The prolonged extreme heat and rainfall shortages that have led to moderate and severe drought conditions across Ohio also have led to reports of two-spotted spider mite – a pest that can cause severe soybean damage or death. Many growers have already reported finding two-spotted spider mites on soybeans, says Ron Hammond, an Ohio State University Extension entomologist who also has an appointment with the Ohio Agricultural Research and Development Center.

Spider mites, which feed on the underside of the foliage with sucking mouth parts and can be destructive when abundant, thrive on plants that are under stress, especially in hot, dry field conditions.

This is significant, considering that all of Ohio except for small portions of four counties near the West Virginia border is experiencing moderate drought, with areas near the Indiana and Michigan borders experiencing severe drought as of July 24, according to the most recent U.S. Drought Monitor.

"Numerous areas in the drier areas of Ohio are already seeing them, and some fields are already being sprayed," Hammond says. "Mites are showing up not only on field edges but also within the field.

"Two-spotted spider mites have the potential to cause more yield loss than any other insect, with the damage caused by the mites being severe enough to kill the entire plant. Growers who have a bad infestation will not see any yield from the affected area."

Typically, Ohio soybeans are affected by two-spotted spider mites in July or early August, because moisture levels earlier in the year are usually high enough to keep the pests at bay, Hammond says.

But the early hot, dry conditions and low soil moisture levels have caused the pests to appear earlier and, because many soybean plants are stressed, the pests have more potential to impact entire fields instead of just the edges, Hammond says.

"If we were having normal rainfall, we wouldn't be worried about mites right now," he says. "In some parts of the state, soils are very dry and crops are starting to suffer."

Currently, topsoil moisture was rated 53% very short, 37% short and 10% adequate, with no surplus, according to the latest USDA weekly crop report.

Hammond says growers should begin scouting their fields for two-spotted spider mites now. Spider mite infestations can first be noticed by yellow stippling on the upper surface of the leaves, he says. The mites themselves, which are found on the underside of the leaves, can be identified using a good hand lens.

"In making an assessment of a spider mite-infested field, it is important that one recognizes the early sign of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when foliage is still green," he says.

 

Spray criteria

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.