Growers can use the following criteria to determine whether to spray for twospotted spider mites:

  • If mites are barely detected on the underside of leaves in dry locations or on the edges of a field and injury is barely detected, this is a non-economic population and growers should do nothing.
  • If mites are easily detected on the underside of leaves along the edges of fields or perhaps on leaves in dry areas throughout a field and most foliage are still green, but yellow stippling caused by mite feeding is becoming detectable on upper side of leaves with the underside showing mite feeding, this is still non-economic but warrants close monitoring.
  • If growers find that many of their plants are infested, with plants showing signs of stippling such as speckling and discoloration of some of the leaves on the field edges or throughout the field, they should consider spraying the entire field if mites are common throughout the field.
  • If growers find that all plants in the area, whether along field edges or within the field, are heavily infested and are discolored with wilted leaves, severe injury is occurring and a rescue treatment will save the field.
  • If growers find extremely high two-spotted spider mite densities, with much of the field discolored, stunted and with many plants dying down or already dead, a rescue treatment will be beneficial only if new growth occurs following late summer rain.

"While growers often wonder if it is worth the costs to spray during a drought, past experience shows that when soybeans are protected from mites, later rains in August will allow the soybeans to recover somewhat and give acceptable yields compared to fields where mites were not controlled," Hammond says.

But, he cautions, if the drought and extreme heat continue, none of these options will provide long-term control.

"Fields may need a second treatment later, so it's important that growers continue to scout their fields," he says. "And if a second treatment is required, growers should use a different material to help reduce the chances of miticide resistance from developing in the mite."

Growers can find more information about when they should spray for two-spotted spider mites and other treatment guidelines (pdf).

"The earlier you recognize mite infestations, the quicker you can deal with them prior to economic losses," Hammond says.