Two Spotted Spider Mite

Two spotted spider mite, Tetranychus urticae , is an occasional pest of soybeans in northern united states. The pest has a worldwide distribution.

Two-spotted spider mite pest outbreaks have been associated with drought conditions; however, every field has a small subeconomic population of the pest. In newyork, field corn is seldom damaged economically. In drier areas where corn is grown, such as Newjersey, Connecticut and New york, spider mites are a frequent and significant economic problem.

The eggs are minute, spherical-shaped, bead-like objects that can be found on the underside of the leaf, usually near the leaf veins. The adults are very small, about 1/60 of an inch, and can be white, green, orange or red. They have four pairs of legs, which is a characteristic that distinguishes them from insects that have three sets. A set of reddish to brownish spots on their back give the species its common name. The larva and nymph look similar to the adult but are smaller. The larvae have only three sets of legs.

Life cycle:Two-spotted spider mites overwinter as adult females in a dormant state under soil clods, in tree bark crevices, in leaf litter, and in clumps of grass. Females become active in early spring and begin laying eggs in the field or on grasses around the field edge. Depending on air temperatures, eggs hatch about 3 to 10 days after being laid. Mites pass through five stages of development: egg, larva, protonymph, deutonymph, and adult. Between the larval and nymphal stages is a resting stage called a chrysalis. Once adults become active in the spring, the population goes through a generation every 7 to 21 days, depending on the ambient temperatures in their environment.

Spider mite development differs somewhat between species, but a typical life cycle is as follows. The eggs are attached to fine silk webbing and hatch in approximately three days. The life cycle is composed of the egg, the larva, two nymphal stages (protonymph and deutonymph) and the adult. The length of time from egg to adult varies greatly depending on temperature. Under optimum conditions , spider mites complete their development in five to twenty days. There are many overlapping generations per year. The adult female lives two to four weeks and is capable of laying several hundred eggs during her life.

Damage:Early symptoms of spider mite injury appear as leaves with a yellow stippled look along the field margins. As the populations continue to build and injury increases, the yellowing spreads across the field and the area of yellow leaves expands and may turn red. The underside of leaves will have significant silk webbing and small, white spots that are the cast skins of the mite.

If the population is not controlled, the yellow leaves will turn brown as the leaf loses moisture and dries up. Continued dry conditions and increasing mite populations can result in the significant loss of leaf area and death of plants. In severe cases, premature leaf senescence will occur. Sublethally injured plants will be stunted (shortened internodes) and have fewer beans. Field corn is typically attacked after reproductive stages are reached, so injury only tends to affect the weight of grain. In silage cornfields, the quality of forage may be affected.

Feeding by the twospotted spider mite, which consists of piercing and sucking of cell contents, occurs on the lower surface of leaves. Damage is expressed as stippling, and bronzing of the leaves and leaf veins. Feeding is particularly damaging during the first 4 to 5 months following transplanting in fall. Their rapid developmental rate and high reproductive potential (about 50-100 eggs per female) enables them to reach damaging population levels very rapidly under good growing conditions.

Mite densities of five per leaflet during this critical period of plant growth substantially reduce berry number and overall plantation yield. Plants that sustain infestations of greater than 75 mites per leaflet may become severely weakened and appear stunted, dry, and red in coloration . The highest mite populations are often observed following the peak spring fruit harvest, and this peak is typically followed by a rapid, natural decline in mite density.

Control:Predators play an important role in keeping twospotted spider mite populations in check. Some predators, such as the mite, occidentalis, and Amblyseius are commercially available for release . Inoculative releases can be made when twospotted mites are first found in the field. Inoculative releases into hot spots may also aid in suppressing infestations. Subsequent innundative releases of predaceous mites may reduce twospotted mite infestations. Following releases of predator mites, it is important to monitor spider mites to determine if they are being maintained below economically injurious levels. Choose insecticides, miticides, and fungicides carefully to prevent killing the predators.

Strawberry cultivars vary in susceptibility to twospotted spider mite. Short-day cultivars are generally more tolerant of mite feeding than day-neutral cultivars, particularly later in the fruit-production season. Vernalization directly promotes plant vigor. Supplemental cold storage can affect a plant’s vernalization. Plants with low amounts of chilling will have low vigor and will often develop intolerable mite infestations. Excessive chilling will promote increased vigor and reduce mite abundance, but other production factors are adversely affected .

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