Watch for Twospotted Spider Mites in Soybeans

July 23, 1999

· Spider mite infestations have been observed in soybeans in several areas of Illinois.

· Some spraying for control of spider mites has been initiated.

· During the next few weeks, watch carefully for increasing densities of spider mites.

You may recall that we printed an article about spider mites in Ohio soybeans in issue no. 14 (June 25, 1999) of the Bulletin. Apparently we are beginning to share their experiences. Although the hot, dry weather has not been exceptionally extended in most regions of Illinois, densities of twospotted spider mites have increased noticeably within the past few weeks. During the latter part of the week of July 12 and early during the week of July 19, we received several reports of spider mite infestations in soybean fields and some observations of spider mites in corn. Recent rains have helped recharge the soil moisture in some areas of the state, but the rains were spotty. Although forecasts for more rain are welcome, people in areas that remain hot and dry need to be especially alert for signs of infestations of spider mites.

On July 15, Randy McElroy, Monsanto Company, reported some spider mite activity in both corn and soybeans near Crawfordsville, Indiana, only about 35 miles east of the Illinois border. On July 16, he reported finding spider mites in soybeans in Edgar County. The infestation extended 20 to 25 yards into the field. We have received other reports of spider mite infestations from Sangamon and Warren counties. It is likely that infestations of spider mites can be found in fields in any area of the state that has not received much rainfall within the past few weeks. We urge you to scout soybean fields in your area right now to determine the extent of infestations of spider mites (and other pests, for that matter) throughout the state. As we have learned in years past, it is possible to spot treat spider mite infestations to prevent their movement throughout entire fields.

Close-up of twospotted spider mite.

Twospotted spider mites on the surface of a soybean leaf.

Twospotted spider mites are not insects; they are related more closely to ticks and spiders. Adult females are oval and have eight legs; an adult male is narrower and has a somewhat pointed abdomen. The mites usually are pale yellow to brown. The food contents within a twospotted spider mite accumulate in two spots on either side of the body; you can see these spots through their cuticle (skin). After mating, females lay light-colored, spherical eggs, usually on the undersides of leaves. The eggs hatch into six-legged larvae, which grow into adults in a matter of days. Spider mites complete their life cycle (Figure 1) from egg to adult in less than a week (4 to 5 days) when temperatures are high. You usually can find all stages of the mites on soybean leaves. In addition, as the mites continue to reproduce, generations overlap and densities can increase rapidly.

Spider mite infestation at the edge of a soybean field.

Twospotted spider mites overwinter in Illinois in undisturbed areas, including grassy and weedy areas adjacent to soybean fields. During most years when rainfall is plentiful and temperatures are not very high, spider mites usually remain in these noncrop areas, reproducing and feeding contentedly. When the weather becomes hot and dry, spider mites begin showing up in the edges of soybean fields. If hot, dry weather persists, densities of spider mites increase rapidly, and the problem spreads into the fields.

Several interacting factors help explain why densities of twospotted spider mites increase during hot, dry weather:

· Drought provides a more favorable thermal environment for the mites' growth and development.

· Drought-stressed plants are more attractive to and acceptable for the mites.

· Drought-stressed plants are physio-logically more suitable for the mites.

· Drought conditions do not favor natural enemies and pathogens of spider mites.

· Drought might induce genetic changes in the mites.

Past research has shown that high temperatures significantly increase spider mites' reproductive rates and decrease generation times. Other research determined that high temperatures increase the mites' metabolic rate by increasing demand for dietary fluids. Some researchers have suggested that outbreaks of twospotted spider mites should not be attributed solely to drought-induced physiological parameters within soybean plants. Research conducted at Iowa State University and published in the early 1990s showed that the spread of a fungal disease occurs only during sustained cool and humid weather. The bottom line is that no single factor causes an outbreak of spider mites when hot, dry weather conditions prevail. The relationship among the mites, soybean plants, and weather is rather complicated.

Spider mites at the tip of a soybean leaf.

As we mentioned previously, symptoms of spider mite injury in soybean fields usually appear first near field edges. The mites use their piercing­sucking mouthparts to remove fluids from plant cells, and the cells collapse, resulting in stippling of the leaves. Soybean leaves injured by spider mites turn yellow, bronze, and then brown as they die. Lower leaves typically show signs of injury first. Webbing usually is evident on the undersides of infested leaves. As the infestation increases, dead leaves begin to fall from the plants, and the injury in the field spreads. Occasionally, symptoms of spider mite injury appear first as yellow spots in the interior of the field. The mites sometimes move by "ballooning," a process by which they extrude a strand of webbing that catches the wind and carries them elsewhere. Consequently, when you scout for spider mites, you need to look throughout the field, even on apparently healthy plants, to assess the extent of the infestation.

Twospotted spider mite adults are quite small (about 1/60 inch, or less than 1 millimeter). We recommend that you carry a good magnifying lens when you begin to scout for these pests. To examine plants for spider mites, hold a sheet of paper or cardboard under the leaves and tap the leaves to dislodge the mites. Yellow or brown specks that move across the paper or cardboard probably are spider mites. Remember, 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 result in soybeans that turn yellow (see the following article).

If the infestation of twospotted spider mites is isolated within a soybean field, a spot treatment with either dimethoate or Lorsban 4E is warranted. However, before you make the decision to spot treat, it is essential that you scout the entire field. Apparently healthy plants infested with spider mites could show symptoms of injury within days after a spot treatment to injured plants. Thus the infestation will continue to spread, and other miticide applications might be necessary.

In 1997 there was some discussion about twospotted spider mites being resistant to dimethoate elsewhere in the United States. We have not confirmed any resistance of twospotted spider mites to dimethoate in Illinois. Nevertheless, you need to be aware of this information. Also, evaluate the performance of a miticide as accurately as possible. If performance seems poor, review your application records. Because the mites usually are on the undersides of soybean leaves, effective results are most likely obtained when you spray with adequate pressure and sufficient gallons per acre.

We mentioned previously that spider mites also had been found in corn. Although we occasionally observe spider mites building colonies on the lower leaves of corn plants in Illinois, the colonies are rarely observed on leaves above the ears. Therefore, economic infestations of spider mites in corn in Illinois are not common. Nevertheless, keep your eyes open.

If you encounter infestations of spider mites in either soybeans or corn, and if you are spraying fields for control of spider mites, please let us know. We all recall how quickly infestations of spider mites can take hold. The more information we have, the better.--Kevin Steffey, Mike Gray

Author: Mike Gray Kevin Steffey