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Dry Conditions Are Ideal for Spider Mite Activity

South Dakota drought map with yellow and orange indicating the severity of drought. Much of western South Dakota is covered in yellow, and there is a strip of yellow across the northern counties.
Figure 1. United States Drought Monitor: South Dakota, July 30, 2020. Courtesy: Richard Heim, NOAA/NCEI.

Several counties in South Dakota are experiencing dryer than normal conditions, with D0 (abnormally dry) D1 (moderate drought) and D2 (severe drought) designations according to the United States Drought Monitor (Figure 1). Therefore, it is important to monitor crops for spider mites, as they thrive during periods of dry weather.

There are two species of spider mites that can be problematic in South Dakota crops. They are the two-spotted spider mite (Figure 1-A) and the Banks grass mite (Figure 1-B). Although both species of spider mites can be an issue for corn, only the two-spotted mite is a pest of soybean. Both species of mites are susceptible to pathogenic fungi that is capable of wiping out their populations. These fungal pathogens require humidity to thrive, which is one of the reasons why spider mites are more of an issue during hot, dry conditions.

Management Considerations

Left: Figure 1-A. Green leaf with two yellow spider mites with dark spots. Right: Figure 1-B. Green leaf with yellow spider mite.
Figure 2. A) Two-spotted spider mites. B) Banks grass mite. Courtesy: Frank Peairs, Colorado State University,

There are several predators that can be effective at managing spider mite populations.

Spider mite issues can often occur after an insecticide is applied for a different pest. Insecticide applications may reduce the number of spider mite predators, which allows spider mite populations to increase rapidly and potentially cause yield loss.

Although there may be spider mites still in the field, management decisions should be made carefully.

For corn, management at or after the dent stage is not economical. Typically, spider mite management needs to occur between the pre-tassle and soft dough stages of corn.

Green leaves with many white spots and discoloration present.
Figure 3. Two-spotted spider mite stippling injury on soybean leaves. Courtesy: Darren Mueller, Iowa State University,

For soybean, the two-spotted spider mite can be an issue throughout the reproductive stages.

The leaves of infested crops will have small yellow or white spots on them, which are the result of spider mite feeding (Figure 3). This type of feeding injury is called stippling. In addition, the leaves may also have webbing on them, which is an indicator for two-spotted spider mites.

Table 1 provides a rating system for determining if management of two-spotted spider mites is necessary. Remember that the rating system should be used when scouting soybean on a weekly basis.

Table 1. Two-spotted spider mites treatment guidelines*

Presence of Mites and Plant Damage Rating and Decision
Mites barely detected on underside of leaves in dry locations or field edges and plant damage minimal or non-existent.
  1. Non-economic
Mites easily detected on underside of leaves in dry locations or field edges but difficult to find within the field. Leaves are still green but with stippling present on some plants.
  1. Non-economic, keep monitoring
Most plants are infested, and most plants have stippling. Speckling and discoloration of lower leaves. Field edges and dry areas exhibit damage.
  1. Treatment is warranted, especially if eggs and nymphs are found with adults
All examined plants are heavily infested with mites, discolored and wilted leaves are easily observed throughout the field. Severe damage evident.
  1. Treatment may be warranted; rescue treatment that may recover yield
Extremely high mite infestation, with the majority of the field discolored. Leaves are bronzing and falling from the plants.
  1. Treatment may not recover yield

*Adapted from Ohio State University.