Soybean growers beware: Spraying spider mites and soybean aphids with pyrethroids can do more damage by causing an increase in mite populations, says a Purdue University expert.
The two-spotted spider mite always is present in soybean fields. Most years the mites are not a pest because they need extended hot, dry weather to increase to damaging population levels.
This year's conditions have been conducive for mite populations to increase, says Christian Krupke, Purdue Extension field crop entomologist.
"A lot of fields have been reporting spider mite populations, and those populations need to be treated aggressively because they can get out of hand much quicker than aphids can and cause a lot of damage" Krupke says. "One of the things to remember when treating for both pests is not to use pyrethroids in areas where you have mites because some pyrethroids can actually make the problem worse.
"Pyrethroids are generally not the best miticides. Lorsban, which is not a pyrethroid, is registered for aphids and mites and does a much better job at targeting the mites while also killing the aphids."
When scouting a field for mites, producers should first look at the crop from a distance, Krupke says. If there is a browning or bronzing of the leaves along the roadside or in other stressed areas of the field, such as in low spots, there is no cause for alarm. However, if this bronzing effect moves further into the field a few rows, then it's time to worry, he says.
Mites typically start populating where plants are stressed or weaker, especially next to roadsides, in low spots or nutrient deficient areas in the field.
In order to rule out physiological conditions like disease or other issues, the best thing to do is pull a soybean plant from the ground and look at the underside of the leaves for a very fine webbing, Krupke says. The mites are so tiny, they are not easily visible on the plant.
"The way to confirm mites is to take a piece of white paper and shake the leaves over the paper," Krupke says. "If there are tiny little moving dots all over the paper, those are mites. That's how you can confirm that mites are contributing to this yellowing or bronzing in the field and can then arrange to treat it if you desire."
Mites aren't the only thing soybean growers need to be aware of, Krupke says.
"So far, aphid numbers have been relatively low, but that is starting to change," he says. "There is an increase in winged aphids, which means they will continue moving, and reproduction will not slow with high temperatures.
"Contrary to what some may think, reproduction of the soybean aphid will not stop at temperatures above 90 degrees. We are seeing the highest numbers in the northwestern and northeastern part of the state (Indiana). This is the last big aphid surge, and we anticipate rising population and more and more fields to be treated over the next two weeks."