Managing spider mite on soybean

Two-spotted spider mites are minute (less than 0.002 inch), greenish, yellowish to orange arachnids with two dark spots on their abdomen. Note they have eight legs, not six like insects. Spider mite adults are about half the size, or less, of the smallest soybean aphid nymph.

These mites attack a wide variety of plants, including several crops (soybeans, dry beans, alfalfa and corn), vegetables, ornamentals and trees. Mites overwinter as eggs and move to crops from permanent vegetation.

Life cycle and population dynamics

Hatching mites colonize the undersides of leaves. Look closely, and you might even see the webbing that earns them the name spider mites. They disperse by spinning a silk thread that's caught by the wind.

Spider mites have a straightforward life cycle, progressing through three stages between egg and adult (Figure 1). Depending on temperature, development takes five to 19 days. Hotter temperatures (more than 90 degrees Fahrenheit) accelerate reproduction, while cooler temperatures slow it down.

With females producing up to 100 eggs each (clear, round and shiny spheres on Figure 1), populations can explode. In fact, there can be as many as 70 times more mites in as little as six to 10 days.

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.

1. It accelerates spider mite movement from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Under drought conditions, cutting sparks a mass movement into adjacent soybean and corn.

2. It improves the food quality of soybean.

3. It diminishes or stops fungal disease activity that attacks 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.

4. It speeds spider mite reproduction, so predatory insects and mites can't keep up.

Spider mites injure leaves by piercing cells and sucking out cell contents. This injury produces white or yellow spots or stippling that’s heaviest on the underside of leaves (Figure 3).

Leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface becomes uncontrolled. Both photosynthetic rate and leaf water status decline with increasing levels of spider mite injury.

As colonies grow, injury intensifies. Entire leaves progress from grayish green to yellow, brown or coppery, and then drop off. Damage begins in the lower canopy and progresses upwards. Spider mite feeding reduces effective leaf area and accentuates drought stress.

This leads to reduced pod set, fewer seeds and smaller seed size. Farmers and crop advisors who aren’t familiar with spider mites may mistake symptoms for drought.

Infestations are typically first observed near field edges or where soybeans are stressed. If you notice lower leaf loss or yellowed or browning spots at the field edge, it’s time for some detective work (Figure 4).

• Examine plants at the field edge first, especially adjacent to roadsides, drainage ditches or alfalfa.

• Pull plants and examine the leaves from the bottom upwards.

• Look at the underside of leaves. Note stippling or webbing. Examine for mites with a hand lens or by tapping infested leaves over a white sheet of paper.

• Determine how far mites and symptoms have progressed up the plant.

When scouting fields for spider mites, note field edge symptoms,  leaf loss and stippling on leaves and presence of mites on plants.

If you verify mite presence, it’s time to move further into the field. Move at least 100 feet into the field before making your first stop. Walk a “U” pattern checking at least two plants in each of 20 locations.

Check fields every four to five days when drought persists. Under these conditions, infestations can quickly develop.

When a soybean field reaches a three on the following scale, spray the middle and upper canopy leaves to protect them.

Infestation scale: 0 to 5

0: No spider mites or injury observed.

1: Minor stippling on lower leaves. No premature yellowing observed.

2: Stippling common on lower leaves. Small areas with yellowing on scattered plants.

3 Spray threshold: Heavy stippling on lower leaves with some stippling progressing into the middle canopy. Mites present in the middle canopy, with scattered colonies in the upper canopy. Lower leaf yellowing is common, and there’s some lower leaf loss.

4 Economic loss: Lower leaf yellowing is readily apparent. Leaf drop is common. In the middle canopy, stippling, webbing and mites are common. Mites and minor stippling present in the upper canopy.

5: Lower leaf loss is common, with yellowing or browning moving up the plant into the middle canopy. Stippling and distortion of the upper leaves are common. Mites are present in high levels in the middle and lower canopy.

Making the decision

The full pod (R4) and beginning seed (R5) stages are critical in determining soybean yield. Spider mite feeding reduces photosynthetic area and accentuates drought stress. This reduces pod set, seed number and seed size.

If leaves drop or plants are killed, pod fill stops in its tracks. Pods on mite-stressed plants are more likely to shatter, which compounds yield loss. Only a 10 to 15 percent reduction in effective leaf area will justify an insecticide/miticide application to avoid yield losses. Unfortunately it's not easy to estimate a 15 percent reduction in effective leaf area.

Damage isn’t reversible, so it's important to protect the middle and upper canopy leaves. At a rating of three, there’s heavy stippling on the lower leaves with some progressing into the middle canopy and very little in the upper canopy.

At this point, mites will be common in the lower canopy, present in the middle canopy and with scattered colonies in the upper canopy. Some lower leaf yellowing will be common with accelerated leaf loss in small areas. In cooler weather, these symptoms will be less pronounced.

While there are many insecticides labeled for soybean, a few have adequate mite activity (Table 1). In addition to dimethoate, bifenthrin or mixtures containing these ingredients, growers have access to two more miticides for use against two-spotted spider mites in soybean.

Some insecticides may aggravate the situation by causing populations to increase. Flaring—when sprayed populations rise above the untreated level— can occur because these insecticides remove predatory mites and insects, and may even stimulate more rapid reproduction.

Considerations and strategies

• Most pyrethroid insecticides, except bifenthrin, aren’t terribly effective against two-spotted spider mites in Minnesota. By eliminating natural enemies, these products may actually flare spider mites. This is why you should check for mite problems within seven to 10 days after spraying for soybean aphids.

• Don’t count on dimethoate to control heavy infestations, even though they’ve performed well against two-spotted spider mite in Minnesota in previous outbreaks. Neither product kills mite eggs, so hatching spider mites begin rebuilding within a few days. Numerous reports of these insecticides failing to control heavy mite populations were reported in 2010. Bifenthrin has a longer residual and may control hatching eggs for a few days. High temperatures shorten the residual of all products.

• Don't bother with edge treatments; make a decision for the whole field.

• Miticides primarily rely on direct contact to kill mites. Because mites usually occupy the underside of leaves, thorough coverage is critical. Don’t skimp on water. Use 20 gallons per acre for ground application or 5 gallons per acre for aerial application (unless the canopy is open). At lower water volumes, performance may suffer.

• Re-scout treated fields five days after application to determine if egg-hatch and re-building populations might require re-treatment. Continue scouting on a regular schedule until soybeans reach stage R6.5, or environmental conditions become unfavorable for mites.

• Don’t re-spray with the same product. Switch products (and modes of action) between applications to ensure miticide resistance won’t develop.

Table 1 shows the insecticide group,  common and trade names of insecticides labeled for two-spotted spider mite (TSSM) control in soybean, the TSSM growth stage that's controlled and any insecticide resistance concerns. The information is summarized from miticide labels to help producers select products. Labels change, so read and follow label directions. The label is the ultimate authority for its application to two-spotted spider mites and may differ from the information provided in this table. 

*Restricted use pesticide
Always read and follow label directions.
Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

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