JoAnne Skelly: How to deal with spider mites

Hot sun, low humidity and drying winds create a hospitable environment for tiny pests called spider mites. With eight legs, these arachnids are related to spiders and scorpions. They are common pests on many yard, garden and indoor plants. Evergreens are highly susceptible, particularly dwarf or blue spruce trees.

Symptoms include graying, bronzing or yellowing leaves or needles or light flecks on leaves. Webs may be evident, but by the time the webbing occurs, the infestation is usually at a critical level. Injury can cause leaves or needles to drop. Severely infested plants may die.

Mites are almost too small to see with the naked eye. A good test for mites is to tap or shake a leaf or evergreen branch over a white piece of paper. If the particles that collect on the paper move on their own, those aren’t dust particles, they are mites. Get a magnifying glass and take a close look at them. They can be clear, red, brown, orange, yellow or green.

With spider mite infestations, people may think a chemical insecticide, or in this case, a miticide is the answer. However, insecticides/miticides should only be used as a last resort, and then preferably only as a spot treatment instead of a total plant application. One reason spider mites become a problem is that their predators, such as lady beetles, predatory mites (yes, there are good mites too), minute pirate bugs, big-eyed bugs and predatory thrips, are killed with insecticides/miticides. You can preserve these beneficial adversaries of spider mites by avoiding broad-spectrum insecticide applications, particularly early in the season.

Since spider mites prefer dry dusty environments, hosing plants off with water early in the season and often throughout the summer can be an important cultural control. I spray off my dwarf Alberta spruce daily with a strong blast from the hose, washing it down thoroughly. In addition, drought-stressed plants go through chemical changes that send out ‘distress signals’ that attract spider mites. Provide plants with optimal moisture for their needs. If daily hosing doesn’t reduce the population of mites to an acceptable level and return your plant to a healthy green or blue, spot treat infested areas with insecticidal soaps or horticulture oils. Be careful with horticulture oils on blue evergreens including blue spruce though. The oil will take away the blue color. Always read and follow the label directions and never spray with insecticides/miticides when temperatures are 90 degrees or above.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.