What started out as some poor insect control now looks more and more like a resistance issue with the use of pyrethroid insecticides for control of corn earworm in soybeans in Virginia.

“A problem still exists with our adult pyrethroid vial test results, says Virginia Tech Entomologist Ames Herbert. Last week’s sample reached 58 percent survivors, the highest level ever recorded for Virginia, and this week’s early sample was still at 50 percent survival, he adds.

Herbert says his research team has had at least one report of lack of control in a soybean field treated with a pyrethroid. “If you are choosing a new product — a brand you are not familiar with, be sure to check the label to see what you are buying and spraying,” he says.

“There are some products that are a mix of two pyrethroids, which is no help if you are concerned about pyrethroid resistance,” Herbert adds.

Herbert stresses that 50 percent or better survival in vial tests does NOT mean half of the corn earworm population is resistant to pyrethroids. The vial tests should be considered an “indicator” of possible problems, he explains.

Testing for resistance in vials in a lab is dramatically different from actual soybean field conditions, Herbert says. “Growers who are using pyrethroids for corn earworm should not increase rates to try and offset a resistance issue — that only works if absolutely no survivors are left in a field, he adds.

The good news is that for the most part, corn earworm populations seem to be pretty light and spotty. Most reports are in the 1-3 worms per 15 sweep range, certainly not excessive pressure, and the moth flight seems to be in decline, the Virginia entomologist says.

“We sometimes have another late summer/early fall corn earworm flight which could result in some new worm infestations, but it if this does happen, most fields will be in the bug-safe late R6-R7 stage.”

With the price of soybeans holding in the $13-$16 per bushel range, growers are urged to take an extra look at their crop to insure maximum yield potential. Scouting is critical to determine when threshold levels are in a field and to insure proper insect identification for proper selection of the most efficacious insecticides.

rroberson@farmpress.com