Hawaii has three species of invasive rats, and none of them are native to the islands. They arrived in waves over centuries, brought by Polynesian voyagers and later by European ships. Today, rats live on every major island in the archipelago, where they threaten native wildlife, spread disease to humans, and remain one of the state’s most persistent pest problems.
Three Species, Three Arrivals
The first rats to reach Hawaii were Polynesian rats, the smallest of the three species. They traveled with early Polynesian settlers, likely stowed in the hulls of voyaging canoes, and now occupy every major Hawaiian island. Polynesian rats are significantly smaller than the other two species and tend to stay close to the ground in agricultural and forested areas.
Black rats (also called roof rats) and Norway rats arrived much later, hitching rides on European trading and whaling vessels in the late 1700s and 1800s. Black rats are agile climbers that thrive in trees and attics, which makes them a particular threat to tree-nesting birds. Norway rats are the largest of the three, preferring ground-level burrows and areas near human settlements. Together, these three species have colonized habitats from sea level to high-elevation forests.
The Mongoose Mistake
In 1883, the sugar industry brought small Indian mongooses to Hawaii Island in an attempt to control rats that were damaging sugarcane fields. The idea spread to Maui, Molokai, and Oahu. It didn’t work. Rats are most active at night, while mongooses hunt during the day, so the two species largely avoid each other. The mongooses’ minor effect on rat populations was far outweighed by their damage to native birds, insects, and other animals.
The Hawaii Invasive Species Council notes that this introduction was carried out by private sugar growers, not as part of any scientific evaluation. It’s often cited as a textbook case of biological control gone wrong, but it was never a proper biocontrol effort in the first place. Hawaii now has both a rat problem and a mongoose problem.
Damage to Native Wildlife
Rats are among the most destructive invasive animals in Hawaii’s ecosystems. They eat the eggs and chicks of native seabirds and forest birds, many of which evolved without any mammalian predators and have no defensive instincts against them. Black rats, with their climbing ability, are especially devastating to species that nest in trees or cliff ledges.
The damage extends beyond birds. Rats eat seeds and fruits of native plants, but unlike the native birds and bats that once dispersed those seeds intact, rats often destroy them during digestion. A University of Hawaii study found that non-native mammals like rats have effectively replaced native seed-dispersing animals across island ecosystems worldwide. Because rats have different mouth sizes and digestive systems than the birds they replaced, they can’t perform the same ecological role. Native plants like the Hawaiian lama tree depend on having their seeds carried to new locations intact, and when rats eat the fruit, the seeds are crushed rather than spread. Over time, this disrupts the ability of native plant species to regenerate.
Rat Lungworm Disease
Rats in Hawaii carry a parasitic worm that causes rat lungworm disease, a serious infection of the brain and spinal cord. The parasite lives in the lungs of infected rats. Its larvae pass through rat droppings and are picked up by slugs and snails, which act as intermediate hosts. Humans get infected by accidentally eating a slug or snail (often a tiny one hidden in unwashed produce), or by consuming raw food contaminated with slug slime.
The infection causes a form of meningitis, with symptoms including severe headaches, neck stiffness, tingling in the skin, and sometimes temporary paralysis. Hawaii confirms roughly six cases per year statewide, with most occurring in Hawaii County (the Big Island). The actual number is likely higher because the disease is difficult to diagnose. Visitors have been infected, though confirmed cases in recent years have been limited to Hawaii residents.
To reduce your risk, wash all produce thoroughly, especially leafy greens and herbs grown in home gardens. Inspect vegetables for tiny slugs or snails. Don’t eat raw slugs or snails on a dare (this has actually caused cases). Controlling slugs and snails around your garden also helps break the parasite’s life cycle.
Leptospirosis Risk
Rats also spread leptospirosis, a bacterial infection transmitted through their urine. In Hawaii, the bacteria contaminate freshwater streams, ponds, puddles, and wet soil. You can become infected when contaminated water enters your body through broken skin, or through the soft tissues of your eyes, nose, or mouth. Hawaii consistently reports some of the highest leptospirosis rates in the United States, second only to Puerto Rico.
The Hawaii Department of Health recommends avoiding swimming or wading in freshwater streams and ponds, particularly if you have open cuts or sores. Submerging your head increases the risk. Don’t drink untreated stream or pond water. If you work outdoors in wet conditions, especially farming, clearing brush, or handling animals, wearing gloves, boots, and long sleeves reduces exposure. Controlling rats around your home and keeping them away from water catchment systems are also key prevention steps.
Keeping Rats Out of Your Home
Hawaii’s warm climate and year-round growing season make the islands particularly hospitable to rats. The state Department of Health recommends exclusion as the single most effective step: sealing any opening larger than half an inch for rats, or a quarter inch for mice. Hardware cloth, sheet metal, and cement mortar all work well. Wood can be used if there are no exposed edges for gnawing.
Beyond sealing entry points, reducing what attracts rats makes a significant difference:
- Food sources: Store pet food in sealed containers and don’t leave excess food in pet dishes. Gather ripe fruit from your yard promptly. Clean up garbage and food scraps.
- Water: Eliminate standing water sources where possible.
- Shelter: Remove woodpiles, stacked boxes, and debris that provide cover. Trim tree limbs that overhang your roof, since black rats use branches as highways onto buildings.
Snap traps and rodenticide bait stations are common control methods. For larger infestations, Hawaii pest control professionals often use diphacinone, a rodenticide that has been studied extensively for safety around birds and marine life and is approved for use in public areas.
Conservation Efforts That Are Working
Hawaii has pioneered some creative approaches to protecting native species from rats. At Kaena Point on Oahu’s northwestern tip, wildlife managers built the first predator-proof fence in the United States, modeled on a design developed in New Zealand. The fence stands about 6.5 feet high with a curved hood at the top, fine mesh between posts, and a buried skirt underground. This combination prevents animals from jumping over, climbing up, squeezing through, or digging under.
Once the fence was in place, teams removed rats, mice, mongooses, and feral cats from the enclosed area using a combination of trapping and carefully targeted rodenticide. The results were dramatic: native seabirds like Laysan albatross and wedge-tailed shearwaters began nesting successfully in far greater numbers, and native plants started recovering. Similar predator-proof enclosures and island-wide rat eradication projects are being planned or carried out on smaller offshore islets around Kauai and Molokai, offering a template for protecting Hawaii’s most vulnerable species.