July 15, 2020
Spider mites can be damaging pests of corn and soybean, especially during hot, dry years (Figure 1). Prolonged moisture stress along with the extreme heat that can occur during the months of July and August can result in infestations of spider mites. Hot, dry growing conditions are ideal for the mites and cause additional stress to the growing crop. Where drought is the leading stressor, the mites increase stress on the plant as they feed on plant sap. The feeding damages leaves and negatively impacts the plant’s ability to perform photosynthesis. Ultimately, early senescence is the leading cause of lost yield potential. There are two common species that can be found on corn: two-spotted mite and banks grass mite, though the two-spotted mite is the more common species attacking soybean.
Figure 1. Enlarged image of a spider mite.
Two-Spotted Spider Mite and Banks Grass Mite
The life cycles of the two species are very similar. Two-spotted spider mites (TSM) are light cream to green in color and can be reddish in color when the mites are under unfavorable growing conditions.1 The TSM life cycle is relatively short and can be completed in five to 14 days with a potential of up to 10 generations per growing season. Under ideal growing conditions, which is temperatures greater than 85° F, humidity less than 90% and low moisture, populations can increase rapidly. This rapid increase in population prevents natural predators from keeping the mite population in check. Natural predators include lady beetle species, immature lacewings, minute pirate bugs and other predatory mites.2 Banks grass mites (BGM) are generally associated with grass plants, such as wheat and corn. In contrast to TSM, the optimum temperature for population expansion for BGM is much warmer, 97° F to 99° F.3 In many situations, BGM moves into corn when adjacent wheat fields begin drying down.
Scouting
If weather patterns indicate the need for scouting, one will find yellow to light-yellow spots on the upper leaf surface. This is caused by the mites feeding on the underneath side of the leaf, causing the visible color changes (Figures 2 and 3). To evaluate the mites, use a light-colored piece of paper attached to a clipboard to shake the leaf on and look for moving mites. In the beginning stages of spider mite infestations, symptoms often begin along the field edge as the mites disperse from the drying plants in the field border to the crop where they then disperse throughout the field. Mites begin feeding on the bottom of the plant and move upward as plant health deteriorates.3
When scouting, observe:
- Crop stage
- Severity of mite colonies in the field
- Location of plant damage
- Near future weather conditions
Figure 2. Soybean leaves showing spider mite feeding injury.
Figure 3. Spider mite feeding on corn leaf. Picture courtesy of Dr. Pat Porter, Texas A&M AgriLife Extension.
Management
If soybean plants are between R1 to R5 growth stages (beginning bloom through beginning seed set) and most plants have mites and heavy stippling and leaf discoloration is observed on lower leaves, a foliar insecticide application may be warranted.1 If corn is between growth stages R1 to R4 (silking through dough stage) and most plants have mites at or around the ear leaf and 15% to 20% of the leaves are discolored, a treatment may be in order.1 Treatments may not be warranted if temperatures are less than 85° F, humidity is greater than 90% and plants have adequate moisture. These conditions are favorable for fungal growth, which can naturally control the mite populations. Mites infected with fungal pathogens will appear red to brown in color.
Scouting early is the ideal time to determine if spider mites are present and if management is needed to avoid economic damage. Consult with your chemical provider to determine the appropriate miticide to use for control.
Sources:
1Hodgson, E. 2018. You “mite” want to scout drought-stressed crops. Iowa State University Extension and Outreach. https://www.crops.extension.iastate.edu/.
2Abbey, T. 2016. Twospotted spider mites. PennState Extension. The Pennsylvania State University. https://extension.psu.edu/.
3Peairs, F.B. 2014. Spider mites in corn. Colorado State University Extension. 5.555. https://extension.colostate.edu/.
Websites verified 6/10/2020.
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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