Cool Weather Spider Mites
The cool weather we have been enjoying may bring a couple of pests to some of the most common plants in our landscapes. Cool weather spider mites are the specific pest to look out for this time of year. Although most people think of them as any other insect that feeds on plants, they are more closely related to spiders than insects since they have 8 legs.
Cool weather mites are dark red (southern red mite) or almost black (spruce spider mite) and with the eight legs spread out would just cover the period at the end of this sentence. The legs of the spruce spider mite are pale yellowish brown. Males are smaller and more slender than females. The eggs of these mites are red or brown, round, flattened, and not visible without assistance from a hand lens or microscope. They have a tiny hair-like structure that sticks up in the center. The larvae of cool weather mites are slightly larger than the eggs and have six legs. Nymphs are similar to adults although some nymphs are smaller.
Spruce spider mites are found throughout North America. Southern red mites are found in the eastern United States and California. Southern red mites prefer azaleas, hollies, and camellias but they have been recorded from a number of shrubs and herbs. Spruce spider mites feed on junipers, spruce, arborvitae, cypress and other needle leaved evergreens (conifers).
Southern red mites feed on the lower leaf surface, causing tissue collapse. Infested leaves turn gray or brown and may fall prematurely. Heavily infested shrubs may die. Spruce spider mites are the most destructive mite pest of ornamental conifers. Infested needles turn yellow or brown and may drop prematurely. Heavily infested shrubs may be noticeably webbed with silk. (This silk is different than a spider web. If you see webs that look like a spider web that is probably what you have. Be glad you have them since spiders feed on many plant-damaging insects.) After several years of heavy feeding by spider mites even large trees may be killed.
Spider mites hatch from eggs and develop through a larval stage and nymph stages before maturing into adults. Males mate with females as soon as the females molt into the adult stage and females soon begin laying eggs. Southern red mites over-winter as eggs glued to the lower leaf surface. If the winter is mild, all stages of this mite may survive. As the weather moderates in late winter, southern red mites increase. Most of the feeding damage occurs in early spring. When populations of predaceous insects and mites are active in summer, southern red mites dwindle away so that only the eggs survive in hot weather. If the summer is mild, all stages of this mite may survive. As temperatures cool in autumn, mite populations build up again.
Spruce spider mites also over-winter as eggs at the base of needles. In April and May the eggs hatch and larval mites begin feeding. Most feeding damage and reproduction takes place in spring and fall. Spruce spider mites tend to feed on older leaves.
Because these mites are active in cool weather, we need to look out for them at the end of winter or summer. October/November and April/May are good months to look for spider mite damage and treat if needed. Two or more treatments of horticultural oil at two-week intervals may be necessary for good control. To determine if spider mites are present you can tap or shake leaves over a sheet of white paper or note card then look closely for dark spots that move around. You may need a magnifying glass to see them. Be sure to read the label of any pesticide product before you purchase it to make sure it is safe for the use you are planning and will control the intended pest. Always make sure to follow the label instructions regarding rates, mixing, and personal protective equipment needed when applying. Since these pests are found below leaves or in older needles it is important to get good spray coverage on the back of leaves and deep into plants. Inspect plants 10 days to 2 weeks after the first treatment to determine if a follow up spray is needed.
If you have questions about gardening or landscaping in Pitt County you can call the Pitt County Master Gardener Extension Volunteers at 902-1705 or email them at email@example.com. More local gardening information is available at http://pitt.ces.ncsu.edu, www.pittcountyarboretum.blogspot.com, www.twitter.com/pittgardening, or on facebook at Pitt County Gardening.