Spider mites (Family Tetranychidae, Order Acari) are not insects; they are closely related to spiders, harvestmen (daddy longlegs), and ticks. Unlike insects which have six legs and three body parts, spider mites have eight legs and a one-piece body. They also lack wings, antennae, and compound eyes. Individual spider mites are almost microscopic, yet when they occur in large numbers, they can cause serious damage. Dozens of species attack shade trees, shrubs, and herbaceous plants.
Symptoms and Damage
Numerous species of mites known as spider mites can infest forest and shade trees. These tiny spider relatives have the ability to spin fine silken webs over the foliage of trees. Some of the more notorious species are also known as red spiders due to their red color. Depending on the species or stage of maturity, colors may vary in different shades of yellow, green, orange, and red. Mites feed by piercing stylets into the surface of the foliage to draw out plant juices. Their feeding destroys the chlorophyll bearing cells at the surface of leaves or needles and results in a stippling or mottling of the foliage.
|Webbing may not always be readily seen on the foliage depending on the level of infestation and species. Mites, which are barely visible to the naked eye, may be detected by shaking and beating suspected foliage over a white sheet of paper. Any mites that are present appear as tiny dots crawling over the paper.
All species of conifers and deciduous trees.
One of the most important species on conifers is the spruce
spider mite, Oligonychus unungius (Jacobi) It attacks hemlock, spruce, arborvitae, pines, and balsam fir and small junipers. Feeding damage occurs as tiny chlorotic flecks on the surface of needles and the foliage appears mottled. Fine webbing is also produced between the needles and foliage may collect dirt and dust. Damaged needles may dry up and drop off. Christmas trees may be severely damaged by this mite. Trees growing on poor sites may be killed.
Spider mites lack chewing or piercing-sucking mouthparts. Instead they have a pair of needle-like structures called stylets which are used to rupture leaf cells. A feeding spider mite pushes its mouth into the torn tissue and draws up cell sap. Small patches of cells are killed, resulting in a stippling or fine flecking on the upper surface of leaves, giving the leaves a "sandblasted" appearance. On heavily infested plants, the foliage will become bronzed, bleached, yellow, or grey. Untreated, such plants lose vigor, become progressively thinner, and may eventually die.
Spider mite damage to foliage is similar on all host plants: fine stippling which progresses to an overall bronzing of the leaves. With a hand lens, egg shells and cast skins are usually visible on the underside of damaged leaves. Mites can be observed by shaking infested leaves over a white piece of paper. The mites are about the size of the period at the end of this sentence.
Spruce Mite. This serious pest is found only on conifers, hemlock, arborvitae, spruce, fir, juniper, and, occasionally, pine. A fine webbing which collects dust and dirt is produced on the foliage where it feeds. Infested plants lose their color and the foliage becomes thin, because severely damaged needles drop prematurely . Treat twice, one week apart, in early May, and repeat in late September if necessary
Life Cycle and Habits
Most species overwinter as eggs although a few, including the honeylocust spider mite, overwinter as adults in bark crevices. The eggs hatch in the spring, and the six legged larvae feed on foliage and can reach maturity within a week. Mature mites have 8 legs, and are less than 1 mm long.
Most species overwinters as an adult in the soil; the honeylocust mite overwinters as an adult in bark crevices on the trunk and branches. Most other common species on trees and shrubs overwinter as tiny round eggs on leaves or bark. These eggs hatch in March or April. First- stage larvae have only six legs, but after molting, they become eight-legged nymphs. Both larvae and nymphs resemble the adults. There may be numerous overlapping generations and populations can build rapidly. Development time from egg to adult varies from five to 21 days depending on the species of mite and the weather. Many generations occur each year. Under optimal conditions, populations can build up very rapidly. Spider mites on conifers and broadleaved evergreens are cool weather pests. They feed heavily and reproduce quickly in spring and fall. Activity is low during the hot part of summer, although damage is often at a maximum and becomes easier to see when other plants are green and growing normally. Spider mites on honeylocust, linden, elm, willow, and oak are destructive in the summer. The two-spotted mite thrives whenever conditions are favorable for plant growth.
Other common spider mites are the European Red Mite, Clover Mite, Hickory Spider Mite, Linden Spider Mite, Elm Spider Mite, Honeylocust Spider Mite, Willow Spider Mite, Oak Red Mite, and the Maple Spider Mite.
Certain lady beetles, thrips, and predaceous mites provide some degree of natural control for spider mite populations, but usually only after mite infestations have become destructive. Natural enemies help keep mites at low levels when conditions are unfavorable for the mites. Most insecticides are not effective on mites and some, especially carbaryl (Sevin), result in increased mite damage by killing their natural enemies. Use a miticide as suggested in Virginia Pest Management Guides, available through your local Extension Agent. Always read the label before applying any pesticide.
If infestations involve only a few small trees, washing with a strong stream of water from a garden hose several times will sometimes reduce mite levels.
If the mite infestation is heavy and control is desired, the application of miticide such as dicofol should give effective control when applied as directed. Diazinon** also registered for use against mites. Refer to the container label for specific use instructions, dosages, and timing.
Mites have overlapping generations meaning egg nymph and adult forms may all be present at one time. Many pesticides are effective only on nymphs or adults hence a second application may be necessary 7 to 10 days after the first. Carefully read the label to determine if a second application is necessary.
*NOTE: ** Some formulations are restricted-use pesticides and may only be purchased or used by certified pesticide applicators.
Disclaimer: This article on Spider Mites may contain pesticide recommendations that, are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The Writer assumes no liability resulting from the use of these recommendations