Spider mites

Spider mites are one of the most serious pests of ornamental crops, especially in hot, dry conditions.The twospotted spider mite is the most important mite species infesting floral crops. It has an extremely wide host range, short generation time, and continuous reproduction when not in diapause.

Damage

Twospotted spider mites feed on the underside of leaves, removing leaf cell contents.This results in a chlorotic, stippled appearance on the leaves of most plants. However, this stippling may not be as apparent on thick-leaved plants such as chrysanthemum. Leaves often dry out and fall off. Large populations can severely defoliate or kill plants. Webbing produced by spider mites can cover foliage and flowers, detracting from the appearance of the plant. Flower quality and yield is reduced on roses. Northern greenhouses may receive a respite from spider mites beginning September and continuing through February, if they go into diapause.

Carmine spider mites do not produce the typical speckling of leaves. Instead, infested leaves become prematurely chlorotic with small, transparent lesions. This type of damage resembles magnesium deficiency. Bright yellow patches develop that eventually turn dark and spread over the entire leaf.This damage may be caused by a toxin injected during feeding. Even small populations of carmine spider mites can cause extensive damage.

Feeding damage from the Lewis mite creates a distinctive stippled appearance on the leaves. Eventually the entire leaf becomes bleached, dies, and falls off.

Description and life cycle

Spider mites are small, usually only 1/50 inch (0.5 mm) long when mature. 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.The carmine spider mite is plum red. Males tend to be smaller and thinner than females.

Twospotted spider mite females lay eggs in webbing on the underside of the leaves. Each female produces up to 120 eggs, depending on the host plant. On chrysanthemums an average of 14 eggs are laid over a 5-day life span, whereas on rose they can lay 112 eggs over 15 days.The pearly white, perfectly spherical eggs hatch into larvae in 4-6 days.The larvae are small and white with only six legs.They molt into eight-legged nymphs and look like the adult. Adults appear after a second nymphal stage.

Lewis mites are slightly smaller than twospotted mites and have up to three spots on each side of their body. Their biology is similar to twospotted mites, but they do not diapause.This mite has been a problem on poinsettia since the mid-1990s, although it will feed on other plants such as cucumber.

Spider mites can produce a generation in 7-14 days in warm temperatures. In cooler temperatures, generations take more than 40 days; below 54°F, development stops. In northern greenhouses adults may go into diapause for several months, hibernating in cracks and crevices from September through February/March.The diapausing mites, which are reddish and do not feed, may be confused with predators or carmine spider mites.

Monitoring

Frequent visual inspection of plant parts is the best method for detecting infestations of spider mites. Because of their small size and cryptic behavior, they are often overlooked until their feeding damage or webbing becomes apparent. They can be found on all areas of the plant, but they most often infest the older, middle-age leaves, and the midrib.

Small-scale growers could consider using plants that spider mites prefer more than the crop plant to detect small populations of mites before damage occurs on the crop—but this is not practical in large operations. Highly preferred plants that can be used this way, as "indicator plants,"include lima beans (tall plants that easily attract blowing mites) and radishes (much shorter plants that show dramatic symptoms of leaf distortion).

Natural enemies

Predators are the only effective natural enemies of spider mites. Numerous species of predatory mites, some beetles, and a few thrips feed on spider mites. Mites of the family Phytoseiidae are the most important predators of spider mites and the most commonly used natural enemies for controlling spider mites in greenhouses.

Predatory mites

Euseius spp. Several phytoseiids in this genus are commercially available, but these predatory mites have not been evaluated for use on greenhouse crops. E.delhiensis (=rubini) will feed on spider mites, broad mites, thrips, and whitefly eggs. E.hibisci feeds on various mites, as well as thrips and sweet potato whitefly eggs. E.scutalis occurs mainly on trees and shrubs in North Africa and the Middle East, where it is associated with tetranychid mites, whiteflies, and tenuipalpid mites. Although it is a general feeder and will attack thrips, spider mites, and whitefly eggs, E. scutalis prefers pollen and tetranychid eggs. E.stipulatus will develop on twospotted spider mite, but it does not lay eggs when feeding on this mite without supplemental food such as pollen or honeydew. Eggs of this species fail to hatch at relative humidities less than 50%.

Galendromus (=Metaseiulus, =Typhlodromus) occidentalis— western predatory mite. This phyto-seiid mite is smaller than Phytoseiulus persimilis (next page) and develops best at cooler temperatures, but it tolerates a wide range of relative humidities (40-80%). It is able to regulate spider mite populations at lower densities and for longer periods of time than P. persimilis, although it will not control spider mite populations as quickly. It can also survive long periods without prey. Nondiapausing strains have been developed that allow control of spider mites through the winter, when days are short. Some strains are resistant to a number of organophosphate insecticides (such as azinphos-methyl, diazinon, and phosmet) and a carbamate (carbaryl). Several different strains are commercially available.

Mesoseiulus (=Phytoseiulus) longipes.

This predator is similar to P.persimilis in activity, but can tolerate warmer tem-peratures—up to 70°-90°F—and relative humidity as low as 40%. It is available commercially.

Neoseiulus (=Amblyseius) californicus.

This predatory mite is similar to P.persimilis, but is smaller, pale, and does not suppress spider mite populations as quickly. However, it is useful for keeping low populations under control because it can survive longer periods without prey. It is commercially available.

Some other species of Neoseiulus, such as the commercially available N.fallacis, feed on a variety of tetranychid mites and are commercially available, but little is known of their utility in greenhouses.

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