Two-spotted Spider Mites: Spider Mites & Soybeans

Prolonged drought always raises the specter of  two-spotted spider mite  (Tetranychus urticae)   infestations in soybeans and corn.  While minor, local outbreaks have occurred in recent years, the last statewide spider mite outbreaks occurred in 1988 and in 2007. Growers and ag professionals tend to focus on spider mite problems in soybean, but often overlook infestations in corn where significant yield loss can also occur.   In soybean, a factor complicating spider mite management decisions in is soybean aphid, and vice versa.  Finally, insecticide and fungicide use in corn and soybeans may aggravate an existing spider mite infestation.

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), green to yellowish to orange arachnids (See close-up photos).  Note the 8 legs, not 6 like insects, and the two spots on the abdomen.  To put their size in perspective, spider mite adults are ca. half the size or less of the smallest soybean aphid nymph you’ve been looking at this week.    These mites attack a wide variety of plants, including crops (soybeans, dry beans, alfalfa, and corn, vegetables, ornamentals, and trees. Mites overwinter as eggs in permanent vegetation. Hatching mites establish colonies on the undersides of leaves that produce the webbing over the leave surface (see photo on right).  This webbing earns them the name “spider” mites.

Spider mites have a straightforward life cycle, progressing through three stages between egg and adult (see Fig. 1). Their development is completed in 5 to 19 days, depending on temperature. Development is accelerated by the >90oF temperatures that we recently experienced.  Reproduction slows down at cooler temperatures. With females producing up 100 eggs each, it’s easy to understand how populations can explode. Populations have been known to increase 70X in as little as 6-10 days.

 Why are spider mite problems worse during drought? 

Spidermites 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. Drought stress 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.
2. Drought stress improves the food quality of soybean for spidermites.
3. Drought diminishes or stops the activity of naturally occurring 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 in their tracks.
4. Hot temperatures also speed spidermite reproduction so that predatory insects and mites can’t keep up. This year we seem to be experiencing a cool drought so be grateful that the temperature effects on soybeans aren’t worse and aren‘t better for spider mites.

 How do spider mites injure plants? 

Spider mites injure leaves by piercing cells and sucking our cell contents. This injury produces a white or yellow spots or “stipling” that is heaviest on the underside of the leaves (see photos of minor and extensive stipling). The leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface is uncontrolled. Research verifies that both photosynthetic rate and leaf water status decline with increasing levels of spider mite injury. As colonies grow and feeding intensifies, entire leaves progress from grayish green to yellow, brown or coppery, and eventually drop off. Damage begins in the lower canopy and progresses upwards. Heavily stippled upper leaves may exhibit deformations reminiscent of herbicide injury. If the infestations is unchecked by disease, predators or miticides, spider mites may kill the entire plant. 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.

 How do you scout for mites in soybeans? 

Infestations typically are first noted near field edges or where soybeans are stressed. If lower leaf loss, yellowed or browning spots are noted at the field edge or in patches within the fields, its time for some detective work. Examine plants at the field edge first, especially adjacent to roadside ditches or alfalfa fields. Pull plants and examine the leaves from the bottom upwards. Look at the underside of leaves. Note stipling, webbing, and examine for mites with a hand lens or magnifying lens. Examine how far up the plant mites and symptoms have progressed. Tap selected soybean leaves with symptoms over a white sheet of paper or cardboard and examine for mites. Note the abundance of moving dark spots (mites) highlighted against the white paper. 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.

Check fields every 4-5 days if drought persists since damaging 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 the soybean field reaches a 3 on the scale above, spray to protect middle and upper canopy leaves. 

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). Two organophosphate insecticides, chlorpyrifos and dimethoate have performed well against two-spotted spidermites during previous outbreaks. Do not count on these products for high levels of control; I found only about 80% control in 1988 Minnesota trials. Both chlorpyrifos and dimethoate will not kill eggs and have a short residual, so hatching spidermites began rebuilding the population in a few days along with re-colonization from adjacent fields and non-cropland areas. Numerous reports of these insecticides failing to control heavy mite populations were reported in 2007.  In one case a field was sprayed 3X with Lorsban before acceptable control was achieved.  Note in the table below that the label for chlorpyrifos specifically requires a switch in products if the first application is not adequate.

For the 2009 growing season, two new products will be available for two-spotted spidermite control. Cobalt 2.55E, labeled in 2007, contains a mixture of chlorpyrifos and gamma- cyhalothrin (the active ingredient in Proaxis). Hero 1.24E, labeled in spring 2008, contains a mixture of bifenthrin and zeta cypermethrin. You’d recognize other products containing these active ingredients: bifenthrin is found in Capture 2E and zeta cypermethrin is found in Mustang Max.   Various formulations of bifenthrin ( are also labeled in soybean.  Performance data on these newer products are sparse.

These miticides primarily rely on direct contact to kill mites. With mites usually occupying the underside of leaves, thorough coverage is critical. Do not skimp on water. For ground applications, use 20 or more gallons per acre. For aerial application, 3 to 5 gallons pre acre are recommended. Sloppy application will be revealed since mites readily reproduce.

 How could spraying for soybean aphid aggravate the spider mite situation? 

Many of the insecticides labeled against soybean aphid are either ineffectual against spidermites, or may even aggravate the situation. Newer pyrethroid insecticides labeled for soybean (Asana XL, Warrior T, Proaxis, Delta Gold, Mustang Max, Baythroid), older pyrethoids (Ambush, Pounce, Pydrin), or their generic counterparts are weakly effective, neutral or even “flare” spidermites. These insecticides can cause populations to rise above untreated levels by removing predatory spidermites and insects, and possibly by triggering more rapid reproduction. The insecticides labeled for soybean appear in Table 1.

Research during the last major spider mite outbreak in 1988 provides an excellent example of flaring by Ambush and the poor performance of other labeled insecticides, such as Furadan or Penncap-M (see table below). The change between spider mite densities at 5DAT and 10 DAT reveals the poor control and the rapid rebuilding of the mite pop:  Penncap-M declined from 44.1% control to 29.6%, Furadan from 27.1% to 11.9%.  The flaring was evident with Ambush, whose control declined from 45.4% reduction in spider mite numbers to a 3% increase over the untreated population.

Warning:  If a pyrethroid was applied for soybean aphids, check for mites within 10-14 days of application to make sure that mite populations are not flaring up again in the field.  Conversely, most pyrethroid insecticides applied for soybean aphid control may cause a flareup of mite problems within a field if mites are present and the drought persists.

 Table 1. Insecticides Recommended for Two-Spotted Spider Mites in Soybean 

 Table 2. Insecticide Performance against Two-Spotted Spider Mites in Minnesota Soybean, 1988. 

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.

Article written by Ken Ostlie, Extension Entomologist and Bruce Potter, IPM Specialist from University of Minnesota Extension.