Two-spotted Spider Mite
Tetranychus urticae Koch
Appearance and Life History
The two-spotted spider mite (TSM) is more closely related to spiders than to insects. There are thousands of species of mites. TSM is one of many phytophagous, or plant feeding species.
The TSM has four life stages; egg, larva, nymph, and adult. Under optimum conditions of high temperatures and low humidity, this mite can complete its development in 5 to 7 days. Under more normal temperatures and humidity, the generation interval is approximately 19 days. The following provides an overview of TSM life stages:
Egg - The spherical, shiny straw-colored eggs are very small. Webbing produced by the TSM, which helps fasten the eggs to the leaf surface, makes the eggs difficult to see. After an incubation period of from 3 (75°F or 24°C) to 19 (50°F or 10°C) days the eggs hatch.
Larva - The six-legged, colorless, larva resembles the body form of the nymph and adult. It is slightly larger than a TSM egg. During the larval stage, little food is consumed.
Nymph - The eight-legged nymph looks like an adult, but is ;smaller and not sexually mature. It goes through 2 nymphal stages, proto-nymph and deuto- nymph, before becoming an adult.
Adult - The adult female is eight-legged and about 1/60 inch (0.4 mm) long. The eight-legged male is only about 1/80 inch (0.3 mm) long. Their color varies from pale yellow to green to orange to brown. When viewed from above, they appear to have 2 pigmented spots, which are actually contents of the gut showing through the body wall. A female TSM will lay from 50 to 100 eggs during her lifetime. Unfertilized eggs develop into males, fertilized eggs into females. The sex ratio can vary considerably, but is usually female-biased.
The adult TSM overwinters in non-crop and weedy areas such as grassy waterways, roadsides, weeds, set-aside acres, and pastures. As temperatures warm in the spring, TSM begins feeding on common hosts such as clover, chickweed, and various grasses. Mating and egg laying soon follow and continue throughout the mite's lifespan. The time required to go through a generation is shortened by hot temperatures. Populations may increase significantly if high temperatures are associated with dry conditions. Dry conditions can reduce naturally occurring pathogenic fungi (primarily Neozygites sp.) and predators that normally keep TSM populations at non-economic levels.
TSM feeds by piercing leaf cell walls with its mouthparts and sucking out the cell's contents. These cells become non-functional, thus the damage is considered irreversible (no longer contributing to plant growth and development). However, this does not mean that plants are unable to produce new leaf growth if conditions conducive to plant development improve.
In the early stages of TSM activity on corn, the feeding damage may not appear to be as pronounced as on soybean. The typical "V" or "U" pattern of infestation is either lacking or is not visible since initial feeding occurs on the lower leaves and is somewhat obscured by the larger upper leaves. As with soybean, the damage in corn will begin near TSM overwintering sites. Depending on the severity of the feeding, symptoms may include some leaf yellowing to brown leaves and a general decline in plant growth. The damage may be enhanced by poor growing conditions and/or soil compaction.
Upon closer inspection of TSM-infested plants, the leaves appear mottled or "sand blasted." Although TSM damage on corn can be very severe in the western Midwest, corn has not been severely impacted by the TSM in the eastern Midwest. Outbreaks of TSM have been noted where corn has been treated with some insecticides and fungicides. These pesticides, although not normally toxic to TSM, greatly reduced the numbers of predators and pathogens that normally keep TSM populations in check.
If hot, dry conditions persist for several weeks, watch for leaf discoloration (yellowing) especially along the field borders or near grassy areas within fields. Carefully inspect these areas for the presence of TSM. Shake some discolored leaves over a white piece of paper. Watch for small dark specks moving about on the paper. Also look for minute webbing on the undersides of the discolored leaves. These are sure signs of TSM activity.
Before considering control, it is very important that TSM are identified as the source of the problem since other crop stresses can cause leaf yellowing, such as diseases, herbicides, nutrient deficiencies, excessive moisture, or insects such as thrips. Once TSM has been positively identified in the damaged areas of the field, it is essential that the whole field be scouted to determine the range of the infestation. Sample at least 5 different areas of the field and determine whether TSM are present by shaking leaves over a piece of paper. If plant discoloration is noted, estimate the average percentage leaf discoloration for the plants within the damaged area. Repeat this procedure in other areas showing damage. Usually the areas that show discoloration first are those areas that are already stressed from factors such as poorly productive soils (sand or clay knobs, etc.) and/or soil compaction.
Corn Insect Control Recommendations: E-series 219-W (PDF)
Control may be necessary when 15% to 20% of the leaf area is covered with TSM colonies, moderate damage is noted, and hot, dry conditions are expected to continue. The greatest benefit from control normally occurs when miticides are applied from the pre-tassel through the soft dough stages of plant development. It is unlikely that the application of a control will be cost effective once the dent stage of plant development is reached. Thorough coverage of corn leaves with a miticide is difficult, but necessary for TSM control.
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.