There are several different species of mites that can cause damage to a wide variety of ornamental plants, including deciduous trees and shrubs, evergreens, and garden plants. As the name implies, mites are so small that they often cannot be seen without the aid of a magnifying glass, which makes identification difficult. Often the first sign of a problem is the foliage’s sticky appearance, which is caused by extremely fine webbing produced by certain species of mites. More commonly, an infected leaf or needle will begin to lose its normal green color, followed by a flecked or stippled pattern. To verify the presence of mites, place a piece of white paper beneath an infested branch and gently shake the foliage. Mites will fall and appear as slow-moving specks on the paper.
Mites are not insects but tiny animals related to spiders. Mites feed by using their needle-like mouthpart to pierce the chlorophyll-bearing cells of a plant leaf and sucking out the plant juices. This repeated feeding action will produce a flecking, bleaching, or stippling pattern on the affected foliage. When significant populations are present, these injured areas can enlarge, turn yellow or brown, dry, and then drop from the plant. Infested deciduous leaves can become deformed or curl downward.
Although mites can appear any time during the growing season, most mite populations will increase quickly during hot, dry weather. Their development from egg to adult usually takes less than a week, and most mite species produce several generations each season. This combination of rapid growth and multiple generations makes them difficult to control.
TYPES OF MITES
Of the many species of mites in existence, the most common garden mites are the two-spotted spider mite, the spruce spider mite, and the European red spider mite.
Two-Spotted Spider Mite (Tetranychus urticae)
Like other members of the Arachnida (spider) class, the adult spider mite has 4 pairs of legs and the ability to produce silk, weaving it into a fine web over foliage or spinning threads on which to propel themselves to other plants. Most adults range in color from white to green or red, with two dark spots on either side of the body. Adult females overwinter in ground litter or under tree bark. The female will live about 2 to 4 weeks and during this time produce from 100-300 pale, translucent-white, round eggs. The two-spotted spider mite is more prolific in warm weather, and will attack more than 180 different trees and shrubs, as well as damage herbaceous plants, weeds, greenhouse plants, and field crops. The mite is especially destructive to winged euonymus (burning bush) in landscapes.
Spruce Spider Mite (Oligonychus ununguis)
In addition to spruce, this mite will also infest arborvitae, hemlock, juniper, Douglas-fir, pine, Siberian larch, and other conifers in both landscape and natural forest settings. Preferring cool weather, the spruce mite is primarily a problem in spring and fall, when it will feed on the older rather than newer needles of the host plant. A small, oval-shaped mite, it is light graygreen when young, turning dark green to black as it matures, although colors can vary. The round, brownish eggs overwinter under bud scales of needles, and under webbing on stems and branches. From the time the eggs hatch in early spring, successive populations develop at 2 to 3 week intervals, with three or more generations possible each year. Damage from mite infestation during the fall or subsequent spring often goes unnoticed until the heat and dryness of early summer. Heavy attacks can cause branch dieback or death of the plant. Damaged needles on spruce turn reddish brown.
European Red Spider Mite (Panonychus ulmi)
The European red spider mite is a warm season mite and a pest of deciduous shade trees, such as elms and silver maples. It also inflicts major damage on fruit trees, including apples, cherries, almonds, and plums. Adults are reddish- brown and oval-shaped with four rows of spines on the back. Overwintering eggs are bright red to orange, with an unusual bristlelike stalk protruding from the center. They are often laid in such large clusters that the twigs and branches appear to be covered with a fine red brick dust. Eggs begin to hatch in early spring with as many as seven generations produced each year.
The presence of a few mites on a host plant usually does not warrant enough concern to require any kind of control. The accumulation of dust and dirt on foliage tends to provide a favorable environment for many mite species, so periodically hosing the plant down with a strong stream of water will effectively remove or reduce the population. Some species, such as the red spider mite, are held in check by the presence of predator mites, but this balance can be upset when the plant is under stress from improper siting, drought, or other environmental conditions. Maintaining healthy, vigorous plants is a preventative measure in keeping mite populations low. Closely monitoring the extent of any damage and the size of the mite population will help determine whether further steps are required.
When infestations are severe enough to warrant more extensive control methods, thoroughly spray the foliage, paying particular attention to the undersides of leaves and needles. Because mites have a rapid growth cycle and are capable of producing several generations per growing season, repeated applications of chemical controls may be necessary, usually at one-week intervals.
Refer to the Illinois Urban Pest Management Handbook (University of Illinois Cooperative Extension Service) for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.