Spider mite outbreak

Juniper, cedar trees in western Nebraska affected

Jennifer Morris, Forest Health Specialist, Nebraska Forest Service

Spider mite infestation

Mats of webbing on the branches and foliage are a sign of trees infested by twospotted spider mites.

Some counties in western Nebraska are seeing high and damaging populations of twospotted spider mites on juniper and cedar trees. These mites can be a nuisance and, in some cases, very detrimental to host plants.

The twospotted spider mite is a warm-season mite active throughout the summer. Infested trees have fine mats of webbing on branches and foliage. Foliage may turn yellow or bronze. During hot, dry weather, mite populations can increase rapidly in just a few days. 

Several other factors can also cause an increase in mite populations. Excessive use of nitrogen fertilizers make foliage more succulent and favorable for mites. Insecticide treatments that kill beneficial predators of mites give the mites an advantage. Mites can also build resistance to a miticide if only one variety is used.

Twospotted spider mites do not overwinter on the host plant, but in grasses or weeds close to the host plant and even in the nearby soil, so forb and grass density may contribute to higher mite populations.

Miticides can help control spider mites when they are present. Applying a product with an active ingredient of bifenazate, hexythiazox, spiromesifen or etoxazole can be effective.  Even certain horticultural oils and insecticidal soaps work if labeled for mites.

Twospotted spider mites are active from late spring to fall, and two or more applications should be made at 7 to 10-day intervals.  Keep in mind that mites can build resistance to a product, and switching between products can be beneficial. 

Mowing routinely around host plants may help, but excessive weed control throughout the year may allow mite populations to move to other hosts.

For more information please contact the Nebraska Forest Service. (www.nfs.unl.edu).