Scientists are collecting insects from onion fields throughout the Rio Grande Valley to determine the extent of resistance that pests are developing to synthetic pyrethroids, a class of widely used insecticides.

Dr. T-X Liu, a vegetable integrated pest management (IPM) entomologist at the Texas A&M Agricultural Research and Extension Center at Weslaco, said growers are reporting a decline in mortality rates of onion thrips treated with two commonly used synthetic pyrethroids, Warrior and Ammo. Synthetic pyrethroids disrupt an insect's central nervous system and cause muscular paralysis.

“Pesticide resistance is fairly common in the battle against insects,” said Liu. “A tiny percentage is naturally resistant to an insecticide, they reproduce to greater and greater numbers and over time they eventually become the majority.

“It's called natural selection and it's the reason we always want to have several weapons in our arsenal to use against pests.”

At this point, Liu recommends growers apply Lannate to control thrips. Lannate, the commercial name for methomyl, is a carbamate insecticide produced by Du Pont and highly effective against thrips, Liu said.

To add to the growers' arsenal, Liu and another IPM entomologist at the center, Dr. Stormy Sparks, are testing several products for their effectiveness. Onion thrips and western flower onion thrips are the area's main onion pests.

The tiny insects feed on onion leaves, reducing the size and value of onion bulbs. Liu said recent tests on thrips from the La Feria area treated with pyrethroids showed only a 20 percent mortality rate, compared to the 50 percent to 70 percent mortality rate the insecticides provided last year.

Western flower thrips collected from the Alamo area showed similar resistance to pyrethroids. “Based on our most recent tests, Lannate is still the best product available,” said Liu. “Neither thrips appear to be developing resistance to Lannate.”

Liu said all products are tested using the manufacturers' field, or recommended, rates. “We're running these tests day and night because it's important to find other effective products to use on thrips,” he said, “especially for the late-season onions that will be harvested through the end of the season in late April and May.”

Dr. Juan Anciso, a Texas Cooperative Extension agent for IPM in Edinburg, said resistance to thrips has been a mixed bag in Valley onion fields dating back several years.

“We've been seeing resistance in some fields and not in others since 1999,” said Anciso. “For now, we're relying on only one product, Lannate, to control both western flower thrips and the synthetic pyrethroid-resistant onion thrips.

“But having only one effective product is not good.”

The Valley's sweet onion harvest got off to an early, mid-March start this season in a relatively strong market. Despite the insect problems, Anciso said, the Valley's onion harvest is progressing rapidly and prices to growers remain relatively high, likely due to cold weather problems in competing onion producing areas.