Spider mites, (Family Tetranychidae, Order Acari), are not insects;
they are very small arachnids closely related to spiders and ticks.
Among plant pests, mites are among the most difficult to control, and
are responsible for a significant portion of all pesticides used on
ornamentals. Individual spider mites are almost microscopic, yet when
they occur in large numbers, they can cause serious plant damage. Many
different species attack shade trees, shrubs and herbaceous plants.
Once present, spider mites are seldom eradicated, but their numbers can
be reduced and managed at levels that are almost undetectable even by
the best of scouts.
Spider mites that commonly cause damage on ornamental plants include the twospotted spider mite, Tetranychus urticae Koch; the tumid spider mite, Tetranychus tumidus Banks; the Pacific spider mite, Tetranychus pacificus MacGregor; spruce spider mite, Oligonychus ununguius Jacobi; southern red mite, Oligonychus illicis
MacGregor. In addition, other spider mites such as the European red
mite, the boxwood mite, the fourspotted spider mite, and many
summer-feeding, tree-specific spider mites may cause damage to
The life cycle of the twospotted spider mite is typical of warm
weather spider mites, including the tumid spider mite. At 85 - 90
degrees F, complete development from egg to adult can occur in as
little as 7 - 8 days, and all life stages may be present throughout the
year, depending on the weather. When temperatures are cooler,
development may proceed more slowly, requiring up to four weeks for
completion. Host plant species, plant nutrition, leaf age, and moisture
stress also influence development. In temperate climates, twospotted
spider mites overwinter as adult mites in the soil, while most other
common spider mites on trees and shrubs overwinter as tiny round eggs
on leaves or bark. Many generations occur each year, depending on the
species of spider mite. Spider mites on conifers and broad-leaved
evergreens are cool weather pests, as are the southern red mite and the
European red mite. These spider mites feed heavily and reproduce
quickly in spring and fall.
Adult female twospotted spider mites are about 0.5 mm long, light
green with two brownish black spots on either side of the abdomen.
Color will vary according to diet and environmental conditions. Males
have pointed abdomens and are more slender than the rounded and plump
females. Females lay between 90 and 110 eggs during their lifetime.
Eggs hatch into six-legged larvae, which then develop into protonymphs,
followed by the deutonymph stage prior to adulthood. Under hot, dry
conditions, twospotted spider mites thrive: more eggs are laid,
development is at a higher rate, and survival of adults is extended.
Conditions of high moisture are known to slow the dispersal of mites.
Spider mites lack chewing or piercing-sucking mouthparts. They use a
pair of needle-like stylets to rupture leaf cells and then push their
mouth into the torn tissue to drink the cell sap. Small groups of cells
are killed, which results in a stippling or speckling on the upper leaf
surface. On plants which are heavily infested, the foliage will often
become gray, yellow, bleached, dry, or bronzed, with leaf drop, loss of
vigor and eventual death if untreated. With a magnifying hand lens,
cast skins, eggshells, and individual mites as well as mite colonies
are visible on the undersides of leaves. Webbing can cover terminal
growth in advanced infestations. Shaking infested leaves over a piece
of white paper permits detection of live mites, which are no bigger
than the period at the end of this sentence.