From apples to zucchini – no matter what types of plants you grow – it's likely spider mites will attack. The most common spider mite, the twospotted spider mite (Tetranychus urticae), has been recorded on more than 300 species of plants, including all of the tree fruit crops, as well as small fruits, vegetables, and ornamentals.

The first sign of infestation by twospotted spider mite is usually a chlorotic, stippled appearance on the leaves, although this may not be as apparent on thick-leaved plants. As the mites feed on the underside of leaves, they remove leaf cell contents, including the chlorophyll that gives the leaves their green color. Without the chlorophyll, those empty cells appear whitish or bronze. Heavily infested leaves turn completely pale, dry up, and fall off. Large populations can severely defoliate or kill plants.

The mites also produce fine silken webs over which to crawl. The undersurfaces of leaves are often completely covered with webbing. When the mites are numerous, their webbing can cover foliage and flowers, detracting from the appearance of the plant.

Spider mites are not insects but are more closely related to spiders. These arachnids have four pairs of legs, no antennae and a single, oval body region. Most spider mites have the ability to produce a fine silk webbing. Spider mites are very tiny, being less than 1/50 inch (0.4mm) long when adults.

Quick Facts...

• Spider mites are common plant pests. Symptoms of injury include flecking, discoloration (bronzing) and scorching of leaves. Injury can lead to leaf loss and even plant death.
• Natural enemies include small lady beetles, predatory mites, minute pirate bugs, big-eyed bugs and predatory thrips.
• One reason that spider mites become a problem is insecticides that kill their natural predators.
• Irrigation and moisture management can be important cultural controls for spider mite

Spider mites are common pest problems on many plants around yards and gardens. Injury is caused as they feed, bruising the cells with their small, whiplike mouthparts and ingesting the sap. Damaged areas typically appear marked with many small, light flecks, giving the plant a somewhat speckled appearance.

Following severe infestations, leaves become yellow and discolored, producing an unthrifty gray or bronze look to the plant. Sometimes there can be a lot of very fine webbing on the plant foliage. Foilage may ultimately become scorched and drop prematurely. Spider mites frequently kill plants or cause serious stress to them.  

Generally mites feed on the undersides of leaves. They use their sucking mouthparts to remove sap from plants, giving the upper leaf surface a speckled or mottled appearance. Leaves of mite infested plants may turn yellow and dry up, and plants may lose vigor and die when infestations are severe. The undersides of affected leaves appear tan or yellow and have a crusty texture. Heavy infestations of the two-spotted spider mite produce fine webbing which may cover the entire plant.

Spider mites (Family: Tetranychidae) are classed as a type of arachnid, relatives of insects that also includes spiders, ticks, daddy-longlegs and scorpions. Spider mites are small and often difficult to see with the unaided eye. Their colors range from red and brown to yellow and green, depending on the species of spider mite and seasonal changes in their appearance.

The most important spider mite is the twospotted spider mite (Tetranychus urticae). This mite attacks a wide range of garden plants, including many vegetables and flowers. The twospotted spider mite is also the most important species on house plants. It is a prolific producer of webbing.

Twospotted spider mites range in color from light yellow or green to dark green or brown. All  have two dark spots visible on the abdomen. Males are smaller and more active than females and have a narrower body with a more pointed abdomen, and larger legs. Females lay relatively large eggs in webbing on the underside of the leaves. Each female produces up to 19 eggs per day and a total of up to 120 eggs, depending on the host plant and temperature. The spherical eggs are transparent when first deposited, but gradually turn yellowish. The larvae that hatch from these eggs after 3-15 days (depending on temperature) are colorless and have only 6 legs. The next two nymphal stages are pale yellowish to green with 8 legs, and the two dark spots can be seen. The mites may mature into adults in as few as 5 days during hot, dry weather.

Many spider mites produce webbing, particularly when they occur in high populations. This webbing gives the mites and their eggs some protection from natural enemies and environmental fluctuations. Webbing produced by spiders, as well as fluff produced by cottonwoods, often is confused with the webbing of spider mites.

The twospotted spider mite is an example of a 'warm season' mite. This pest has been reported from over 180 host plants including field crops, ornamental plants, house plants and weeds.

The females overwinter in the soil or on host plants. The females become active in April and May when they seek out the undersides of leaves on suitable hosts. Each female may lay over 100 eggs. A single generation may require as much as 20 to as few as five days, depending on the temperature. These mites prefer hot, dry weather and often do not reach damaging populations in cool, rainy periods.

In the summer, the adults and nymphs are white with two greenish spots. However, overwintering females usually turn reddish-orange and can be mistaken for other mite species.

Outdoors, twospotted spider mites overwinter as females resistant to low temperatures. These females are bright orange or red and do not feed. Development stops when temperatures are below about 55°F.

The eight-legged female mites are yellow to dark green with two to four dark dorsal spots as shown below: 
R. Bessin, Univ. of Kentucky

One of the biggest problems with recognizing that spider mites are causing a problem is the fact that they are small – usually only 1/50 inch (0.5 mm) long when mature – and they spend most of their time on the underside of the leaves. Typically mites are discovered only when damage on the plant becomes noticeable.

Life Cycles and Habits

Spider mite species seem to be warm weather or cool weather active pests. 

Two-Spotted spider mites do best in dry, hot summer weather. Spider mites develop from eggs, which usually are laid near the veins of leaves during the growing season. Most spider mite eggs are round and extremely large in proportion to the size of the mother. Left: Twospotted spider mites, with eggs.

The eggs hatch in days to weeks into the first stage, called a larva. Larvae are round bodied and have only three pairs of legs. The larvae feed for a few days, seek a sheltered spot to rest and then molt into the first nymphal stage. The first nymph now has four pairs of legs. The first nymphs feed a few days, rest and molt into the second nymph. The second nymphs feed, rest and molt into the adult stage. The males are usually the size of the second nymph and have pointed abdomens. The females have rounded abdomens and are the largest mites present.

Males are smaller with more pointed abdomens. The tiny, spherical, eggs are laid on the undersides of leaves, often under the webbing produced by the mites. A six-legged, colorless larva that emerges resembles the nymph and adult, but is only the size of an egg. Both of the eight-legged nymphal stages look like the adult, but are smaller and not sexually mature. Under optimum conditions of high temperature and low humidity, the life cycle may be completed in 7 days. Females can lay 200 eggs.

Most spider mites spend the winter in the egg stage but the twospotted spider mite overwinters as adult females resting in protected places.

There is some variation in the habits of the different mites that attack garden plants, trees and shrubs. Outdoors, the twospotted spider mite and honeylocust spider mite survive winter as adults hidden in protected areas such as bark cracks, bud scales or under debris around the garden. Other mites survive the cool season in the egg stage. As winter approaches, most mites change color, often turning more red or orange. This habit may be why they are sometimes called "red spiders."

Most spider mite activity peaks during the warmer months. They can develop rapidly during this time, becoming full-grown in as little as a week after eggs hatch. After mating, mature females may produce a dozen eggs daily for a couple of weeks. The fast development rate and high egg production can lead to extremely rapid increases in mite populations.

Other species of spider mites are most active during the cooler periods of the growing season, in spring and fall. This includes the spruce spider mite and most of the mites that can damage turfgrass. These cool-season spider mites may cease development and produce dormant eggs to survive hot summer weather.

Dry conditions greatly favor all spider mites, an important reason why they are so important in the more arid areas of the country. They feed more under dry conditions, as the lower humidity allows them to evaporate excess water they excrete. At the same time, most of their natural enemies require more humid conditions and are stressed by arid conditions. Furthermore, plants stressed by drought can produce changes in their chemistry that make them more nutritious to spider mites.

Control

Early detection of spider mites, before damage is noticed, is important.  The tiny spider mites can be detected by taking a piece of white paper or cardboard and striking some plant foliage on it.  The mites can be seen walking slowly on the paper. If 10 or more mites per sample are common, controls may be needed.

Biological Controls

Various insects and predatory mites feed on spider mites and provide a high level of natural control. One group of small, dark-colored lady beetles known as the "spider mite destroyers" (Stethorus species) are specialized predators of spider mites. Minute pirate bugs, big-eyed bugs (Geocoris species) and predatory thrips can be important natural enemies.

A great many mites in the family Phytoseiidae are predators of spider mites. In addition to those that occur naturally, some of these are produced in commercial insectaries for release as biological controls. Predatory mites often have fairly high requirements for humidity, which can be limiting. Most suppliers provide information regarding use of the predator mites that they carry. 

In addition to predatory mites there are numerous insects (lacewings and lady beetles) that prey on spider mites.  If predators are used, do not apply pesticides that will kill them.

Water Management

Since rainy weather seems to knock off spider mites, using a forceful jet of water from a hose (syringing) can perform the same task. A regular syringing can keep spider mites under control on most ornamental plants in the landscape. This technique also helps conserve natural predators.

Adequate watering of plants during dry conditions can limit the importance of drought stress on spider mite outbreaks. Periodic hosing of plants with a forceful jet of water can physically remove and kill many mites, as well as remove the dust that collects on foliage and interferes with mite predators. Disruption of the webbing also may delay egg laying until new webbing is produced. Sometimes, small changes where mite-susceptible plants are located or how they are watered can greatly influence their susceptibility to spider mite damage. Use of overhead-sprinkler irrigation may provide some short- term relief of mite infestations.

       Above: Twospotted spider mite injury to eggplant.

Chemical Controls               

One reason that spider mites become problems in yards and gardens is the use of insecticides that destroy their natural enemies. For example, carbaryl (Sevin) devastates most spider mite natural enemies and can greatly contribute to spider mite outbreaks. Malathion can aggravate some spider mite problems, despite being advertised frequently as effective for mite control. Soil applications of the systemic insecticide imidacloprid (Merit, Marathon) have also contributed to some spider mite outbreaks.

Many insecticides used for control of insect pests severely reduce numbers of beneficial insects that keep mite populations in check. Therefore, apply insecticides only as-needed, rather than at regularly scheduled intervals. When possible, select pesticides which will have the least impact on beneficial insects.

"Soft Pesticides" Most spider mites can be controlled with insecticidal oils and soaps. The oils, both horticultural oil and dormant oil, can be used. Horticultural oils can be used on perennial and woody ornamentals during the summer at the 1 to 2 percent rate. Higher rates of horticultural oil (3 to 4 percent) or dormant oil are useful for killing mite eggs and dormant adults in the fall and spring. The insecticidal soaps are useful in the warm season. Remember that mites are very tiny and soaps and oils work by contact only. Therefore, thorough coverage of the plant is necessary for good control.

Miticides Spider mites are usually not killed by regular insecticides, so be sure to check the pesticide label to see if "miticide" is present. Pesticides claiming "for mite suppression" are usually weak miticides and will not perform well. There are few products available to the homeowner. Dicofol (=Kelthane) is registered for over-the-counter use but is difficult to find. Acephate (=Orthene), dimethoate (=Cygon), chlorpyrifos (=Dursban), diazinon, disulfoton (=Di-syston), and malathion have over-the-counter product labels but are considered weak miticides.

Avermectin (=Avid), bifenthrin (=Talstar), dienochlor (=Pentac), fenbutatin-oxide (=Vendex), fluvalinate (=Mavrik), oxamyl (=Vydate), oxydemeton-methyl (Metasystox-R), oxythioquinox (Morestan), and propargite (=Omite) are restricted use pesticides available only to licensed applicators.  

(Control Strategies provided by Ohio State University Extension.)

Chemical control of spider mites generally involves pesticides that are specifically developed for spider mite control (miticides or acaricides). Few insecticides are effective for spider mites and many even aggravate problems. Furthermore, strains of spider mites resistant to pesticides frequently develop, making control difficult. Because most miticides do not affect eggs, a repeat application at an approximately 10- to 14-day interval is usually needed for control. Table 1 includes a summary of pesticides that may be useful for managing spider mites.

Resistance to pesticides has increased the difficulty of controlling of these pests. Because mites primarily occur on the undersides of leaves, applications of contact miticides need to be directed at both the lower and upper leaf surfaces. Mite eggs are resistant to some miticides, so repeated applications are often necessary to control infestations.

Chemical control is often the best option for battling spider mites. 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 mites are on the underside of the leaves, so that's where the soap spray needs to be applied.

Insecticidal soaps are very effective and should be the first choice for most spider mite problems. However, other chemicals with residual activity can be used when there is no danger of children or pets contacting the leaves. Most insecticides no longer work against spider mites because of resistance problems. The product ISOTOX IV has both an insecticide and a miticide and can be used on outdoor ornamental plants. Products called miticides (or acaricides) are made to control spider mites, but these products can be hard to find or are not sold to homeowners.