Spider mites probably are one of the worst insect problems in most gardens during hot summer weather. They suck juice from the foliage, causing severe injury to plants. They are all the more serious because they multiply and are rapidly and are so tiny they can barely be seen without a magnifying glass.
Tomatoes, beans, corn and similar annuals can be killed in a relatively short time, at most a matter of weeks. Perennials, on the other hand, often lose their leaves or needles as an immediate effect but ultimately, a year or two later, will likely die from a serious loss of vigor if the feeding has been heavy.
Spider mites construct an extensive system of webbing, much more dense than the average spider web, over which they travel. Adults fasten their eggs directly to the leaf surface or to the webbing. In a normal infestation all stages of development are present at the same time.
The webbing also gives considerable protection to the mites. It not only facilitates travel but also enables them to keep off leaf surfaces that have been treated with miticides. The webbing also intercepts spray particles. Even with intensive spraying mites can remain unaffected by the sprays when they are protected by the canopy of webbing.
Mites may pass the winter in the egg or adult stage depending on the species. They usually develop rapidly in warm weather and a generation may be completed in 20 days.
An exception is the spruce spider mite, which may build up large populations on the old needles of conifers - especially arborvitae, juniper, hemlock and spruce - in the cool weather of spring and fall.
To check these trees for mites, hold a piece of white paper under a branch. Strike the branch 3 or 4 times. The mites present will drop onto the paper where they can be seen crawling about on the white background. They can be seen best with the aid of a magnifying glass.
When they are numerous on other plants, their feeding activities first cause leaves to appear white to yellow, and finally brown.
For mite control on tomatoes, specialists recommend spraying with Kelthane. Treat when mites appear in damaging numbers. In all cases, read the directions on the label and follow them closely.
For mite control on strawberries, cucumbers, cantaloupes, squash, pumpkins, watermelons, lima and snap beans, specialists recommend spraying with Kelthane or Spectracide. Spray when damage appears.
Mites cause leaves and pods of lima beans to turn rusty red and many leaves to drop. Use of sevin for control of bean insects, such as the Mexican bean beetle, favors the mites as much as the hot weather. Sevin kills predators of the mites (without harm to the mites) allowing them to increase at even faster rates. The hot weather also reduces the activity of predators.
A curious thing noted by researchers is that mites are not likely to attack plants of low nutrient level. Potted tomato plants, grown in a special potting mixture of low fertility, were ignored by mites although they were nearby in large numbers.
Dormant sprays in late winter and early spring with superior 60 or 70 second oil, before temperatures reach 80 degrees F., give good mite control. These highly refined oils act by smothering the overwintering stages, both eggs and adult mites. These sprays may reduce mite populations to such a low level they will not build up to serious proportions during the summer.