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EUROPEAN RED MITE Panonychus ulmi (Koch) The European red mite is an introduced pest in the US. It is a pest of nut, pome and stone fruits, as well as some berries, damaging leaves and causing fruit russetting. All active stages of the European red mite injure the foliage by feeding with piercing mouthparts and removing cell contents, including chlorophyll. Moderate to high numbers of mites can cause the leaves to initially turn pale, then bronze as feeding continues. Heavy mite feeding early in the season can reduce tree growth, yield, and also affect fruit bud formation for the following year. Some apple cultivars, such as 'Red Delicious' and 'Braeburn', are especially prone to mite buildup and injury. Description: The adult female mites are brick red with white spots at the base of six to eight hairs on their back. The male mite is more slender and lighter in color than the female, with a more pointed abdomen. Eggs are red, globular and somewhat flattened (onion-shaped) with a slender stalk on the upper side.
Life Cycle: European red mites overwinter as eggs laid in roughened bark around the bases of buds and spurs on small branches. During the summer, eggs are laid on the underside of leaves. Egg hatch in the spring is closely correlated with bud development and begins close to the tight cluster stage. During the summer, eggs require 7 to 14 days to hatch. European red mites can have six to eight generations per year, depending on the temperature. Summer generations may develop in as few as 14 days. Monitoring: To monitor for mites, examine five hardened-off leaves from each of four scaffold limbs per tree. Commercial orchardists should examine at least five trees per acre. Certain varieties, such as Red Delicious, are more likely to develop large numbers of mites, so be sure to make samples representative of the varieties in the orchard. Using a hand lens, count all active stages of pest and predatory mites. Predatory mites are more active and are tear-drop shaped. Determine the average number of mites per leaf. The economic threshold for the mites varies with the time of year. A miticide is recommended early in the year (until April 1) if numbers of active mites exceed an average of five per leaf, during April and May when mite numbers exceed 10 per leaf, or the rest of the season if mite numbers exceed 15 per leaf. Control: European red mites are rarely a problem on backyard apple trees. Predatory mites help to maintain the European red mite at nondamaging levels. This mite is considered a secondary pest, as it typically only builds to damaging levels after its natural enemies have been depleted by insecticide applications used to control codling moth or other pests. Minimizing insecticide usage and selecting insecticides that are least toxic to beneficial organisms will help to minimize problems with this mite. Overwintering mite eggs should be controlled through the use of a delayed-dormant oil treatment, anytime between just before bud swell until 12 mm (1/2 inch) green. Control with dormant oil improves the closer to egg hatching. Management of mites during the growing season is based on scouting and the use of miticides or summer oil treatments as needed. Often when heavy summer infestations exist, a second miticide treatment may be required 10 to 14 days later. Horticultural oils provide an alternative to traditional synthetic miticides, are able to kill all mite life stages, and are less toxic to the applicator. While effective control can be obtained with summer horticultural oil treatments, caution is advised as these may be incompatible with some other pesticides within seven to ten days of application (particularly products containing sulfur and the fungicide Captan), are phytotoxic at higher temperatures (above 90 degrees F and high humidity), and may affect fruit finish on some varieties. TWOSPOTTED SPIDER MITE Tetranychus urticae (Koch) The twospotted spider mite is widely distributed and a common pest of orchards and nursery plants. When environmental conditions are hot and dry, spider mites multiply rapidly and become a pest of fields crops such as corn, soybeans and cotton. Initial infestations tend to occur in fields bordering clover fields or grassy areas. Field perimeters and corners tend to exhibit the earliest symptoms of infestation. Dispersal over a wide area occurs when spider mites are carried on a balloon of their webbing by the wind. Twospotted spider mites feed on the underside of the foliage with sucking mouth parts and may be very destructive when abundant. Under hot and dry field conditions, spider mites thrive on plants that are under stress. The juices which the mites obtain from stressed plants are rich in nutrients and the mites multiply rapidly. Description: Adult twospotted spider mites are only about .35 mm (1/60 inch) in length, and have a black spot on each side of their bodies, which range in color from white to light red. The eggs of the mites appear like small clear or pale marbles when viewed through a hand lens.
Life Cycle: Twospotted spider mites overwinter as adult females. Egg laying begins in late April or May; the eggs hatch in five to eight days into the protonymph stage, which later molts to a deutonymph stage. These nymph stages have four legs. The time from egg to adult normally requires about three weeks, but may take less time under hot and dry conditions. Depending on weather conditions, 5 to 10 generations may occur within a growing season. Spider mite reproduction and population multiplication will persist until cool weather of late summer leads to a reduction of population activity. Monitoring: In assessing the degree of a spider mite infestation, it is important that one recognize the stippling or speckled effect on green foliage which is the early sign of mite feeding. It is essential to use a good hand lens to view the relative abundance of mites in egg, nymph and adult stages. Control: When conditions are optimal for spider mite outbreaks, early detection facilitates timely and effective treatment. Since mite development is linked to host plant stress, cultural practices and varieties which limit plant stress in times of drought will also minimize the development of spider mites. Narrow-range oil sprays should provide effective control. Twospotted spider mites are known to have developed resistance to miticides quite rapidly if a miticide is applied repeatedly to a population. Alternate miticides and avoid unnecessary spraying.
Adapted from Ric Bessin, University of Kentucky College of Agrictulture and Harold Willson, Ohio State University Extension, 1999
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