Gardening in Steamboat: The curse of the red spider
January 24, 2011
Steamboat Springs — The leaves of your favorite houseplant are turning brittle, yellowish, tan or bronze and have tiny holes on the undersides. Also there is mysterious webbing where the leaves meet the stalks of the plant. Leaves are falling off, and the plant looks like it is going to die. Putting a piece of white paper under the remaining leaves and shaking the foliage briskly results in maybe a few more leaves falling on the paper, but also tiny specks of black, brown or red, and they are moving.
What you are seeing is the dreaded spider mite of the family Tetranychidae. Mites are common on outside garden plants, vegetables and fruits (Tetranychus urticae — two-spotted spider mite), and inside houseplants. There also are mite species that infest evergreens, shade trees and lawns.
The first line of defense against these mites is to increase the humidity around the plants. Hose or shower the leaves to wash the dust, webbing and mites off every week or so. The warm, dry conditions that the mites like are the conditions that their predators do not like. Stressed plants can change their chemistry and become more nutritious to the mites, said Colorado State University etymologist Whitney Crenshaw.
Prune and throw away infested leaves and stalks (and maybe the entire plant).
Then you might want to bring out the chemical arsenal. No. 1 on the list might be neem oil, which is extracted from the seed of the tropical tree Azadirachtin indicta. It is labeled a miticide and is effective against all stages. Please note: this product will kill "nontarget" insects (ladybugs and honeybees) and is toxic to fish.
No. 2 in the arsenal could be horticultural oils known as summer oil and dormant oil. These products work by smothering the pests or affecting their feeding. These oils are fairly selective in that they do not harm beneficial insects and are the least toxic to the gardener. Follow the directions and test on a small part of the plant first as the oil may make the plant sensitive to sunlight.
No. 3: Sulfur may be dusted or sprayed on the plants (don't forget the undersides). Don't mix with the horticultural oil.
No. 4: Insecticidal soap, ready-mixed or mixed with soft water, rainwater or distilled water. Hard water deactivates it — now I know why it never worked for me.
Lastly, try the systemic guns such as Orthene. Don't use these chemicals on anything edible. I have used this product, and it wasn't effective for mites.
Keep a lookout for these pests before they multiply, which they do quickly.
Hopefully, by following the cultural changes and limited spraying, you can rescue a prize houseplant.
Barb Sanders is a master gardener through the Routt County Extension Office. Call 970-879-0825 with questions.