Western flower thrips aren't supposed to be an economic threat to alfalfa, but they're causing big headaches for Kent McMullen, Eltopia, WA.

Millions of the tiny bugs are dining on his crop, stealing both yield and quality, he says. And he's not their only victim. He's certain that thrips are damaging alfalfa throughout the Columbia Basin, although he suspects many growers don't know they have the problem.

“I've seen thrips from the north end of the basin at Ephrata to the south end by Pasco,” says McMullen, who grows 320 acres of alfalfa. “So I don't think there's a grower in the basin who isn't impacted.”

Western flower thrips are so small (0.06" long as adults) that they're barely visible. They scrape plant surfaces with rasping mouthparts, then suck the exuding juices. They've long been recognized as troublesome in greenhouses and on garden crops, but their damage is thought to be mostly aesthetic in field crops like alfalfa.

“There is no evidence that feeding by thrips causes a reduction in either yield or quality of alfalfa hay,” according to University of California Integrated Pest Management Guidelines.

But McMullen has way too many of them, in part, he believes, because a series of mild winters have favored thrips survival. Thousands of adult thrips take to the air when he swaths, and he sees clouds of them above his fields on summer evenings.

“When the heat of the day is gone, about 5:30 or 6, they become airborne,” he reports. “They rise up and drift with the air currents.”

On alfalfa plants, thrips larvae and adults tend to concentrate at the growing point of stems, feeding on the most succulent growth. Symptoms of heavy thrips feeding include cupped, or puckered, leaves and stunted plants, according to McMullen.

“I'm sure it has an effect on protein, fiber, TDN and tonnage,” he says.

He first noticed the problem five years ago. Thrips were less numerous the following year, but 2003 is the third consecutive year of heavy thrips infestations on his farm. The bugs thrive in hot weather, so they're most damaging in his second and third cuttings.

With help from his crop advisor, Tom Morris of Simplot Soilbuilders, McMullen tested several insecticides last summer. But the $8,000 investment yielded little help. Thrips numbers were high after every chemical application, either due to lack of control or reinfestation.

“I think a lot of the problem is coverage — getting it where they're at,” says Morris. “And they can multiply and come back in very quickly.”

McMullen and Morris did their own insecticide testing because they couldn't find any information on thrips control in alfalfa. But they hadn't talked to Bob Hammon, Colorado State University area extension agent in Grand Junction, CO.

Hammon says thrips “absolutely” can cause significant damage to alfalfa.

“In hot, dry climates like we have here in western Colorado, they can cause problems at times,” he says.

He's also concerned about migration of thrips to other crops, especially onions. When infested alfalfa fields are cut, there's often a flush of thrips into onions. Although western flower thrips may not damage onions, only trained eyes with a microscope can distinguish them from onion thrips, which are a serious threat.

Hammon says insecticides can reduce western flower thrip populations in alfalfa, but won't control them. In 2001 trials, he got about 50% control with single applications of these products: Dimethoate, Furadan, Lannate, Metasystox-R, Sevin, Capture and Pounce.

About half the thrips population is pupating in the soil at any given time. So two chemical applications five days apart should be more effective than one. The best time may be shortly after the first irrigation following a cutting.

Since adult thrips are mobile, evaluate the effectiveness of an insecticide treatment by counting larvae, Hammon advises.

He says to consider spraying if forage quality is a top priority; hot, dry weather is expected; and you're collecting at least 100 thrips per sweep-net sweep.

“I've seen a thousand per sweep before, and it's scary,” says Hammon. “You start itching as you're running out of the field.”

McMullen figures he's had that many in his fields, too, but says it's hard to get an accurate count because they're so small. He's trying to make growers and chemical company reps aware of the situation, hoping a solution can be found. But he doesn't plan to use insecticides again soon, fearing quick reinfestation from nearby fields that aren't sprayed.

“All you need is a breeze from the right direction and you're reinfested,” he says.