Identification & Distribution

Adult twospotted spider mites are very small (ca. 1/60 inch in length), eight legged arthropods (nymphs have 6 legs) with a black spot on each side of their bodies. Color of the mites is variable ranging from white to light red. The eggs of the mites appear like small clear or pale marbles when viewed through a good hand lens.

The twospotted spider mite is widely distributed and a common pest of orchards and nursery plants. When environmental conditions are hot and dry - such as during a drought, 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.

Field Symptoms

The major crop affected during periods of draught by spider mites in the Midwest is soybeans. Spider mites feed on the underside of the foliage with sucking moth 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.

Soybean foliage infested with spider mites initially exhibit a speckled appearance. As plants become heavily infested, foliage turns yellow, then bronze, and finally the leaves drop off the plants as the effect of heavy feeding leads to dehydration and death of the plant.

Figure 1. Spider Mite Damage on Soybean

In a year of a spider mite outbreak - when mite populations are widespread and rapidly multiplying, a field warranting rescue treatment may appear relatively green and healthy. Severely infested fields appear discolored and a potential yield loss may have already occurred due to a loss in vigor of plant growth. Heavily infested stands will exhibit a loss in plant stand.

Test plot experiments at the Ohio Agricultural Experiment Station demonstrated a 47% yield loss between soybeans treated and soybeans untreated where a severe infestation occurred.

Life Cycle

Twospotted spider mites overwinter as adult females. Egg laying begins in late April or May and hatch in 5 to 8 days into the protonymph stage which later molts to a deutonymph stage. These nymph stages have four legs. Thereafter follow the adult stage with 8 legs. The time from egg to adult normally requires about 3 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. In the spider mite outbreak of 1988, many heavy infestation totally collapsed around late August. A population collapse may be due to an epizootic of a fungal organism that attacks mites.

Sampling & Assessment

Sampling procedures have not been established for spider mites on soybeans, but a scheme for evaluating an infested field was used in the 1988 outbreak which provide some basis for as scale based on observable symptoms and conditions as follows:

1. Mites barely detected at field perimeter or other dry loci. Multiple plants need to be inspected before mites are found.
   Assessment: Clearly non-economic.
2. Between ratings 1 and 3.
3. Presence of mites easy to detect at field perimeter and dry spots. Difficult to detect within field. Foliage is still green but stippled feeding injury with few mites under lower side of leaves detectable, but not on every plant.
   Assessment: Clearly non-economic, but warrants monitoring.
4. Similar to level 3, but egg laying is abundant.
5. Entire field (all plants) exhibit some sign of infestation with speckling and some discoloration of lower leaves. Foliage exhibiting various levels of stippled feeding injury on relatively healthy foliage. Field perimeter and dry spots exhibit severely damaged plants.
   Assessment: Rescue treatment warranted at this point, especially if immature mite stages are in abundance.
6. Similar to level 5, but with heavy egg laying present.
7. Infestation widespread with discolored and wilting foliage easily detected throughout field. All plants heavily infested when examined closely. Severe damage evident.
   Assessment: Effective rescue treatment will save field.
8. Similar to level 7 with heavy egg laying present.
9. Total field discoloration and drying down of foliage. Significant foliage and stand loss evident.
   Assessment: Field may be beyond point of recovery if rescue treatment applied. However, new growth may resume if treated.

In making an assessment of a spider mite infested field, it is important that one recognize the early signs of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when foliage is still green. In addition, it is essential that one uses a good hand lens to view relative abundance of mites in egg, nymph, and adult stages.

Management Options

When conditions are optimal for spider mite outbreaks, early detection facilitates timely and effective rescue treatment.

Since mite development is linked to host plant stress, cultural practices and varieties that limit plant stress in times of draught will also minimize the development of spider mites.

Spider mite activity may be adversely affected by the onset of rains depending on the level of mite infestation established. Rains may have a negative effect on a minor infestation. However, well established mite populations may tolerate significant rains especially if host plants are already in a condition of stress.

When rescue treatment is required for control of spider mites, the efficacy of a control treatment is improved significantly is the treatment is applied by ground rigs with sufficient carrier applied at high pressure in a manner to penetrate the foliage.

It is important to recognize that spider mites are not normally a significant problem on Midwest crops. However, when conditions are conducive to spider mite development, periodic scouting of fields will detect outbreaks early and facilitate timely and effective control actions.

Copyright 1993, The Ohio State University

OPMS Circular #FC-24 TSSM
Prepared by: Harold R. Willson, Dept. of Entomology
January 1989