Dealing with spider mites on houseplants

Question:

The leaves on some of my houseplants have suddenly died. Some are brown and some dried out while still green. I have noticed some very fine cobwebs around these leaves. Is it possible that spiders are killing my plants? I haven't seen spiders there, but the cobwebs are present.

Answer:

Spiders do not feed on plants, but they will eat insects and other things that feed on plants. The cobwebs you have described were probably made by spider mites. These relatives of spiders (they are arachnids with eight legs like spiders) do feed on plants, can cause the sudden death of leaves, and in time may kill the plant if not treated. Their webbing is very fine and difficult to see, and has no structure, so the term cobweb is appropriate. You may see some extremely small mites moving along the webbing if you look closely or use a magnifying glass.

There are miticides for controlling spider mites, but most are not for indoor use. It is easier to just put the houseplant in the bathtub or shower and spray with a strong, but fine mist of water. The shower head may be too coarse, but a handheld sprayer filled with water will work. If you wish, you can add insecticidal soap to the water to enhance its effect. Washing the spider mites from the plant frequently will reduce their numbers and their damage. The insecticidal soap will kill some of the mites and assist in washing them from the plant.

If the plants cannot be moved to the bathtub or shower, you can spray the plant with insecticidal soap. When you do this, protect your upholstery, draperies, and carpeting with plastic covering to avoid damaging the fabrics. When you apply insecticidal soap, apply it to the top and bottom of the leaves and stems. If you can delay, apply the insecticidal soap to a small portion of the plant to check for phytotoxicity (some plants are damaged by the soap). If the sprayed part of the plant does not show symptoms of damage (drying or discoloration) within a few days, it is probably safe to spray the whole plant. Make a fresh solution of the insecticidal soap at this time. This treatment should be repeated once or twice a week for two to three weeks. If the spider mites reappear, continue the treatments.

Whenever using insecticidal soap or any other pest control product be sure to read and follow the label directions. These directions will list some plants that may be injured by the product and will advise you as to the proper quantity of product to be mixed with water for best results.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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