Don't give spider mites a chance

Spider mites are a common plant pest at this time of year, as they like it hot and dry. But some things we do might make it even harder to control them. I think that's called shooting yourself in the foot.

Spider mites come from the family Tetranychisae and are classified as arachnids, or spiders. They have lots of cousins you may know, including other spiders, ticks, daddy-longlegs and scorpions. Spider mites won't bite you, however.

There are many kinds of spider mites, which attack different types of plants, from spruce, juniper, pines and arborvitae to turf grasses. Today, however, I'm just going to talk about those you might find in your vegetable garden, and on some ornamentals, including houseplants.

These rascals are almost impossible to see with the naked eye, as they are smaller than a grain of sand. We usually first notice them by the damage they do. A few spider mites won't be particularly harmful, but in large numbers they can cause serious damage or even kill a plant.

Some of the plants most likely to host a convention of spider mites include cantaloupe, cucumbers, beans, eggplant, tomatoes and blackberries. They are also a threat to houseplants, often causing distorted leaves.

Spider mites have a pair of needle-like structures called stylets. They use these to split and break leaf cells, then they push their mouth parts into the split tissue and suck out the sap in the cell. This leaves a tiny tan spot on the leaf, making the plant look like it has little, tan freckles. You might be able to see webbing on the underside of the leaf, too, depending on the number of mites.

These creatures reproduce at an astonishing rate, taking only a week to go from egg to adult. Each mature female may produce a dozen eggs daily for a couple of weeks, which can result in a real population explosion. That's another reason to keep on eye on your plants.

Hot, dry conditions greatly favor spider mites, and they can develop rapidly during this time. Hot, dry conditions also stress plants, making them more susceptible to attack.

The overuse of insecticides, especially systemics, will often worsen the problem, as they will kill off most of the spider mite's natural enemies, which include lady bugs, lacewings, pirate bugs and big-eyed bugs (yes, that's their real name).

If you already have spider mites on your plants, here are a few things you can do. Water plants regularly so they don't become stressed. Hosing affected plants, especially on the underside of the leaves, with a strong stream of water helps disrupt the reproduction cycle. Repeated application of insecticidal soap or neem oil may help. Although they are chemicals, they are not as broad-spectrum as most so will do less damage to the natural predators.

But most important, perhaps, to stop shooting yourself in the foot step away from those broad-spectrum insecticides.

Coming up: Cliff Bennett of Chet's Garden Center will teach a class on the fall care of perennials. The class will be held from 7 to 9 p.m. on Wednesday, Sept. 11, at the Southern Oregon Research and Extension Center, 569 Hanley Road, in Central Point. The cost is $5. Call 541-776-7371 to register.

Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. Email her at

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