Spider mite infestations can be common under hot, dry conditions, particularly on drought-stressed corn. Yellowish or whitish spotting (stippling) on leaves is evidence of their presence. Timely rainfall, irrigation, natural enemies, and miticides can help control infestations and protect yield potential.
Spider mites feed on plant sap on the undersides of leaves which results in yellowish or whitish spots across the upper leaf surface. The symptoms are referred to as stippling (Figure 1). Fine silken webs, that can be easily seen under low magnification are produced by the mites.
Banks grass mite (BGM) (Figure 2) and two-spotted spider mite (TSM) (Figure 3) are the common spider mites found on corn with the TSM having a broader host range, while the BGM is associated with only grasses.
Banks Grass Mite:
Two-spotted Spider Mite:
Determining which species is present is important because of miticide resistance. An Extension entomologist and/or local Channel® TA should be consulted to help determine if resistance has developed to any of the commonly used miticides. Since both species may appear in a field at the same time, treating one species could result in increased populations of the other.
The injury by the mites, in conjunction with drought stress, increases the stress on the plant and negatively impacts photosynthesis. Early plant death can result in lost yield potential which has been estimated to be 23% for corn harvested as a grain crop and 17% for silage tonnage.3
Guidelines for potential chemical control include: 1) Treat if damage is visible in the lower 1/3 of the plant and mite colonies are present in the middle 1/3 of the plant, 2) treat if active mite colonies are found on 1/3 of the leaves of 50% of the plants, 3) treat if 15 to 20% of the leaf area is covered with mites and their damage. A pre-tassel preventative treatment may be warranted if the field has a spider mite history, temperatures are expected to exceed 95 °F, plants are drought-stressed, the field was previously treated and natural predators were reduced, and mites were found on the majority of the plants early in the growing season.
1 Cullen, E. and Schramm, S. 2009. Two-spotted spider mite management in soybean and corn. University of Wisconsin Extension. A3890. http://corn.agronomy.wisc.edu/.
2 Peairs, F. B. 2010. Spider mites in corn. Colorado State University Extension. 5.555. http://www.ext.colostate.edu/.
3 Bynum, E., Porter, P., and Vandiver, M. 2012. Insights from 2012 spider mite problems for future management decisions. Institute of Ag Professionals. Proceedings of the 2012 Crop Pest Management Shortcourse & Minnesota Crop Production Retailers Association Trade Show. www.extension.umn.edu/AgProfessionals.
Texas A&M AgriLife Extension. Texas A&M University. https://www.extension.umn.edu/agriculture/ag-professionals/cpm/2012/docs/UMN-Ext-CPM12-Bynum.pdf.
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