By: Jennifer Morris, Forest Health Specialist
Spider mites can be quite the nuisance and in some cases very detrimental to host plants. Some western counties in the state are seeing high and damaging populations of twospotted spider mites on juniper and cedar trees. 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.
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. Non-selective 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. They overwinter 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. Applications of 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 2 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.