07-11-16 CSU Ext Pest Alert: Monitor cornfields for spider mites…

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Monitor cornfields for spider mites

Written by: Assefa Gebre-Amlak and Frank Peairs, Colorado State University

Spider Mite courtesy of CSU ExtThe Colorado State University field studies show that spider mites can be serious corn pests in Colorado especially during hot and dry years causing significant dry matter and grain loss.

There are two species of spider mites that attack in Colorado, Banks grass mite (BGM) and two-spotted spider mite (TSM). Most corn growing areas are affected by BGM alone but north central Colorado, especially Weld and Larimer counties and productions areas along the South Platte River are prone to serious mixed-infestations. Spider mites feed on the undersides of leaves, eventually killing the leaf and leaving it with scorched or burned appearance.

Banks grass mite is commonly found in corn from mid-whorl through the grain filling growth stages, while two spotted mite is rare on corn before flowering. Factors that encourage Banks Grass mite infestations include host drought stress, elevated temperatures, low rainfall, and low humidity.

Most banks grass mite problems occur in the drier corn-growing areas and always associated with grasses, for example many problems in corn start when adjacent wheat fields dry down. Whereas two-spotted mite occurs in more humid growing areas such as along river bottoms.

Webbing on leaves and discoloration are often the first signs of an infestation. Initially, Banks Grass Mites are most abundant on the lower third of the plant and density declines as the infestation moves up in the plant. Mites damage corn and small grains by piercing plant cells with their mouthparts and sucking the plant juices.

Proper irrigation to avoid drought stress is the key cultural practice for avoiding mite outbreaks but once mite infestation are established, irrigation cannot reduce mite densities in corn.

Banks grass mite builds up on the plant from the bottom up. Chemical treatment is justified when there is visible damage in the lower third of the plant and small colonies are present in the middle third of the plant before hard dough stage. For effective products for both species of spider mites management and detailed economic threshold, check the High Plains IPM guide: www.highplainsipm.org.

Submitted to the BARN by:

Jeannie S. Lambertson, Administrative Assistant, Colorado State University, GOLDEN PLAINS AREA EXTENSION; 181 Birch Avenue, Akron, CO  80720; 970-345-2287; email: jeannie.lambertson@colostate.edu