Spider mites harm plants but are easy to control

Spider mites are really bad this year, encouraged by our hot dry weather. We often see them indoors on houseplants in winter, when our homes are dry and warm. We don’t often see them on plants such as those in the vegetable garden or ornamental beds in summer, though.

This year is different.

They weave tiny webs from stalk to petiole, and sometimes extensive webs on the under side of leaves. Both webs and the mites are hard to see, but hold a piece of white paper under leaves and tap on them. If you see dots moving around, you’ve got spider mites or thrips. If you can find the webs, you’ll definitely know it’s spider mites. They tend to live in colonies on the undersides of leaves, and when a colony has a large number of mites, their feeding on leaves is noticeable, even without magnification. Leaves look stippled, and ultimately may be bronze to red in color.

When they weave a web on the underside of the leaves, they lay eggs in the web, eggs that hatch in three days. Spider mites are true arachnids, with eight legs, but the newly-hatched mites only have six legs. In other respects, they just look like smaller versions of their parents. In large numbers, they can cause severe damage on plants.

How do you control them? Remember, they thrive in dry heat and dust. Set a nozzle to mist, and hose off the leaves on affected plants, especially the undersides of the leaves. I’d do that in morning so that leaves have a chance to dry out before evening cooling. They’re vulnerable to being washed off with water, and they’re killed by soapy sprays (true soap, not detergent). Spider mites have a number of enemies in the insect world, those we’re most familiar with are ladybugs. Both adult and larval lady bugs consume spider mites.

Curly top virus

Curly top virus has been found in the Treasure Valley, on sugar beets and pepper plants, according to a Pacific Northwest Pet Alert. This devastating disease is transmitted by a beet leafhopper that has once bitten a leaf containing the virus. The leafhopper will carry the virus (and transmit it) for the rest of its life, but it will not transmit it to its eggs. The virus affects many other plants, such as tomatoes, squash, beans, cucumbers and some ornamentals. When it infects a mature, heavily-fruited plant such as a tomato, it’s almost like a death in the family. Leaves curl upward, exposing dark purple veins, the plant feels rough and leathery, and the plant develops a sickly yellow color. Symptoms in other plants are distorted leaves and stems, and, in some cases, a stunted plant.

This disease occurs mainly in dry deserty or semi-desert conditions in the U.S., from Canada to Mexico (including the chile pod growing area of New Mexico), and in some locations in the Middle East, such as Saudia Arabia. Leafhoppers can ride the winds for up to 200 miles, so proximity to beet fields is not a necessary requirement for the spread of the disease. Winds blowing infected leafhoppers have sent them down the Columbia river canyon to the Willamette Valley and somehow through the Sierras to the East Bay Area of California.

The fruit is safe for humans to eat I’ve been told, but I don’t eat it. Other viruses or bacteria may distort leaves, so distortion is not the defining factor of diagnosis. Some folks immediately pull out the diseased plant, others do not. Since these leafhoppers are so transient, sprays do not help. They dislike shade, although full sun for at least six hours daily is necessary for vegetable garden exposure. Some folks have found dense plantings of dill or other odorific herbs around tomatoes repels leafhoppers (or at least seems to). Or plant resistant tomato varieties such as Payette, Owyhee, Super Star VF, Red Lode VF, Ida-Red VF, Parma VF, Bicentennial VF, Rowpac VF, Columbia VF, Roza VF, and Salad Master VF. Those with the VF designation have also been found to be resistant to verticillium and fusarium diseases.

This story was originally published August 23, 2017 at 3:32 PM.