Hot and dry conditions set up our crops and landscapes for a tiny pest called the two-spotted spider mite. They are a very tiny creature, about 1/50th inch long, yellow-orange in color, with two dark spots, one on each side of the body.
Spider mites live through the winter as eggs on vegetation. Larvae hatch and complete development in one to two weeks depending on the temperature. Under high temperatures ( greater than 90°F) colonies can reach high numbers in less than two weeks. After hatching, the mites build colonies on the undersides of leaves and produce a somewhat disorganized webbing over infested leave surfaces. This webbing gives them the name "spider" mites.
Spider mites use piercing-sucking mouthparts to feed on the underside of leaves and needles in a manor very similar to how mosquitoes feed on humans. When there are a lot of spider mites, the combined feeding efforts produces tiny white or yellow spots or "stippling" on leaves and needles. Some plants will take on a bronzed or yellowed discoloration. These symptoms may be confused with drought stress.
Sometimes, an event, such as mowing grass hay, a dry lawn or tall weeds can set the spider mites on a migration to look for new sources of food. I recall several fields that surrounded a newly baled hay field near Millersburg a few years ago. There was corn on the east side of the field, soybeans to the south, and vegetables to the west of the hay field. The first 15 to 20 feet of each neighboring field were being devastated by mites moving out of the hay field. The damage was visible just driving down the road.
If you find discolored leaves and suspect spider mites, hold a white sheet of paper or paper plate under the leaves and shake the branch or leaves. If mites are present, you will tiny specks the size of a grain of pepper begin moving around on the paper.
When the mite population is high, natural enemies are not effective at controlling spider mites. Using pesticides like carbaryl (Sevin)and imidacloprid for mite control can kill these natural enemies as well, making the mite issue worse.
In landscaping and gardens, insecticidal soap and horticultural oil can be effective against mites and have little impact on people, animals and nontarget insects.
These products will only kill mites that the pesticide directly contacts. They do not have any residual activity, and may need to be reapplied a couple of times. Thorough coverage of the plant is important. Target the underside of leaves as well as the top.
Effective active ingredients of residual pesticides include malathion, bifenthrin, deltamethrin and lambda cyhalothrin. Use these pesticides only when necessary, as they might affect a variety of insects. Select a product that is labeled for the type of plants affected. Water plants thoroughly before spraying pesticides for spider mites. Spray in the early morning or early evening. These steps will reduce the risk of further stressing plants and causing injury.
For outbreaks on field crops, visit Purdue Extension Entomology page https://extension.entm.purdue.edu/publications/pubs/f_crop.html to select a miticide for control.