By Nate Royalty, Ph.D.
What do they look like?
Spider mites, also known as webspinning mites, are arachnids, so they are more closely related to spiders than to insects. Mature adults have oval bodies, eight legs and red eyespots near the head. Most spider mite species are yellow-orange in color, although with some species, such as spruce spider, citrus spider, or European red mites, the adult females are red. The adult females of most commonly occurring spider mites also have distinctive dark spots on the abdomen; on twospotted spider mite, the most commonly occurring spider mite pest, the paired spots are large and very distinctive to the naked eye.
There are three immature stages. Newly hatched larvae look similar to adults, except they have only six legs and spots that are absent or very faint. The two nymphal stages have eight legs, and, as they mature, the spots become progressively darker as the mites feed. Spider mites produce a clear, shiny chrysalis in between molts, in a manner similar to butterflies.
Spider mites are much smaller than most insect pests; adult females, the largest life stage, measure less than 1/20 of an inch in length. So plant damage often is the first indication of a spider mite infestation. Most spider mite species produce webbing, which is easy to detect if large populations are present. This webbing serves as protection from natural enemies and helps produce a favorable microenvironment on the leaf surfaces. Often one can see numerous spider mite eggs, which are small, creamy-white, opaque spheres scattered throughout the spider mite webbing.
Host material and range
Numerous species of spider mite attack trees, shrubs and herbaceous plants throughout the United States. Spider mite problems are most common when the temperature is warm and humidity is low (i.e., in summer), however some species, such as spruce spider mite, are active during the spring and fall.
The twospotted spider mite is the most commonly occurring spider mite; it feeds on hundreds of different plant species. Other spider mites are very host specific, and the common names reflect this — the spruce , boxwood, hickory, linden, elm, honeylocust, willow, oak red, citrus red, and maple spider mites all are common pest problems in urban landscapes.
Spider mites feed on ornamentals by piercing plant leaves with their needle-like mouthparts (called stylets) and by sucking the fluids out of the damaged plant cells. Early signs of mite feeding are tiny yellow or brown spots, known as “stippling.” As the feeding damage increases, foliage becomes bronze or yellow. Heavy infestations cause premature leaf drop. Long-term effects of heavy mite feeding are reduced growth. Herbaceous plants can be killed by the long-term effects of spider mite feeding.
As water-stressed plants are prime targets for spider mites, proper irrigation practices are very important in managing mite populations. Dusty areas also encourage outbreaks, so regularly apply water to dusty pathways and yard areas. Non-selective insecticides such as pyrethroids that are used to control insect pests can often “flare” mites by killing mite predators that keep spider mite populations under control. And insecticides can flare mites by increasing the rate of mite reproduction, so unnecessary insecticide treatments should be avoided. If an insecticide application is warranted, monitor the treated plant for mites for a few weeks after application.
Miticides can be applied to susceptible ornamentals early in the season (March through May), targeting the eggs. The eggs of many species overwinter on the trunk and twig bark of the host plant. Horticultural oil aids in control of the eggs, but can cause phytotoxicity on some ornamentals. Miticides also can be applied when adults and immatures are detected later in the season.
Because of their small size, spider mites often are not detected until present in heavy numbers, which makes effective treatment more difficult. Another challenge in treating spider mite-infested ornamentals is that mites usually feed on the underside of the leaf. This makes spray applications especially difficult because it is necessary to directly treat the undersides of the leaves for effective results.
Forbid miticide, manufactured by Bayer Environmental Science, features translaminar activity, meaning that a spray application on the top of the leaves will kill mites on the underside as well. Forbid features the lipid biosynthesis inhibitor mode of action; treated mites are unable to maintain proper water balance, so they dry up and die. Always read and follow label instructions.
What can you do?
Early detection is important, so be on the lookout for the early signs of spider mite feeding such as spotting or stippling of leaves. Look on the undersurface of leaves for the distinctive webbing and the spotted adult females. A hand lens will aid in identification. If chemical treatments are necessary, contact a local licensed pesticide applicator.
Nate Royalty, Ph.D. is product development manager — insecticides, Bayer Environmental Science.