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.
of conifers and deciduous trees.
One of the most
important species on conifers is the spruce
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
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
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
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
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.
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.
Some formulations are restricted-use pesticides and may
only be purchased or used by certified pesticide applicators.
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