Spider Mites

by Marcie Crockett
Spider mites are a major pest on a wide variety of garden and landscape plants.   They are minute arthropods whose feeding can cause yellowing, stunt growth and   reduce yields.

Identification – Most Common Species

Twospotted Spider Mite – Tetranychus urticae

The “two spot” spider mites are usually yellow/tan/ greenish in color   (Figure 1), and by maturity they grow two dark spots on their shoulders, one   on each side.

Spider mites can float along with wind currents, or be carried by pets, clothing   or infested plant material. Adult two-spotted spider mites are very small (ca.   1/60 inch in length), eight-legged arthropods. The color of the mites usually   is green or greenish-yellow but can vary from white to light red. The eggs of   the mites appear like small clear or pale marbles when viewed through a good   hand lens.

Spruce Spider Mite – Oligonychus ununguis

The spruce spider mite is common throughout the Northwest. It feeds almost    exclusively on conifers, often reaching tremendous numbers. The spruce spider   mite is generally dark green in color. Trees damaged by this mite show light-yellow   stippling on the needles or leaves. In heavy infestations webbing will be present   and needles turn brown and drop from the tree. Overwintering eggs are deposited   near needle bases on the tree.

Hosts

From apples to zucchini, no matter what types of plants you grow, it’s likely   something spider mites will attack. The most common spider mite, the two-spotted   spider mite has been recorded on more than 300 species of plants, including   all of the tree fruit crops, as well as many small fruits, vegetables, and ornamentals.

Life Cycle

The two-spotted spider mites overwinter as adult females. Egg laying begins   in late April or May. Larvae hatch from the eggs, which develop to the nymph   and finally the adult. Larvae, nymphs and adults all feed on the plant. The   time from egg to adult normally requires about 3 weeks, but may take less time   under hot and dry conditions. Depending on weather conditions, 5 to 10 generations   may occur within a growing season. Spider mite reproduction and population multiplication   will persist until cool weather of late summer leads to a reduction of reproductive   activity. In the spider mite outbreak of 1988, many heavy infestations totally   collapsed around late August. A population collapse may be due to predators   or an outbreak of a fungal organism that attacks mites.

Special Species Notes: Adult females have the ability to go dormant for a time   after the daylength shortens, then re-emerge to lay more eggs a few weeks after   the days lengthen again. That’s one reason spider mites keep reappearing crop   after crop on indoor plants.

Damage

The first sign of spider mite presence is little yellow speckles on leaf surfaces   (Figure 2). Spider mites can be found on either leaf surface although they commonly   infest the underside.

Their eggs, best seen with a magnifier, will be scattered around at random.   Spider mite eggs are all perfectly round, the same size, ranging from clear   to amber in color. With larger infestations a fine webbing, crawling with mites,   covers the plant tops (Figure 3). In severe infestations, the leaves become   brown and dry.

Monitoring

Indoors, especially in greenhouses, two-spotted spider mite can be a difficult    pest to control. It’s very important to know if mites are developing on your   plants before damage becomes severe. Frequent visual inspection of plant parts   is the best way to keep track of whether mites are present. They can be found   on all areas of the plant, but they most often infest the older, middle-age   leaves, and the midrib. Once their feeding damage or webbing becomes apparent,   populations are already well established and more difficult to control. On houseplants,   spider mites often become a problem in November when the heat comes on and the   air is drier. On outdoor plants predatory mites are important natural controls   of spider mites

Management-Biological control

Predaceous mites can keep spider mites under control if broad-spectrum insecticide   applications are avoided. Heavy rain and cold weather also suppress mite numbers.

 

 

 

 

Management-Cultural control

Broad-leaved weeds like mallow, bindweed, white clover, knotweed, and others   are hosts for spider mites and can enhance mite numbers. Suppression of these   weeds with cultivation or grasses may reduce mite problems. Spider mites may   be washed from plants with a strong stream of water. Water plants properly,   as drought-stressed trees are more susceptible. Avoid excessive nitrogen applications,   as this encourages mites.

Management – Insecticides

Frequent applications of insecticidal soap can be very effective. Soap has   no residual activity and only affects the mites it is sprayed directly on, so   thorough coverage of the foliage and repeated applications are essential. Most   of the spider mites are on the underside of the leaves, so that’s where the   soap spray needs to be applied.

Also recommended for this area are:

  • Beauvaria bassiana -a fungus
  • Horticultural oil
  • Dimethoate (cygon)

Read and follow label directions and precautions making certain that the product   that you purchase is registered for the target pest.

Sources: Ohio Pest Management & Survey Program Field Crops Pest Management   Circular # 24,Department of   Entomology, University of Wisconsin – Madison. Susan Mahr and Phil Pellitteri,   and PNW Insect Management Handbook 2003

March 31, 2003

Posted in Insects