Twospotted spider mites (Figure 1) are present in our field crops every year. Under certain circumstances, their populations can increase dramatically and result in significant yield losses. Two important conditions that favor their buildup include:

1. hot dry weather, and
2. use of insecticides.

The widespread need to manage soybean aphid and the continued droughty weather will put the region at greater risk for mite population increases. The following information is intended to provide a better understanding of spider mite biology, why our risks for problems will escalate, and what management decisions should be considered.

Description:
Twospotted spider mites are distributed worldwide. They are not insects; they are closely related to spiders They have an extensive host plant range that includes soybean, dry edible bean, alfalfa, corn, and others from our region. Mites are very small at a length of only 2/100th of an inch (0.3 to 0.4 mm). Mites range in color from pale yellow to green to orange. Two dark spots are visible through the transparent body. The eggs are round and yellow (Figure 1). Though mites are best seen with magnification, they can be seen with the unaided eye. This is best done by tapping an infested leaf over a white surface. The mites appear to be small dust specks, but they will actively move about. Crushing the small mites will leave a small, reddish-brown spot on the white surface.

Life Cycle:
All the life stages of the twospotted spider mite occur on plants. They overwinter in the region as adult females in plant material. They migrate to plants in the spring by walking or when aided by wind which catches them while they congregate on the top leaves of a plant. The adult females lay eggs on the undersides of leaves. Females can lay from 2 to 6 eggs/day. Fertilized eggs produce females; unfertilized eggs produce males. Eggs hatch in 4 to 14 days. Temperatures in the low 90’s F are optimum for reproduction. They hatch as a six-legged nymph, followed by two eight-legged nymphal stages. Time from egg to adult is dependent on temperature and can be as short as 4 days.

Plant Injury:
Spider mites feed by inserting their styletlike mouthparts into leaf cells. The contents of these individual cells are removed. The resulting symptom is white or yellow spots referred to as stippling (Figure 2). It can be seen on both upper and lower surfaces of the leaf. As mite populations increase, injured leaves are susceptible to water loss at the feeding wound sites. The combination of feeding, water and heat stress can turn leaves yellow to brown in color. Spider mites also produce webbing. This webbing is often visible on heavily infested leaves, trapping dust that makes plants look very dry and dirty (Figure 3).

more photos of plant injury . . .

        
   

Field Monitoring:
When environmental conditions favor the increase in mite populations, monitoring fields for their presence and assessing plant damage levels is strongly recommended. Often initial infestations are along field margins. Look for mites on the leaves in the lower to middle canopy. To confirm the presence of mites, use a 10x hand lens or tap leaves over a sheet of white paper. Assessing damage will be more of a challenge. Generally, the attempt to assess leaf area loss is recommended. Estimating green leaf area lost to mite feeding can be done by estimating leaves lost, or potentially lost, as a percentage of the whole. This would be a similar approach to estimates that are made to assess leaf loss when grasshoppers or bean leaf beetle are munching holes in leaves.

Treatment Decisions:
Treating for spider mites can be a testy decision. The environmental conditions that favor mite infestations are not the best for optimum soybean yields. In addition, when conditions continue to favor mites, multiple treatments may be required to keep the infestations suppressed. Unfortunately, most of our treatment decisions in the next few weeks will be driven more by aphids. The spider mites will be a second consideration, though an important one when it comes to the selection of an insecticide.

The economic threshold is not well defined for spider mites. Keeping in mind that a leaf loss of 20 to 30% during pod fill can reduce yields, it is necessary to back that off to a lower value to avoid actual losses. University of Illinois entomologist Dr. Mike Gray has recommended a treatment timing when 10 to 15% of the leaves are discolored during pod fill. You can read more details on justification for this threshold in the 2005 Pest Management and Crop Bulletin newsletter at:

http://www.ipm.uiuc.edu/bulletin/article.php?id=354

Natural control:
Spider mite populations are often suppressed through natural biological control agents. Predatory mites (Acari: Phytoseiidae) colonize the same plants and feed on the twospotted mites. The aphid predators such as Lady beetles, minute pirate bugs, and lacewing also feed on mites. Unfortunately, insecticide applications will disrupt all these predator species.

The most important biological control agent is the fungal pathogen, Neozygites floridana. Spores from the fungus attach to the mite’s body, germinate, and infect the body. Infection leads to death in 1 to 3 days. Environmental conditions necessary for the fungus to be active are temperatures cooler than 85 F and humidity of at least 90% for periods of 12 to 24 hours. Rain itself does not trigger the fungal infections; the resulting high humidity is still necessary. When conditions are not favorable for the fungus, infected, dead mites become the resting source, housing the fungus until environmental conditions trigger the resumption of growth and spore release. When epizootics occur, they can reduce populations of mites very rapidly.

Insecticide Selection:
Comments have already been made by other authors about the importance of proper insecticide selection when mites are a concern.

The pyrethroids (e.g., Asana, Asana, Baythroid, Decis, Mustang Max, Proaxis, Taiga, Warrior) kill off the natural enemies that help control spider mites, stimulate movement of the mites, spreading the infestation, and there is some data from orchard systems to indicate they may increase the reproductive rate of existing female mites and increase the number of females being born into the population (again, raising the reproductive rate of the colony). This worsening of mite infestations can occur in as little as 7-10 days after application. The organophosphate (OP) insecticides (e.g., dimethoate, chlorpyrifos - Lorsban and Yuma, Penncap-M) have provided a better kill rate, and are a better choice for mite management. However, there is still a risk of continued problems when environmental conditions remain hot and dry. To illustrate the difference between OP and pyrethroid insecticides, a small study conducted in 2005 comparing an OP (Lorsban) and a pyrethroid (Asana) resulted in 99+% control of mites by the OP while the pyrethroid had populations equal to or significantly greater than the untreated check (Figure 4).

Another question that is quite popular now is whether there is benefit to mixing a pyrethroid and an organophosphate? If aphids are the only concern, probably not. Where aphids and mites are an issue, then yes. Lorsban does well on both aphids and mites and should provide the best control with a single insecticide. If there is need for mite control under the industry incentive programs where pyrethroids are being used for a second time, the treatments would probably be enhanced in their performance with the addition of Lorsban. In this situation, a lower rate of Lorsban at the 8 oz/a rate would be expected to provide adequate results (2 EE label issued 4/15/05), though follow-up scouting is highly recommended to assess future infestation levels.

Phillip Glogoza, REE - Crops, Moorhead Regional Center
and
Ian MacRae, Extension Entomologist, NWROC