Spider mites reported in South Dakota crops

Adam Varenhorst
South Dakota State University Extension
Twospotted spider mites traveling on webbing that is present on a leaf.

With drought conditions continuing in South Dakota, it isn’t surprising that two-spotted spider mite infestations are being reported from several counties throughout the state.

With hot temperatures in the forecast and limited chances for precipitation, we can expect spider mite populations to thrive in infested fields, and it is likely that they will show up in additional areas.

There are two species of mites that we monitor for during dry conditions. These mites are the two-spotted spider mite and the Banks grass mite. Although both species of spider mites can be an issue for corn, only the two-spotted mite is a pest of soybean and alfalfa.

Both species of mites are susceptible to pathogenic fungi that can wipe out their populations. The catch is that 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.

Scouting and management

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 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. For soybeans, 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. This type of feeding injury is called stippling. Leaves may also have webbing on them, which is an indicator for two-spotted spider mites.

The following is 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.

Two-spotted spider mite treatment guidelines

Non-economic

  • Mites barely detected on underside of leaves in dry locations or field edges and plant damage minimal or non-existent

Non-economic, keep monitoring

  • 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

Treatment is warranted, especially if eggs and nymphs are found with adults

  • Most plants are infested, and most plants have stippling. Speckling and discoloration of lower leaves. Field edges and dry areas exhibit damage.

Treatment may be warranted; rescue treatment that may recover yield

  • All examined plants are heavily infested with mites, discolored and wilted leaves are easily observed throughout the field. Severe damage evident.

Treatment may not recover yield

  • Extremely high mite infestation, with the majority of the field discolored. Leaves are bronzing and falling from the plants.

If spider mites are observed, make sure to select an insecticide or miticide that will reduce the populations. Except for the active ingredient bifenthrin, most pyrethroid-class insecticides are not extremely effective at managing spider mites.

In some cases, applying pyrethroids will lead to a spider mite population increase. The residual of all products will be reduced during high temperatures, so fields need to be scouted five to seven days after application.

For spider mites, the entire field should be treated if spider mites have reached a density that requires treatment.