Spider mites benefiting from our hot dry weather

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

We are getting lots of plant samples that show very high numbers of spider mites and severe spider mite feeding injury. We’ve seen spider mite injury on Viburnum, Astilbe, Rose, Amelanchier, Burning bush, soybeans, spruce and hemlock in the last week alone. Some samples have shown the beginning stages of injury while others show advanced stages where there is very little green tissue left on the leaves.

Spider mites have piercing/sucking mouthparts. To feed they abrade or poke the leaf surface with their pointed mouthparts and then suck up the liquid that oozes from these wounds.  The affected tissue more or less dries up and the result is a yellowish stippling of the leaf or needle surface. As the feeding injury progresses, the stippled areas coalesce and the entire leaf turns a brownish color. This is often referred to as “bronzing.” This yellowing and bronzing may appear similar to other damage symptoms like drought or a nutrient deficiency, so it is always a good idea to look for evidence of the mites, which will include the mites themselves, cast skins, clear spent egg shells and viable eggs. Our own Jan Byrne proudly holds the lab record for the most severe case of spider mite injury on a broadleaf ornamental.

Spider mites are among the most common and destructive of all plant pests, especially during summers that are particularly hot and dry, like the one we are having this year. Some species are very specific and feed on only one type of host plant. Others, like the twospotted spider mite, seem to be able to feed on many plant species including weeds, vegetables, flowers, field and forage crops, brambles and other small fruits, tree fruits, greenhouse plants, and certain trees and shrubs. Spider mites feed and reproduce whenever conditions are favorable for plant growth, from early spring until late fall. Most spider mites prefer new growth. Hot weather favors rapid development and reproduction. The twospotted spider mite, for example, can go from an egg to an adult in only five days at 75ºF. The female lives two to four weeks and produces about 100-300 eggs.

Most of the time spider mite populations are kept in check by natural enemies and do not require pesticides. Occasionally, during favorable conditions that produce damaging mite populations, a miticide application may be warranted to protect the host plant from serious injury. Because of their rapid development and reproductive potential, repeated treatments at 10-day intervals may be required for some plants to bring about satisfactory control.

Female spider mite
A female twospotted spider mite
under glass.

Spider mite egg skins
Numerous spruce spider mite egg
skins on hemlock.

Close up spider mite
A close up of a female twospotted
spider mite.

Spider mites with egg
Twospotted spider mites with an egg.

Feeding injury
Severe spruce spider mite feeding injury
on hemlock. Photo by Steven A. Gower, MSU Diagnostic Services.

Close up of feeding injury
Close up of spruce spider mite feeding
injury on hemlock. Photo by Steven A. Gower,
Diagnostic Services.

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