Two-spotted Spider Mite
Two-spotted spider mites (Tetranychus urticae) are present in soybean crops every year, but they are not often a significant problem. The risk of significant two-spotted spider mite infestations increases considerably during periods of hot weather and prolonged drought. For farmers and crop advisors not familiar with spider mites, the progression of symptoms from silvering, yellowing, browning, lower leaf loss and death, may be mistaken for drought symptoms.
Two-spotted spider mites have a wide host range, including soybeans, dry beans, alfalfa, corn, vegetables, ornamentals, and trees.
Mites overwinter as adults which lay eggs in the spring. Hatching mites establish colonies on the undersides of leaves that produce the webbing over the leave surface, prompting the name “spider” mites.
How drought affects spider mite populations
Spider mite populations are held in balance by natural enemies, weather and host quality. Drought triggers spider mite outbreaks in soybean and corn by upsetting this balance in four ways.
- Accelerates spider mite movement to soybean and corn from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Cutting initiates mass movement into adjacent soybean under drought conditions.
- Improves the food quality of soybean for spider mites.
- Diminishes or stops the activity of fungal diseases that attack mites, such as Neozygites. Disease outbreaks are fostered by cool, highly humid conditions that favor spore formation and mite infection. Hot dry weather stops these diseases.
- Speeds spider mite reproduction so that predatory insects and mites can’t keep up.
Source: Managing Two-Spotted Spider Mites on Soybeans and Corn in Minnesota – University of Minnesota Extension
Spider mites pierce the leaf cells with their mouth parts and suck plant fluids from the leaves. This injury produces a white or yellow spots or “stipling” that is heaviest on the underside of the leaves. The leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface is uncontrolled. If the infestation is unchecked by parasites, predators, or miticides, spider mites may kill the entire plant.
Adult spider mites are tiny arthropods with eight legs, not six, like insects, and no wings. Most are greenish-white or yellow, and occasionally orange or brown, and have a pair of distinct dark spots on the outer edges of the back.
Spider mites have a relatively simple life cycle, progressing through three stages between egg and adult. The two-spotted spider mite overwinters 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, followed by 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.
The best time to scout is in July and August, especially during periods of drought. Spider mite populations can build rapidly, especially if the weather gets hot.
As you scout soybeans for other pests and problems, always give a look to the plants near the edges of the fields or where soybeans are stressed. If spider mite populations are increasing, they will probably be noticed there first.
Soybean foliage infested with spider mites will initially appear speckled or “stippled”. As plants become heavily infested, the foliage turns yellow, then bronze, and finally the leaves drop off the plants from the bottom up. The effect of heavy feeding by mites leads to dehydration and death of the plant.
For farmers and crop advisors not familiar with spider mites, the progression of symptoms from silvering, yellowing, browning, loss of lower leaves, and plant death may be mistaken for drought symptoms. Heavily stippled upper leaves may also exhibit deformations that look like herbicide injury.
Checking for mites
Look for yellowing plants that appear sandblasted, and then look for the mites and webbing on the undersides of the leaves. Hold a clipboard with a white sheet of paper under the leaves and tap the leaves to dislodge the mites. Yellow, brown or black specks that move across the paper are probably spider mites. Crushing the mites will leave a small, reddish-brown spot on the white surface.
A 10X hand lens is necessary to clearly see the mites, although with good eyesight mites can be seen unaided, once your eyes are adjusted.
Determining treatment levels
Sample both injured and healthy plants to determine if the infestation is spreading. Also, be certain that the yellowed soybeans are caused by spider mites. Other factors can cause soybeans to turn yellow.
If mite presence is verified, it’s time to progress into the field. Move at least 100 feet into the field before making your first stop. Walk a “U” pattern checking at least 2 plants at each 20 locations. Assess mite damage using the following scale:
- 0 — No spider mites or injury observed.
- 1 — Minor stipling on lower leaves, no premature yellowing observed.
- 2 — Stipling common on lower leaves, small areas or scattered plants with yellowing
- 3 — Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. (Spray Threshold)
- 4 — Lower leaf yellowing readily apparent. Leaf drop common. Stipling, webbing and mites common in middle canopy. Mites and minor stipling present in upper canopy. (Economic Loss)
- 5 — Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
No specific threshold levels have been established for the two-spotted spider mite. Most Extension entomologists recommend treatment only if damage and mites are detected throughout the field.
Use the previous scale following guide, consider treatment when injury progresses to a rating of 3. Fields with ratings of 5 or worse may not be salvageable. Check fields every 4-5 days if drought persists since damaging infestations can develop quickly.
Spider mites are widely distributed throughout the region.
Spider mite populations are generally suppressed through a common fungal pathogen, Neozygites floridana, that attacks all stages of mites and is host-specific to spider mites. Spores from the fungus attach to the mite’s body, germinate, and infect the body. During early infection stages, mites have a discolored, waxy or cloudy appearance and mite death occurs within 1 to 3 days.
The environmental conditions necessary for the fungus to be active are temperatures cooler than 85°F and high humidity. Rain itself does not trigger the fungal infections; the resulting high humidity is still necessary. When conditions are not favorable for the fungus to grow, it will produce resting spores in infected, dead mites until environmental conditions trigger the resumption of growth and spore release. When the fungal parasite is active, it can reduce populations of mites very rapidly.
Predatory mites that colonize soybean and corn plants and feed on the two-spotted mites provide additional control. Aphid predators such as lady beetles, minute pirate bugs, and lacewings also feed on mites.
Unfortunately, insecticide and fungicide applications will disrupt all these predator and parasite species.