Issue: March 9, 1998

Spider mites on junipers

A gardener friend told me that I need to spray my junipers because they are going to get spider mites. What are spider mites and should I be worried? What do I use to spray them?

Junipers may get spider mites, but don't spray unless you discover that they are indeed infesting your juniper. So, what are spider mites? They are very small, eight-legged critters which damage many of our plants by sucking the "sap" from the plants. Though each is very small, the spider mites reproduce very rapidly and can infest plants by the thousands or more. Collectively they are capable of turning one side or all of a juniper brown in a couple of weeks. As I said, they are very small, so you are more likely to notice the symptoms and signs of spider mite infestation rather than the mites themselves.

Signs to watch for are yellowing or browning of the junipers, a trashy or lint-like coating on the twigs, and a very fine webbing which coats the twigs. This webbing does not look like spider webs which stretch from one branch to another; rather, it just coats the tips of the branches and the juniper leaves. This webbing collects dust and other debris blowing in the air creating the "trashy" appearance that I mentioned. The webbing is not intended to trap insects as is a spider's web; it is to protect the mites from predatory insects and to maintain a higher humidity next to the plant to protect the mites from desiccation.

The symptoms to observe are the yellowing, browning, and dying of the juniper leaves and twigs. You can usually identify the presence of the mites by the webbing and other signs before the plant symptoms develop and thus avoid visible damage to the plants.

If you think the mites may be present, you can confirm their presence by watching for very small specks moving on the webbing. These specks are the mites. If your vision is not very good, you may need a magnifying glass to see the mites themselves. Another trick is to hold a sheet of stiff construction paper under a branch you suspect to be infected. Use a dark or light colored paper rather than a medium colored paper. Then, with a gloved hand, sharply strike the branch supposedly infested. Now the mites may be observed as small moving specks on the construction paper. It is much easier to use a magnifying glass or hand lens over a piece of construction paper than trying to see the mites on the branch. Be sure the wind is not blowing or the mites will blow off the paper, or specks of dust will appear to be moving around on the paper and you will misdiagnose the mites thinking they are there when they are not a problem. Check several branches at several locations on the juniper. If you find only a few mites or none, you need not worry now. Just keep checking on a weekly or bi-weekly basis.

If or when you find the mites present, you can begin treatment. The first line of defense is the use of water to wash away the mites' protective webbing and to wash many of the mites from the plant. A strong jet of water from the garden hose is sufficient for this treatment. Do this every few days as long as the mites remain present. If this does not seem to be effective, there are several chemical sprays to use. Read the labels of products at your local garden store, looking for horticultural oils or miticides which are labeled for use on junipers or ornamental shrubs. Then use the product according to the directions. Several treatments may be needed to effectively clean up the problem. These treatments should be applied at about three-day intervals unless the product directions tell you otherwise. The reason for these repeated treatments is to eliminate any mites which hatch from eggs following the earlier treatments. Three treatments at three-day intervals should be effective. Following the series of treatments, check again once every two weeks or so using the construction paper method to be sure there has been no reinfestation.

Remember, mites are not insects, so most insecticides will not be effective at controlling mites. Some are labeled for mite control and will be effective. The advantage of using the water jet treatment is that it will not eliminate the predatory mites and other natural enemies of the mites and, by leaving the "good guys" on the juniper, you will reduce the opportunity for the mites to reinfest the juniper. By washing the mites off, you have just bought time for the "good guys" to bring the problem under control for you.

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.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!