Tiny Bugs Can Take Big Bite Out of Vidalia Onions

By for CAES News

A thrips JUST ONE THRIPS doesn't look like much, and just one isn't; a single insect isn't much larger than the period at the end of this sentence. But thousands of thrips can invade an onion field and prevent the bulbs from reaching large or jumbo size.

Georgia onion growers don't usually think of insects as a major problem. But thrips can take a bite out of the sweet onion crop, said a University of Georgia scientist.

Even a few causeÿ some damage

"Even a relatively small population can have a significant yield impact," said UGA entomologist David Riley. "Thrips damage the leaves. Without healthy leaves, the bulbs can't develop to their potential."

Riley said he's seen thrips populations increase during the past three years of his research at the Tifton campus of the UGA College of Agricultural and Environmental Sciences.

"Thrips aren't going to wipe out the Georgia onion crop," he said. "But they definitely can reduce yields."

Treatment needed and effective

Riley said this year one-third of Georgia's 17,000 acres of onions may need thrips treatments they're not getting.

"With effective control of the higher thrips numbers this year," he said, "farmers could add about $4 million every year to the value of the crop."

In 1998, Riley's research revealed that fields with effective thrips control produced more and larger Vidalia-type onions. For about $100 per acre in treatment cost, farmers can pocket another $900 in onion value from fields where thrips are at damaging levels. Without treatment, the farmers harvested fewer jumbo and large onion bulbs.

J. Cannon, UGA CAES
Vidalia onions
IMMATURE ONIONS like these are most prone to thrips damage during March, April and May. The thrips damage the leaves, weakening the plant and preventing full bulb growth.

The damage they cause

David Adams, a UGA Extension Service entomologist, said the thrips weaken the plants. "They use their rasping mouthparts to damage the plant cells and lap up the juices," he said.

When it hurts the farmer, it hurts the consumer, too, Riley said. "If the supply is lower," he said, "consumers will have to pay more for these onions."

Research efforts paying off

But with Riley's research data, farmers can be sure their thrips control dollars will pay off in the long run. He's working to learn what thrips species damage onions and when it's most effective to apply insecticides.

Adams said that the insects cause varying levels of damage at different times in the season.

"If a lot of thrips are in the field early in the season, they can damage young plants as the bulbs are forming," he said. "Effective control is vital then."

Later in the season, he said, a few thrips may not cause as much damage. "However, in Georgia, thrips populations are highest during March, April and May," he said. "So control efforts should be aimed at thrips during the spring."

Spotting the problem

The damage thrips cause is hard to see. The insects are no bigger than the period at the end of this sentence. It's hard to spot them.

Bo Herndon, an onion farmer in south central Georgia, said he has to look hard to see thrips in his fields.

"I've really got to get down and look close," he said. "And even then, you'll probably see the damage first. They're easier to see if there are a lot of them. And at that point, it's time to treat -- fast!"