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Outbreaks of spider mites are common in hot, dry conditions, particularly on drought-stressed corn. Spider mite feeding reduces the photosynthetic abilities of the leaf and can eventually kill the leaf. Prevention includes proper irrigation to avoid drought stress and removal of alternate grass hosts. Miticides, applied when economic thresholds are reached, can help to control infestations and protect yield potential.
Figure 1. Spider mite feeding damage (stippling). Photo courtesy of Dr. Pat Porter, Texas A&M Agrilife Extension.
Two-spotted spider mite (TSM) Tetranychus urticae Koch. Adult TSM are yellow with two irregularly-shaped dark spots on the abdomen. TSM overwinter in sheltered areas such as field margins. Infestations usually occur sporadically throughout the field and are more common in humid areas like river bottoms. This species is often found in corn near a neighboring alfalfa field. Problems with TSM usually occur later in the growing season, rarely before flowering.
Spider mites overwinter as females. When the weather warms, females lay pearly-white, spherical eggs. Mites have three immature stages followed by an adult stage. Generation times depend on temperature and can range from 4 to 20 days.1,2 Under ideal conditions, spider mite populations can increase 70-fold in one generation.2
Spider mites feed on the undersides of leaves and damage corn by removing plant sap, resulting in leaf discoloration characterized by yellow or whitish spotting (stippling) across the surface of the corn leaf (Figure 1). This damage reduces the photosynthetic abilities of the leaf and increases water loss. Spider mite feeding can eventually kill the corn leaf, leaving it with a scorched or burned appearance.
Scout field edges where mite outbreaks are most likely to begin. Continue in 5 to 10 locations throughout the field, examining lower, middle, and upper leaves for stippling. Spider mites produce a fine network of silken webs on the undersides of the leaves that can be easily seen under low magnification. Using a magnifying glass, check the undersides of leaves for adult mites and webbing.
Select proper miticide. Miticide resistance is widespread, particularly in regions with long histories of miticide use. A grower’s best option is to consult with an extension entomologist and/or local agronomist before choosing a product to determine if resistance has developed to any of the commonly used miticides.
Spider mite outbreaks can be common in hot, dry conditions, particularly on drought-stressed corn. Proper irrigation to help avoid drought stress and removal of alternate grass hosts are the key cultural practices to control or prevent outbreaks. Natural enemies, including predatory mites, lady beetles, minute pirate bugs, lacewing larvae, thrips, and fungal diseases, normally keep spider mite populations in check. However, spider mite populations can flare when pesticides, applied for control of other corn pests and diseases, kill the natural enemies, particularly when hot, dry conditions coincide with the treatment. If an insecticide application is necessary, consider including a miticide in the application if spider mite colonies are already present.
Treatment of spider mites on corn is usually justified when the following conditions are met:
Economic thresholds (ET) for treatment based on the percentage of infested leaves per plant, the market value of the crop, and the costs associated with treatment are provided in Table 1. This is a simplified version of the ET table developed by Extension Entomologists T.L Archer and E.D. Bynum, Jr., at Texas A&M University and does not take into account percentage of leaf damage. Other guidelines for determining when to implement chemical control include:
A field survey should be conducted before and after a miticide is applied to evaluate the efficacy of a treatment. Closely examine 25 infested leaves and mark them so that the same leaves are reexamined after treatment. If a treatment is effective, no live adult mites should be found. However, eggs present during a treatment may not be killed (most miticides do not kill the eggs) and may begin to hatch, resulting in a new generation of immature mites. In some cases, retreatment may be necessary before immature mites become adults and begin laying eggs.
A preventative pre-tassel miticide treatment may be beneficial if:
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/. Web sources verified 08/16/16. 140415023019