Despite gains in limiting births, race to eradicate rats in Hawaii far from over

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University of Hawaii at Hilo researchers have shown they can decrease the birth rate of rats.


University of Hawaii at Hilo researchers have shown they can decrease the birth rate of rats.

The goal was to decrease rat births humanely while avoiding poisons that can be harmful to humans and pets, and also avoiding traps that can make consumers cringe at the thought of finding a dead or injured rodent.

But their study, which observed rats fed infertility compounds made by Senestech, is just a single step in what likely will be a complex and lengthy effort to get rid of the bothersome invasive creatures.

“There’s a huge number of questions before you get to answer a bigger question like that,” said Shane Siers, supervisory research wildlife biologist with the U.S. Department of Agriculture’s Hawaii Field Station.

The UH-Hilo research team captured wild rats from the Big Island for the study, got them used to captive living and fed them a milky liquid placebo.

“All of the rats were acclimated to feeding on that (placebo),” Siers said.

Then, compounds — nontoxic to humans — were added to block sperm production in male rats and egg production in female rats. The infertility compounds were only put into treatment rats’ food and skipped in the control group’s food.

Males and females, from the control and treatment group rats, were kept separate until time to feed treatment rats the infertility concoction. At 58 days, the food was switched back, for all rats, to placebo.

After male and female rats were given access to each other, 70 percent of control females gave birth; rats treated with the infertility compounds produced no “pups.”

In a second study period, 70 percent of untreated control rats again produced pups; no treated rats had pups.

During a third period, control rats still had a 70 percent birth rate. Treated rats, at 128 days after first exposure to infertility compounds, had birth rates return to about 25 percent — 70 days after the infertility compound was stopped.

“It was already starting to wear off,” Siers said. “Reproduction began to occur again, but at a slower rate.”

He said for the compounds to be effective for the average consumer, bait stations will need to be refilled repeatedly throughout long time periods, along with other rat control methods, in a process called “integrated pest management,” or IPM.

IPM is a plan that integrates various methods, each of which is partly effective on its own. Used together, the methods yield better results.

“A fundamental principle of IPM is to select those methods, or combination of methods, that are feasible, efficacious and yet most protective of non-target resources, including wildlife, the public and the environment,” a state Department of Land and Natural Resources report says.

Why do Big Island rats need to be controlled?

Because they carry diseases that affect humans, such as rat lungworm disease, which is caused by a parasitic worm that can cause neurological harm. Rats also eat food crops including bananas, lychee, coconuts, coffee, sugar, pineapple and vegetables, according to the DLNR. They also eat or damage 5-10 percent of the Big Island’s macadamia nut crop.

“Rats are consummate reproducers,” Siers said. “They’re very efficient at turning food mass into rat biomass.”

The loss of palm tree forests also is blamed on rats, which eat palm seeds and flowers.

Two species, the fan palm loulu and kanaloa palm, are now nearly extinct, with a single wild kanaloa tree still living “on a rodent-free sea stack off Kahoolawe,” according to the DLNR.

What’s next for rat research?

Some scientists, Siers said, want to add anti-parasitics to rat bait to break the life cycle of diseases such as rat lungworm. The hope is to also make infertility bait more attractive to rats than human food.

Human food, Siers said, can be more tasty and energy-rich for rats than bait. That creates a problem when rats have a choice between bait or pizza, Siers said.


If there’s a choice, he said, “the rats are going to eat the pizza.”

Email Jeff Hansel at

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