Spider mites are tiny 8-legged animals closely related to spiders. Several kinds are important pests of ornamental crops, shrubs, and bedding plants. Under favorable conditions spider mites can build up rapidly and seriously threaten plant health.
All of these spider mites feed on plant leaves by piercing leaf tissues and sucking the green liquid that oozes out. Leaves appear bronzed after the green color is lost from many tiny feeding spots. Heavily infested leaves and branches can become covered with an almost invisible webbing.
The best way to confirm a spider mite infestation is to hold a sheet of white paper under a branch and then to tap the branch sharply. If present, they will fall off and be seen as tiny specks crawling over the paper. Crawling is the characteristic that most clearly distinguishes mites from the grit that can also be knocked off a plant.
The two-spotted spider mite is oval in shape, about 1/50-inch-long and may be brown or orange red, but a green, greenish-yellow or an almost translucent colour is the most common. The female is about 0.4 mm in length with an elliptical body that bears 12 pairs of dorsal setae. Overwintering females are orange to orange red. The body contents (large dark spots) are often visible through the transparent body wall. Since the spots are accumulation of body wastes, newly molted mites may lack the spots. The male is elliptical with the caudal end tapering and smaller than the female. The axis of knob of aedeagus is parallel or forming a small angle with axis of shaft.
Spider mite development differs somewhat between species, but a typical life cycle is as follows. The eggs are attached to fine silk webbing and hatch in approximately three days. The life cycle is composed of the egg, the larva, two nymphal stages (protonymph and deutonymph) and the adult. The length of time from egg to adult varies greatly depending on temperature. Under optimum conditions (approximately 80ºF), spider mites complete their development in five to twenty days. There are many overlapping generations per year. The adult female lives two to four weeks and is capable of laying several hundred eggs during her life.
All mites have needle-like piercing-sucking mouthparts. Spider mites feed by penetrating the plant tissue with their mouthparts and are found primarily on the underside of the leaf. All spider mites spin fine strands of webbing on the host plant. The mites feeding causes graying or yellowing of the leaves. Necrotic spots occur in the advanced stages of leaf damage. Mite damage to the open flower causes a browning and withering of the petals that resembles spray burn.
When two-spotted spider mites remove the sap, the mesophyll tissue collapses and a small chlorotic spot forms at each feeding site. Continued feeding causes a stippled-bleached effect and later, the leaves turn yellow, gray or bronze. Complete defoliation may occur if the mites are not controlled. Spider mites are the most common mites attacking woody plants and the two-spotted spider mite is one of the most economically important spider mites. This mite has been reported infesting over 200 species of plants. Some of the more common ornamental plants attacked include arborvitae, azalea, camellia, citrus, evergreens, hollies, ligustrum, pittosporum, pyracantha, roses, and viburnum.
Spider mites feed on plants one cell at a time. Small numbers of spider mites cause leaf stippling. Left unchecked, spider mite infestations build up quickly in warm temperatures and can destroy plants when infestations are not detected early enough. Thin-leafed plants are more susceptible to spider mites than plants with thick or waxy leaves.
Managing spider mites
Spider mites can threaten the health or appearance of your plants. Therefore, it is important to balance cultural, biological, and chemical control methods all season.
Inspect your plants once every two weeks for spider mites. This means placing a sheet of paper under a branch and hitting the plant sharply. Use an index card to sample bedding plants.
If you are monitoring every 2 weeks you should consider applying a pesticide when an average of 2 dozen mites fall from a branch each time you strike it over an 8 x 11 inch sheet of paper. Keep in mind that spider mites must migrate to stems of deciduous woody plants.