Managing Two-Spotted Spider Mites
on Soybeans and Corn in Minnesota
by Ken Ostlie, Extension Entomologist and
Bruce Potter, IPM Specialist
Updated July 2012
Prolonged drought always raises the specter of two-spotted spider mite (Tetranychus urticae) outbreaks in soybeans and corn. As the 2012 drought intensifies in Minnesota, infestations are reaching treatable levels. Spider mite outbreaks are rare but have occurred more frequently in recent years (1988, 2007, 2009 and 2010). Farmers and their crop management advisors may need a refresher on two-spotted spider mites, population dynamics, scouting, treatment thresholds, and miticide options. To make matters worse, trying to manage both spider mites and soybean aphids creates decision headaches.
Even though rainfall may occur during a spidermite outbreak, don’t assume rain will eliminate aphid or mite problems. Heavy rains in the 1-2” range certainly remove a lot of stress on the plant, but you need to make sure that ongoing aphid or spider mite problems don’t continue to rob yield. Scout now for spider mites, their signs, and damage, especially if you’re contemplating a spray for soybean aphids. Spraying for soybean aphid with most pyrethroid sprays (e.g. Asana, Baythroid, Decis, Delta Gold, Mustang Max, Proaxis, or Warrior) could aggravate the situation. Pyrethroid insecticides perform poorly against spider mites and could even aggravate the situation by “flaring”, or increasing spider mite populations. The only products that are recommended for spider mites in soybean include insecticides containing chlorpyrifos, dimethoate and bifenthrin. With dimethoate performance quite variable against soybean aphid, only chlorpyrifos, bifenthrin and mixes containing these ingredients will work well against dual infestations. Chlorpyrifos is not labeled for spider mites on corn, but corn has additional labeled options including propargite, and spiromesefin, If you previously sprayed for aphids in the last two weeks, be sure to check the fields for spider mites and rebounding aphid infestations.
With drought continuing in many parts of Minnesota and reports of spider mites reaching treatable levels in some areas, let’s quickly overview two-spotted spider mites, scouting and decision-making.
What are two-spotted spider mites?
Two-spotted spider mites are minute (<0.002 inch), greenish, yellowish to orange arachnids with two dark spots on their abdomen (see close-up photos). Note their 8 legs, not 6 as in insects. Spider mite adults are ca. 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. Hatching mites colonize the undersides of leaves. Look closely, and you might even see the webbing that ears them the name “spider” mites (see photo on right). They disperse by spinning a silk thread that's caught by the wind.
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Spider mites have a straightforward life cycle, progressing through three stages between egg and adult (see Fig. 1). Depending on temperature, development takes 5 to 19 days. Hotter temperatures (>90°F) speed reproduction while cooler temps slow it down. Recent hot temps have accelerated reproduction. With females producing up 100 eggs each, it’s easy to understand how populations can explode, increasing up to 70X in as little as 6-10 days.
Why are spider mite problems worse during drought?
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.
- Speed spider mite reproduction so that predatory insects and mites can’t keep up.
How do spider mites injure plants?
Spider mites injure leaves by piercing cells and sucking out cell contents. This injury produces white or yellow spots or “stippling” that is heaviest on the underside of the leaves (see photos). Leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface becomes uncontrolled. Research verifies that 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, then drop off. Damage begins in the lower canopy and progresses upwards. Spider mite feeding reduces effective leaf area and accentuates drought stress. The result is reduced pod set, fewer seeds, and smaller seed size. For farmers and crop advisors not familiar with spider mites, symptoms are simply mistaken for drought.
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How do I scout for spider mites in soybeans?
Infestations typically are first observed near field edges or where soybeans are stressed. If lower leaf loss, yellowed or browning spots are noted at the field edge, it’s time for some detective work.
- 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 tap infested leaves over a white sheet of paper.
- Determine how far mites and symptoms have progressed up the plant.
If mite presence is verified, 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 2 plants at each of 20 locations.
Assess mite damage using the following scale:
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 or scattered plants with yellowing.
3 – Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. (Spray Threshold)
4 – Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic Loss)
5 – Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
Check fields every 4-5 days when drought persists. Under these conditions, infestations can develop quickly.
When should spider mites be sprayed in soybean?
Full Pod (R4) and Beginning Seed (R5) stages are critical in determining soybean yield. Spider mite feeding reduces photosynthetic area and accentuates drought stress. The result is reduced pod set, seed number, and seed size. If leaves drop or plants are killed, pod fill is stopped in its tracks. Pods on mite-stressed plants are more likely to shatter, which compounds yield loss. Only a 10-15% reduction in effective leaf area, yield losses will justify an insecticide / miticide application. Unfortunately it’s not easy to estimate a 15% reduction in effective leaf area.
When a soybean field reaches a 3 on the scale above, spray to protect middle and upper canopy leaves.
Damage is not reversible so it’s important to protect the middle and upper canopy leaves. At a rating of 3, there is heavy stippling on lower leaves with some stippling progressing into 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. Note: In cooler weather, these symptoms will be less pronounced.
Treatment is recommended only if damage and mites are detected throughout the field. Edge treatments are not effective since mites are usually spreading throughout the field before any visual symptoms are noted. During the severe outbreak in 1988, fields progressed from symptoms visible at the field edge to severe leaf loss throughout the field in only 2 or 3 days.
What miticides control two-spotted spider mites in soybean?
While there are numerous insecticides labeled on soybean, only a few have adequate mite activity (see table below). Your choices are basically limited to chlorpyrifos, dimethoate, bifenthrin or mixtures containing these ingredients. Many of the insecticides labeled against soybean aphid are either ineffectual against spider mites, or may even aggravate the situation. Most newer pyrethroid insecticides labeled for soybean (Asana XL, Warrior T, Proaxis, Delta Gold, Mustang Max, Baythroid), older pyrethoids (Ambush, Pounce, Pydrin), and their generic counterparts are weakly effective, neutral or even “flare” spider mites. Flaring, the rise of sprayed populations above untreated levels, occurs because these insecticides remove predatory mites and insects, and may even stimulate more rapid reproduction.
The insecticides labeled for two-spotted spider mites in soybean appear in the table below.
Important points about miticides:
- Most pyrethroid insecticides, except bifenthrin, are not terribly effective against two-spotted spider mites in Minnesota. By eliminating natural enemies, these products may actually “flare” spider mites so check for mite problems within 7-10 days after spraying for soybean aphids.
- Chlorpyrifos and dimethoate have performed well against two-spotted spider mite in Minnesota in previous outbreaks, but don't count on control of heavy infestations. Neither product kills mite eggs, so hatching spider mites began 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. Residual of all products is shortened by high temps.
- Don't bother with edge treatments—make the decision for the whole field.
- Miticides primarily rely on direct contact to kill mites. Since mites usually occupy the underside of leaves, thorough coverage is critical. Do not skimp on water. Use 20 gpa for ground application; 5 gpa for aerial application (unless the canopy is open). At lower water volumes, performance may suffer.
- Re-scout treated fields in 5 days after application to determine if egg hatch and re-building populations might require re-treatment. Continue scouting on regular schedule until soybeans reach stage R6.5, or environmental conditions become unfavorable for mites.
- Do not re-spray with the same product. Switch products (and mode-of-action) between applications to ensure that miticide resistance will not develop.
Table 1. Miticides Recommended for Two-Spotted Spider Mites in Soybean
(* = Restricted Use Product)
| Chemical Name | Trade Name(s) | Application Rate (lb AI/acre) | Re-entry Interval | Pre-Harvest Interval | Use Comments |
Chlorpyrifos |
*Lorsban 4E, |
0.25 - 0.5 |
24 hrs |
28 days |
Check 3-5 days after application. If new nymphs, switch product. Do not make a second application to spider mites. Do not make more than within 10 days. Do not feed or graze treated soybean plants. |
Chlorpyrifos + Gamma- Cyhalothrin |
*Cobalt 2.55E, |
0.26-0.52 |
24 hrs |
30 days |
Do not graze or feed to livestock. |
Dimethoate |
*Dimethoate 4E, 4EC, 400 |
0.50 |
48 hrs |
21 days |
Do not feed or graze within 5 days. |
Zeta |
*Hero 1.24E |
0.10 |
12 hrs |
21 days |
Do not graze or feed treated forage to livestock. Do not make applications less than 30 days apart. |
Bifenthrin |
*Bifenthrin 2E, |
0.08-0.10 |
12 hours |
18 days |
Do not graze or feed treated forage to livestock. Do not make applications less than 30 days apart. |
Please note: The information provided in this table is summarized from miticide labels to help make decisions regarding product selection. Labels change so read and follow label directions on the product you purchase. The label is the ultimate authority for its application to two-spotted spider mites and may differ from the information provided in this table. Read and follow label directions!
Table 2. Insecticide Performance against Two-Spotted Spider Mites in Minnesota Soybean, 1988.
| Insecticide | Rate (AI/acre) | Mites per Leaflet (10 DAT) | % Control | % Leaf Injury | Yield (bu/acre) |
Cygon 400 |
0.50 |
71 c |
84.5 |
14.8b |
24.7ab |
Lorsban 4E |
0.50 |
150 c |
68.1 |
15.5b |
26.0 a |
Penncap-M 2 FM |
0.50 |
403 b |
29.6 |
22.6a |
21.5bc |
Furadan 4F |
1.00 |
541ab |
11.9 |
23.3a |
21.8bc |
Ambush 2E |
0.15 |
675 a |
- 3.0 |
23.3a |
18.8 d |
Untreated |
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552ab |
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25.1a |
19.6cd |
| Insecticides applied on 8/13/1988 to soybeans (Weber) at R5.5 (beginning seed) with stipling evident up to the 2-3 upper leaves. Mite counts taken from 4th and 8th leaves from the top. | |||||
The following graph illustrates yield response to several insecticides applied for spidermite control in a field in 2007. Note that Asana XL, Mustang Max, and Warrior T did not get a significant yield response, while Lorsban (chlorpyrifos), Dimate (dimethoate), and Hero (bifenthrin + zeta cypermethrin) all yielded significantly better. Cobalt performance was intermediate in this trial at Rosemount, MN against a mite infestation that slowed and stopped within a week of application.
Fig. 2. Yield response to insecticide application against two-spotted spider mites in soybean.
Because of spatial variation in yields, a projected yield for each plot was interpolated from unprotected check plots (3/rep) using an inverse weighting function. The deviation in actual yields from this weighted average was subjected to statistical analyses. Means followed by the same letter do not differ (p<0.05).
Will rainfall stop spider mite infestations?
Single rainfall events during a drought, unless drought-breaking, are usually not sufficient to derail a rapidly growing spider mite infestation. Rainfall exerts its biggest impacts not by washing mites off leaves, but by slowing down infestations in several ways. First, it may alleviate drought stress on the soybean or corn. Alleviating moisture stress lowers its quality for spider mites and indirectly reduces reproductive rate. Significant rainfall also enhances soybeans’ or corns’ ability to tolerate spider mite feeding. Second, cooler temperatures behind a cool front will slow spider mite activity and reproduction, allowing predators begin catching up. Third, prolonged rainfall or elevated humidity that leads to heavy persistent dews may create the highly favorable environment for a fungal disease outbreak. Unfortunately, after a prolonged drought period there’s little disease inoculum so disease outbreaks may not occur immediately.
If you’ve made a decision an infestation warrants spraying, don’t hold up a spray waiting for rain. Instead, if rain is unlikely to occur before the spray dries, go ahead with the spray. Reducing spider mite pressure will allow the crop to take full advantage of the moisture.
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Last Modified
7/30/12 4:03 PM
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