Agriculture

Roof rats are invading California farms and destroying fruit, nuts and irrigation lines

Roof rats are being found on California farms.
Roof rats are being found on California farms. UC Agriculture and Natural Resources

Roof rats, a common problem for city dwellers, are migrating to California farms and nibbling on everything from avocados to irrigation tubing.

University of California scientists say last year's wet weather created a perfect environment for the quick-breeding rats to flourish.

Ample rain helped produce an abundance of weeds and weed seeds, a good source of food for rats. And it didn't take long for the rats to discover an even bigger bounty in the Valley's orchards and fields.

"Rodents are everywhere and they are opportunists," said Rachel Long, a University of California, cooperative extension adviser. "They move in from their surrounding urban habitats to take advantage of any food source they can find. And once that food source disappears they search for food elsewhere."

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Roof rat damage on an avocado. University of California


In Long's 25-year career, she'd never come across roof rats in an orchard until she got a recent call from a grower in the Sacramento Valley who was puzzled by what he thought were squirrels digging holes in his pistachio orchard.

After consulting with Niamh Quinn, UC Cooperative Extension human-wildlife interactions adviser based in Irvine, Long figured out it wasn't squirrels that had moved in. It was roof rats.

Long learned that roof rats adapt depending on their environment. In urban areas, they prefer to live above ground and make their nests in trees and in people's attics. But in rural areas, they burrow under ground.

Roger Baldwin, a UC Cooperative Extension wildlife specialist who has studied the migration of roof rats onto farms, says he has seen an increase in roof rat activity this year.

"It seems to be a good year for rats in a number of different areas and crops throughout the state. I've received more questions and comments about rats this year than perhaps the last 10 years combined," Baldwin said.

Although it's unknown how much damage the rats have caused this year, UC farm advisers do know that the rats have been found munching on pomegranates, citrus, avocados, pistachios and irrigation tubing.

Long has issued a warning to farmers to be on the look out for the destructive pests. Unlike squirrels, roof rats are not active during the day and their holes are slightly smaller, typically about 3 to 4 inches in diameter. They may also have nut shells in front of them.

Roof rats are prolific breeders, so if you think they've moved into your field, its best to get a handle on the problem quickly.

"A few rats can turn into a hundred very quickly," she said.

For more information on controlling roof rats, you can download a guide by Quinn and Baldwin's, Managing Roof Rats and Deer Mice in Nut and Fruit Orchards at http://anrcatalog.ucanr.edu/Details.aspx?itemNo=8513.

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