April 7, 2000

As growers in the Lower Rio Grande Valley begin harvesting this year’s onion crop, many are reporting the treatments they’ve used successfully for years on onion thrips are not as effective this season.

In response, scientists at the Texas A&M Research and Extension Center in Weslaco are conducting large-scale tests of both old- and newly-developed insecticides to find alternatives.

"It’s likely that thrips are developing resistance to the old stand-by pyrethroids that onion growers have been using for years," said Dr. T-X Liu, a vegetable integrated pest management entomologist at Weslaco.

He said reports of failure began during last year’s crop and increased in numbers this year. Fields affected by onion thrips take on a pale, whitish color as leaves dry and lose their dark green color. A high thrips density in a field stunts plant growth and reduces onion bulb size.

"One problem we’re seeing," said Liu, "is that growers who apply their insecticides with tractor or airplane sprayers aren’t mixing enough water. Those droplets help get the insecticide down into the center of the plant where thrips thrive."

Dr. Marvin Miller, a plant pathologist at the A&M Center in Weslaco, said despite the thrips problem, growers are poised to harvest a healthy crop.

"Market commodity prices are lower than we’d like to see," said Miller, "but Valley onion growers have produced a beautiful crop of onions this year, including our famous 1015 onions."