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From apples to zucchini – no matter what types of plants you grow
– it's likely
something 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.
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.
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.
Outdoors, twospotted spider mites overwinter as females resistant to low temperatures. These females are bright orange or red and do not feed. Indoors, mites continue to feed and reproduce through the winter, but at a slower rate than at other times of the year. Development stops when temperatures are below about 55°F.
Fortunately for us in the Midwest, spider mites are not particularly problematic outdoors on most plants. Mites develop best under hot, dry conditions and we generally have high enough humidity to prevent them from doing too well. During periods of drought or in specific microclimates, however, they can be devastating. Even as little as a month without significant rain during the growing season can favor a mite outbreak.
Indoors, especially in greenhouses, twospotted spider mite can
be a difficult pest to control. It's very important to know if mites are
developing on your plants. Frequent visual inspection of plant parts is the best
way to keep track of whether mites are present. They can be found on all areas of the plant, but they most
often infest the older, middle-age leaves, and the midrib. Once their feeding damage or webbing
becomes apparent, populations are already well-established and more difficult to
control. On houseplants spider mites often become a problem in November when the
heat comes on and the air is drier.
Predatory mites of the family Phytoseiidae are the most important predators of spider mites. Naturally-occurring species often keep spider mites under control on outdoor plants. The are also the most commonly used commercially available natural enemies for controlling spider mites in greenhouses. In hobby greenhouses they will be effective only if the humidity is generally above 70%. They are not recommended, however, for controlling heavy infestations indoors, or problems on individual plants.
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. The miticide KELTHANE cannot be sold or used in Wisconsin.
