June 2, 2008
New Black Cutworm Threshold - from Loretta Ortiz-Ribbing
The long, cool, wet spring weather has been a challenge for getting crops in the ground, but as I type this article, most corn and soybean are planted. Planting soybean is still in progress. Soybean seeds need warmer soil conditions than corn. The weather delays for corn and soybean planting are pushing all management and production issues for wheat, hay, corn, and beans into a tight window causing some headaches and time crunches.
Corn is slowly coming up and growing. Some fields had crusting or low germination problems and tillage or replanting practices are being employed, but a big concern now is insects. Be sure to scout for black cutworm damage. Adult black cutworm moths have been flying into Illinois and laying eggs in corn fields or weedy areas for the past month. According to Dr. Kevin Steffey, University of Illinois Extension Entomologist, that means black cutworm larvae that survive this wet weather, will continue to feed and cut young corn plants well into June. Black cutworm adults are still being captured so even replanted corn will be threatened by damage from this insect. More information on the black cutworm can be found in the Pest Management and Crop Development Bulletin at http://ipm.uiuc.edu/bulletin. Check the May 23 issues and other May issues for information on black cutworms and other recent insect pests.
Current cutworm recommendations are to continue scouting for this pest by checking 50 plants in 5 locations within a field for leaf-feeding, plant cutting, wilting or missing plants. Feeding by small cutworms on the shoot of the corn plant will produce a row of small holes across the leaf when it unrolls. Be sure to identify the worm as a black cutworm and not a sandhill or dingy cutworm. Sandhill and dingy cutworms rarely cut corn plants. Compared to black cutworms, dingy cutworms are smaller, pale gray to brown in color with a reddish tinge. The dingy cutworm has four spots of equal size on each abdomen segment, while the black cutworm has one pair of spots that are smaller than the second pair of spots. The sandhill cutworm is about the same size but much paler and almost translucent in color. It also has white stripes down its back and sides.
Scouting will help you determine thresholds for making spray decisions, by recording the number of cut or missing plants and determining a percentage. It is still true that worms one inch or larger, those almost done feeding, should only have insecticides applied if 5% or more of the plants are cut. But a new, lower threshold for black cutworm has been determined by Iowa State for larvae ¾ of an inch or less in size. Evaluating current technologies and corn prices shows that 1 to 2 % cut plants or loss of corn plants, with a corn value of $5.00 to $6.00 per bushel, would just cover the cost to control small black cutworms ¾ of an inch or smaller. Information on the lower threshold and photos of the different types of cutworms can be found at http://www.extension.iastate.edu/CropNews/. Click on the Archive for weekly issues and select May 19, 2008.
Posted by John Fulton at 2:23 PM |
