U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3109

October 2005

Brown Treesnake (Boiga irregularis) 

Fact Sheet for Pacific Island 

Residents and Travelers 

Printed on recycled paper

An Invasive Pest Species 

The brown treesnake is an introduced species on Guam that 

has become a serious pest. The snakes probably arrived on 

Guam hidden in ship cargo from the New Guinea area, about 

1100 miles to the south. The first sightings were inland from the 

seaport in the early 1950s. Snakes became conspicuous throughout central Guam by the 1960s, and by 1968, they had probably 

dispersed throughout the island.

In the absence of natural population controls and with vulnerable prey on Guam, the snakes have now become an exceptionally common pest, causing major ecological and economic 

damage on the island.

Extinction of Birds

The brown treesnake has virtually wiped out the native forest 

birds of Guam. Ten species of birds, some found nowhere else, 

have disappeared from the island, and several others persist in 

precariously low numbers close to extinction. Some introduced, 

non-native bird species better adapted to resist snake predation 

are still currently found on Guam, but these introduced species have not filled the ecological niches made vacant when the 

native bird species disappeared. 

Other 

Damages

Snakes crawling on electrical 

lines frequently 

cause power 

outages and 

damage electrical lines maintained by the 

Guam Power 

Authority and 

Naval Public 

Works. Since 

1978, more than 

1200 power outages have been 

caused by snakes, and the resulting damage to electrical equipment is a significant economic burden to nearly all civilian and 

military activities on Guam. The power interruptions cause a 

multitude of problems ranging from food spoilage to computer 

failures. These problems have caused enormous productivity 

and economic losses. 

The snakes feed on a wide variety of animals including lizards, birds, and small mammals as well as bird and reptile eggs. 

Snakes frequently invade poultry houses, homes, and yards to 

consume domestic poultry, eggs, pet birds, and small mammals 

associated with residential areas.

Abundance

Scientists 

estimate that up to 

13,000 snakes per 

square mile occur 

in some forested areas of Guam. Many snakes are killed by 

automobile traffic and island residents, others are electrocuted 

while climbing on electrical lines. Despite this mortality, snake 

abundance remains high in nearly all forested and urban habitats 

throughout Guam. Pigs and monitor lizards eat snakes, but these 

predators are ineffective in lowering the snake population in 

most areas of the island.

Danger to Humans

The brown treesnake is aggressive when threatened. It will 

often raise the head and neck region in a striking position, flatten the head and neck to appear larger, and attempt to bite as it 

lunges forward. Adults can reach lengths of 8 feet or more and 

weigh up to 5 pounds.

The brown treesnake is a mildly venomous species that 

will also constrict resisting prey. The venom flows down teeth 

located in the back of the snake’s mouth. Brown treesnakes are 

not known to be fatal to humans, but special precautions should 

be taken to keep snakes away from infants and small children. 

Bites should be carefully cleaned. Anyone bitten should be 

monitored closely, and if any unusual symptoms appear, the 

victim should seek medical attention.

The snake is relatively strong and holds its prey by wrapping its body around the animal. Snakes can be easily removed 

by unwrapping them starting at the head or tail. The brown 

treesnake poses no danger of strangulation or injury from constriction to adult humans, but with infants and small children, 

the possibility of suffocation or envenomation exists.

Danger to Other Islands

Snakes are frequently accidental stowaways in cargo and 

flights leaving Guam, and unless discovered they may establish 

populations on other islands. Economic and ecological problems 

like those currently present on Guam would be likely to develop 

if the brown treesnake were to reach other Pacific Islands.

In addition to Guam, brown treesnakes have been sighted 

on Saipan, Tinian, Rota, Kwajalein, Wake, Oahu, Pohnpei, 

Okinawa, Diego Garcia and the mainland U.S. (Texas and 

Oklahoma). To date, this snake is not known to be established 

on any of the islands except Guam, but frequent reports of snake 

sightings on Saipan indicate that the brown treesnake possibly 

has become established there. The Governments of Hawaii, 

Guam, Palau, Commonwealth of the Northern Mariana Islands, 

the Federated States of Micronesia, and the Republic of the 

Marshall Islands have taken steps to help prevent the spread of 

the snakes to other islands.

This snake died by electrocution as it crawled 

from the power pole across the insulator to the 

nest of a bird (drongo) situated on an active 

power line. A snake crossing an insulator can 

cause a power outage.
What Can Be Done? 

Travelers, cargo handlers, 

and Pacific Island residents alike 

share in the responsibility to 

protect island environments from 

this pest species. Careful inspection of materials, cargo, and baggage shipped from or throughout 

Guam is necessary to prevent the dispersal of snakes to other 

islands.

With increased awareness and careful inspection of cargo 

arriving from Guam, it may be possible to prevent the spread 

of the brown treesnake to other islands. The cooperation of 

U.S. civilian and military federal agencies, other Pacific Island 

governments, and a wide range of Guam’s government agencies 

is needed to address this important problem.

The USGS Brown Treesnake Rapid Response Team is 

headquartered on Guam and can respond to brown treesnake 

sightings throughout the Pacific region. The response team has 

approximately 35 trained members in the Marianas and Hawaiian Island chains. Team members at the USGS lab on Guam 

are prepared to respond to snake sightings within 24 hours of 

notification, and can be deployed for extended time periods.

Origins of the Snake

The brown treesnake is native to eastern Indonesia, the 

Solomon Islands, and New Guinea, as well as the northern and 

eastern coasts of Australia. The brown treesnake does not occur 

in the Philippines or farther west than Sulawesi, Indonesia. 

Thus, it is incorrect to refer to the snakes on Guam as Philippine 

rat snakes.

Habits and Hiding Spots 

The snake is an able climber 

and is most at home in trees 

and shrubs, where it forages at 

night to find lizards, birds, and 

mammals using its keen sense of 

smell and sensitive night vision. 

It frequently invades homes, 

commercial buildings, and other 

urban habitats in search of food 

and hiding spots. Any site that 

provides protection from bright 

light and high temperatures is a potential daytime retreat. 

Snakes can coil their flexible bodies in extremely confined 

spaces when hiding.

Reproduction

The brown treesnake reproduces by depositing up to 12 

leathery-shelled eggs, perhaps as often as twice a year. The eggs 

are abandoned by the female in caves, hollow trees, or in other 

places where they hatch if adequately protected from drying and 

overheating.

The young snakes are about 18 inches in length when they 

hatch about 90 days later, but they may reach lengths of 3 feet 

in about one year. Snakes are about 4 to 5 feet long and about 3 

years old when they first reproduce.

Other Snakes on Pacific Islands 

Not all snakes are bad; Palau has four native snakes that do 

not cause major problems. In Palau, native snakes should not be 

indiscriminately killed or confused with the brown treesnake. 

Prior to the arrival of the brown treesnake, Guam had only one 

other snake, a tiny blind snake, Ramphotyphlops braminus, shown at right, 

which is harmless to man, lives in the 

soil, and feeds on the eggs and young 

of termites and ants. The blind snake 

is dark grey or black in color, and is 

the only snake occurring on many islands in the central 

Pacific region.

Sightings should also be reported to local government 

officials, such as the state department of agriculture or state fish 

and game agency. Snake sightings on Hawaii should be reported 

to the U.S. Fish and Wildlife Service, Brown Treesnake Coordinator, P.O. Box 50167, Honolulu, HI 96850, Tel. 808-792-9414.

When possible, the snake should be captured, killed 

and preserved for positive identification. Specimens can be 

preserved in 70% ethanol or 50% isopropyl alcohol, available 

in any drugstore. If a dead snake is found, it should be placed 

in a plastic bag and frozen until it can be transferred to a local 

biologist or government agency. Please include a label documenting the date, where the snake was found, who discovered 

it, your contact information, and circumstances surrounding its 

discovery.

Send preserved specimens, photographs, or information to:

Gordon Rodda

USGS Fort Collins Science Center

2150 Centre Avenue, Building C

Fort Collins, CO 80526-8118

Tel. 970-226-9471 gordon_rodda@usgs.gov

 For more information on brown treesnakes, visit

http://fort.usgs.gov/resources/education/bts/bts_home.asp

Report Snake Sightings!

PACIFIC ISLANDS

BTS Hotline: (24/7): 671-777-HISS (4477)

Landline: 671-355-4015 or 671-637-7834

James Stanford, Coordinator

USGS Brown Treesnake Rapid Response Team

P.O. Box 8255, MOU-3, Dededo, Guam 96912

James_stanford @ usgs.gov Fax 671-355-4016

Any sightings on islands thought to lack snakes should 

be reported immediately to the BTS Hotline.

NOTE: The Hotline is a Guam cell phone number. Cell 

phone service can be sporadic in the islands. If you reach the 

Hotline’s voice mail recording, please leave a message, then 

use one of the two landline numbers.

MAINLAND U.S.

Landline: 970-226-9242 (Colorado)

Teri Kman

USGS Fort Collins Science Center

2150 Centre Av. Bldg. C, Fort Collins, CO 80526

Teri_Kman@usgs.gov Fax 970-226-9230

Because of the great time zone difference in Guam, please 

leave a message with the Hotline number if necessary, 

then call the Mainland (970) number. This will help ensure 

a timely response. We also are available to answer questions 

or assist you with concerns regarding snake sightings during 

normal business hours.