They are ubiquitous. They float on air currents and move readily between all kinds of plants including orchids. Sometimes we are tipped off to their presence by webs. Other times we see damage on leaves.
We call them spider mites but those found on our orchids may well be other mite genera. They breed prolifically in warm weather and low humidity. They are also tiny, some as small as one fiftieth of an inch.
As temperatures rise, mites lay more eggs. And the eggs hatch more quickly. At 48°F, eggs hatch in 19 days. At 70°, they hatch in 3–7 days. The female lives about 68 days during which she can lay 100 eggs. Infestations proliferate quickly. Thus, early control is important. Spider mites are not insects. They have eight legs, unlike insects which have six. So, insecticides don’t work.
When I looked for information on non-toxic spider mite control, the commercial products popped up first. And, by and large, they were the same that I’ve mentioned before such as neem oil and soap products (e.g., Safer).
Among the easiest things you can do is to hose off your orchids regularly, taking care to wash the undersides of leaves. Make sure to keep humidity up around your orchids. Use humidity trays if you grow indoors. Some people recommend washing the entire plant with sudsy water. Repotting apparently doesn’t help.
Other recommendations include spraying with 70% rubbing (isopropyl) alcohol combined with a mild liquid detergent (but alcohol can damage buds) or using neem or horticultural oils or insecticidal soaps. If you use an oil, be sure the temperature is below 85°F and, to prevent burn, don’t apply it on a sunny day. And since spraying won’t kill the eggs, you must repeat treatment every 1–3 weeks.
Learn more about mites in an article by Paul Johnson, Ph.D., Insect Research Collection, South Dakota State University, on the American Orchid Society’s website: https://www.aos.org/orchids/orchid-pests-diseases/mites-on-cultivated-orchids.aspx.
Comments and input wanted
Please email me your questions or tips about pest problems or observations and put “NWOS Pest Column” in the subject line. I’d especially like to hear from those of you who grow indoors.
Kathy Murray, firstname.lastname@example.org