Bats Could Help Walnut Growers Control Costs

Walnuts are one of the highest value crops in California -- bringing in almost $2 billion every year. But keeping the crop free of destructive insects can be costly.

Scientists are studying if walnut growers could reduce their pest control costs by employing hungry bats.

Sean McNamara is walking toward a white barn at his family’s walnut orchards outside of Winters.

"So here are three boxes," says McNamara.

Walnut Farmer Sean McNamara hangs pheromone traps to combat the codling moth, a pest that ruins walnuts. Andrew Nixon / Capital Public Radio

He looks up about 15 feet at bat boxes attached to the side of the barn.

"This tiny box four inches deep, by two feet tall, by one foot wide, can host a colony of up to 100 bats," says McNamara.

Bats eat insects, lots of them. One pregnant bat can consume two-thirds of her body weight in insects a night. McNamara says his bats provide free insect control for the orchards. The codling moth is particularly destructive -- its larvae can burrow inside the walnut.

Codling Moths larvae can burrow deep and destroy walnut. Photo / The Regents of the University of California

"When those walnuts go into grading at our processor handler, they will make a random sampling and downgrade you for large levels of codling moth," says McNamara. "You could get up to 10 percent codling moth damage, which would be a significant financial hit."

Scientists want to learn how much bats might improve a walnut grower’s bottom line. There is only one way to determine a bat’s consumption of codling moth.

"We are here for glorified poo in a bag," says Katherine Ingram, a wildlife ecologist with UC Davis. "We allow the bats to defecate in a bag so that we can test the guano sample for codling moth DNA."

She and her graduate students set out to capture a bat in McNamara’s orchard. They set up what looks like a badminton net, with fine mesh netting. It allows them to catch the bat without hurting it. It’s placed strategically in the orchard -- in what could be a bat flyway.

At dusk, they wait. At the bat house, bats start to chatter. Then all at once, they fly out. In no time, Ingram catches two bats. One gets really tangled in the net.

Ingram talks to it like a mother, even though it’s biting her.

"OK buddy, all right," she says. "Oh, you got a good hunk of my knuckle."

The bat’s little teeth bite down.

"This is a Mexican free-tailed bat, which is one of the common ones," says Ingram.

It takes awhile to get it out. But once free from the net, it’s placed in a breathable Ziploc bag.

"You are a warrior, you are a warrior little bat," Ingram says. "So this guy’s angry, that guy's nice."

But soon, Ingram gets what she came for.

"Angry bat gave us a poo," she says.

Ingram determines the bat’s sex, weight and species. Then she lets it go. The guano will be sent off to the lab to test for codling moth DNA. But she can’t capture every species. To really get an idea of how bats forage in walnut groves, she arms herself with a unique recorder.

"These recorders can pick up bat calls for up to ten meters away, so a ten-meter radius," she says.

In another walnut orchard, Ingram carries a 10-foot-tall pole with a recorder and microphone attached.

"So I’ve chosen to put my unit up on a baby walnut," she says.

Ingram says the recorder picks up 700 to 1,000 bat calls a night. The recordings and a software program call Sonibat help her determine what species are foraging.

 Katherine Ingram with UC Davis places a bat recorder on a walnut tree. Amy Quinton / Capital Public Radio

"Based on the recordings that I’ve already gotten I think there is at least 10 species of bats using these orchards," she says.

She puts the recorders in conventional and organic orchards of varying ages. Ultimately she hopes to determine the financial benefits.

"Bats eating coddling moth may have a substantial enough impact to prevent one or two sprayings of pesticide per year," she says.

Pesticides can cost some walnut farmers around $160 per acre, per application. Ingram hopes to finish her research next year.