Frank Peairs, Colorado State University, Bugwood.org
The two-spotted spider mite, Tetranychus urticae, is an occasional pest of soybeans in Pennsylvania. The pest has a worldwide distribution. Over the past 15 years, pest outbreaks have been associated with drought conditions; however, every field has a small subeconomic population of the pest. In Pennsylvania, field corn is seldom damaged economically. In drier areas where corn is grown, such as western Kansas, Oklahoma, Nebraska, and Texas, spider mites are a frequent and significant economic problem.
Two-spotted spider mite. E.G. Rajotte - Penn State
The Problem in Pennsylvania
Over the last 15 years, soybean acreage has increased from about 250,000 to 450,000 acres. With this increase, the frequency of spider mite injury has also increased. Since 1985, spider mite outbreaks have occurred about one in four years. In outbreak years, about 10 to 15 percent of soybean fields required an insecticide treatment to prevent yield reductions.
Outbreaks of the pest are associated with extended hot and dry weather. The combination of hot and dry conditions lead to mite population explosions. High temperatures reduce the time required for a mite to complete its life cycle from egg to adult. Therefore, the pest can complete more generations during the growing season. Unlike many pests, mites complete multiple generations during the growing season. Almost all of the individuals in the population are females. Each female produces about 300 offspring during her lifetime, which can last up to 30 days.
When conditions are dry, natural diseases of the mite population do not help slow population growth. These factors allow mite populations to go from a few individuals to millions within a few generations. For this reason, mite outbreaks appear to come out of nowhere. A moderately infested field can lose 10 to 15 percent of its yield, while heavily infested fields can lose substantially more and, in rare cases, the plants can be killed.
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.
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.
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.
Mites injure the plant by piercing the individual cells with their mouthparts and consuming the cell's contents. Some researchers believe that salivary enzymes destroy the cells, and the mechanical injury causes increased water loss from the plant.
Spider mites are difficult to manage because of their rapid outbreak. The only cultural practice that can help is to keep weeds down around fields. Most mite populations first develop on grasses and other plants found along the margins of fields. The prereproductive female mites use a silken thread as a kite string to migrate into the field. By crawling to the top of plants and releasing the silken thread into the wind, they are picked up and transported up to 350 feet.
The only successful method of pest management is to scout soybean fields weekly and watch for evidence of developing populations. If hot, dry weather conditions are predicted into the near future, then immediate action should be taken. If heavy rains are predicted, it is likely that the population in the field will collapse after the rain. In large fields when damage is seen only along the field margins, a spray directed to the injured area and into the field about 100 feet farther may contain the problem. In small fields, the entire field should be sprayed.
The current arsenal of miticides available for soybeans does not provide long-term protection from the pest. Since the pest recovers rapidly after treatment, fields need to be monitored continually for resurging populations. It is seldom economical to treat a soybean field more than two times. Few cornfields in Pennsylvania are damaged economically by the pest.
Refer to the current issue of the Penn State Agronomy Guide for recommendations on spider mite control.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Dennis Calvin, Associate Professor of Entomology
September 9, 2000