Spider Mites in Soybeans

Twospotted spider mite (Tetranychus urticae) outbreaks in soybeans can occur in hot and dry weather conditions. Scouting fields can help detect outbreaks early and facilitate timely and effective control measures.

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Lifecycle

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Twospotted spider mites are very small greenish, yellowish to orange arachnids with two dark spots on their abdomen (Figure 1)2. Adults can barely be seen with the naked eye and have eight legs. Spider mites progress through three stages (egg, nymph, and adult), and complete multiple generations per year. Adult females overwinter and can produce up to 100 eggs each. Egg to adult development takes 5 to 19 days, with the lesser amount of time at hotter temperatures2. They establish colonies on the undersides of soybean leaves. Hatching mites in the colonies produce a webbing on the leaf surface. They can be carried on a balloon of their webbing by wind and dispersed over a wide area.Adult twospotted spider mite and stipling of soybean leaves.
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Favorable Conditions for Spider Mites

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Drought can trigger spider mite outbreaks in soybeans. Hot and dry conditions reduce natural fungi that infect mites and increase their reproductive rate. Under drought stress, soybean plant tissue becomes more concentrated with nutrients that foster good growth of spider mites. Additionally, movement to soybeans from other vegetation increases as ditches or forages dry out or are mowed2.
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Injury to Soybeans

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Typically, spider mite damage is first noticed near field edges or where soybeans are stressed. They injure soybean leaves by piercing cells and sucking out the contents. This produces white or yellow spots or “stipling65533;? (Figure 2)2. Stipling may be more noticeable on the underside of the leaves. Feeding damage begins in the lower canopy and progresses upwards. As mite colonies grow and feeding intensifies, plants take on a yellowed then bronzed appearance. The leaves may eventually die and plants could defoliate with continued mite pressure. If left untreated, a 40 to 60% potential yield loss may occur, with greater losses if the plants are drought stressed5.​
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Scouting for Spider Mites 

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Soybean fields should be scouted weekly for evidence of developing spider mite populations. Fields should be checked more often if drought conditions persist since damaging infestations can develop quickly. Recognizing speckling or stipling on the lower leaves when foliage is still green is important for early detection of mite feeding. When scouting for mites, look at soybean plants at the field edge first, especially adjacent to drainage ditches, alfalfa, or corn. Examine leaves from the bottom of the plant upwards. Look at the undersides of leaves and note any webbing, speckling or stipling. Spider mites can be spotted by shaking plants over a white sheet of paper and look for moving specks on the paper. A hand lens is useful to observe the relative abundance of mites in egg, nymph, and adult stages. Examine how far the plant mites and symptoms have progressed. Then walk a “U65533;? pattern, at least 100 feet into the field, checking plants along the way.​
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Management

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Spider mites are generally controlled by naturally-occurring enemies such as lady beetles, lace wings, and most importantly, the fungus Neozygites floridana3. The fungus flourishes with temperatures under 85° F and humidity of 90% or greater. These conditions are generally scarce in the drought situations therefore spider mites become troublesome. The importance of these natural controls should be considered when contemplating making a fungicide or insecticide application to an infested field. Reduced levels of the natural controls, combined with the tremendous reproductive ability of spider mites could lead to an increase in the spider mite population.​
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Stipling of soybean leaves caused by the feeding of twospotted spider mites.

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There are currently no economic thresholds established for spider mites3. However, an insecticide application is often recommended when stipling is commonly observed in the lower canopy and is extending into the middle canopy, and when mites are present in the middle canopy with a few colonies in the upper canopy3. Treatments should be made before mites cause much leaf bronzing and leaf drop. If spider mites are found early, it may be feasible to just make an insecticide application to the outside border of the field, making enough passes to extend beyond any noticeable damage from the spider mites1.

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Chlorpyrifos (the active ingredient in Lorsban®-4E) and dimethoate are two widely recommended insecticides available for control of spider mites. It is important to have adequate water volume and spray pressure since mites are on the undersides of leaves1. Dimethoate can be systemic in soybean plants; however, systemic activity is reduced in drought-stressed plants4. Chlorpyrifos does not have systemic activity4. These products will not control eggs and have short residuals, so mites can begin rebuilding their population in a few days2. Although more than one application may be necessary to maintain control under certain conditions, it is important to follow all label restrictions. The Lorsban 4E label states that if large amounts of eggs are present, the treated area should be scouted in 3 to 5 days and if newly hatched nymphs are present, an application of a non-chlorpyrifos product should be made. Additionally, only 3 applications of chlorpyrifos are allowed annually, and there must be at least 14 days between applications. The reapplication interval for dimethoate is 7 days. The preharvest intervals are 28 and 21 days for chlorpyrifos and dimethoate respectively. There are synthetic pyrethroids labeled for spider mites, but generally they are not recommended due to the lack of field data for their control of twospotted spider mites6. Additionally, their application may control the beneficial insects, increasing the potential for growth of the spider mite population.

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What About Rain?

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Rain can help reduce spider mite populations, but expectations should be set appropriately. Rain washing mites off of the plants is less beneficial than the other effects rainfall can have on plant and fungal growth2. Rainfall, preferably repeated rainfall in respectable amounts, can improve plant health, allowing for the plants to better tolerate injury from spider mites. The reproductive rate of spider mites can be reduced due to rain diluting the nutrient content of the soybean plant available for spider mites and/or cooler temperatures that often accompany rain. With longer periods of rain and/or high humidity that encourages long lasting heavy dews, there could be increased growth of the fungus Neozygites floridana to help suppress the spider mite population. After long periods of drought the fungal population may take time to rebuild due to low inoculum levels. The bottom line would be that if a field warrants an insecticide application, and there is a chance of rain the forecast, it is not recommended to cancel the insecticide application to wait for the rain2. If the application is made and rain comes, the soybeans will likely be better able to take advantage of the precipitation.​

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