "a mesh cylinder about 50 cm long by 20-30 cm in diam with inward-pointing funnel ends" that is capable of capturing multiple snakes while requiring only periodic maintenance.

As a part of servicing the traps, each trap is supplied with an attractant, often a live mouse or gecko in a separate secure inner chamber. These lure animals have to be protected from rain and temperature extremes, which adds additional time and consideration to the trapping process. Also, raw potatoes providing moisture and mixed grain or a parafinized grain block serving as food for the mice must be replenished at regular intervals.

Photograph of person checking a trap suspended in favored snake habitat.' USGS photo by T.H. Fritts.

An additional obstacle to trapping is the lack of reliable and adequate supplies of mice on oceanic islands. This potential limiting factor is particularly important when considering that trapping efforts may need to be expanded quickly in response to snake sightings in new areas. To overcome this and other difficulties encountered in maintaining traps and live bait, research is underway to define the sensory responses of brown tree snakes and find or create alternate attractants. Incorporating an artificial bait into the trapping scheme would save employee time and considerable expenses, but this subject is a complex one that promises to warrant attention for some time to come.

On Guam, numerous traps must be employed in fairly close proximity to dent the snake population even in specific and focused areas. To effectively capture brown Treesnakes in a given area, traps must be set 15-25 m apart. At $7-$40 (U.S.) per trap and several thousand traps per square mile, the overall cost for this type of control for labor and materials adds up fast. But, like hand capture, at present this tactic is focused in and around air and ground transport areas and wildlife enclosures. Using trap capture in these areas is particularly appealing because, as opposed to visual searches and hand capture, where high motivation, experience, and focused attention are necessary on the part of the individuals at work, only reasonable diligence is necessary to service snake traps.

Trapping of brown Treesnakes is conducted continuously on Guam and Saipan, and infrequently on Tinian, Rota, and Oahu. The Wildlife Services program on Guam trapped over 17,600 snakes from 1994-1998, but so far no snakes have been captured in traps on other islands. The difference in capture success is undoubtedly related to snake abundance but probably also results from more widely available mammalian or avian prey on these islands, making baited traps less attractive to snakes.

Control and Containment Strategies on Guam: Trapping

Canine Detection Teams

Habitat Modification

Physical Barriers

Trapping

Visual Searches

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