No spider mites in corn yet

Spider mite injury is being found in Iowa soybean, but no injury has yet been reported in corn. Some areas, particularly in western Iowa, are under drought stress and these fields also may develop economic infestations of spider mites. If mites do appear, deciding whether to use a pesticide to control the population may be necessary.



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Corn under drought stress is more susceptible to injury from spider mites.

University of Nebraska entomologists Bob Wright, Jack Campbell, and Gary Hein have produced an economic threshold table based on spider mite research in Texas. This table also may be useful for Iowa producers.

To use this table, first check 10 plants from five locations in the field and record the number of leaves with mites. Calculate the percentage of leaves with mites. Determine approximate market value of crop [=expected yield (bushels per acre) multiplied by the expected corn price (dollars per bushel)] and control costs (dollars per acre). If the percentage of leaves infested with mites is greater than the first value in the table (the black numbers), estimate the percentage leaf area infested with mites. Refer to the second value in the table (the red numbers); if the value for your field is greater than the second value, applying a pesticide for mite control would probably be profitable. However, corn that has reached the dent stage is unlikely to benefit from a pesticide application for mite control. Insecticides labeled for mites in field corn are Capture and dimethoate. Read and follow all label directions.

Economic injury levels for spider mites on corn based on percentage of infested leaves and percentage of total leaf area damaged.

Control

Cost/Acre

($)
Market Value ($)
200 300 400 500 600 700
5 15  8 10  5 7  4 6  3 5  3 4  2
10 29  16 20  10 15  8 12  6 10  5 8  4
15 44  23 29  16 22  12 8  9 15  8 13  7
20 59  31 39  21 29  16 24  13 20  10 17  9
25 74  39 49  26 37  20 29  16 25  13 21  11

Source: University of Nebraska.

This article originally appeared on page 156 of the IC-488(19) -- July 29, 2002 issue.

Updated 07/28/2002 - 1:00pm