• Soybeans nearing pod fill stage
• Soybean spider mite problems
• Northern corn rootworm extended diapause
• Mid to late season crop disease problems

Figure 1 has accumulated GDD’s on the Y-axis and Date of Planting on the X-axis. The line with the "Squares" tracks GDD’s for the southern area of my eight county region and the line with the "Diamonds" tracks GDD’s for the northern area. The top line (-) shows estimated accumulated GDD’s in seven days (July 29).

Two-Spotted Spider mites have been showing up in many Iowa soybean fields. I have not had any reports in this area yet, but we need to be looking. Population normally first develop in fields that are under physiological stress. Most spider mites are greenish white to yellow, although some can be orange or red. Spider mite injury to soybean can resemble herbicide injury or a foliar disease; however, characteristic signs are tiny yellow spots, or stipples, on leaves. As the injury becomes more severe, leaves turn yellow then brown or bronze, and finally die and drop off. Because they are small and stay on the undersides of leaves, spider mites can be difficult to see. One way to spot them is to tap leaves against a white piece of paper and look for moving yellow dots on the paper. Be careful not to confuse spider mites with thrips, which are noticeably elongated insects that move more rapidly than spider mites. More information on spider mites can be found at ISU’s Crop Management newsletter http://www.ipm.iastate.edu/ipm/icm/2001/7-23-2001/twospotsm.html

Grasshoppers have also been prevalent in area soybean fields. There are no good economic thresholds for grasshoppers in soybeans, but consider treatment if grasshoppers are present and defoliation reaches 40 percent in the preblooming stages or 20 percent in the pod-forming and pod-filling stages. Often times rainfall is the best treatment to control grasshopper populations.

Corn Rootworms are now mostly pupated and extended diapause has been observed in many fields this year. This is a northern corn rootworm phenomena and is seen in corn following soybeans. Rootworm eggs typically hatch the year following corn production; hence rootworm is normally a problem in continuous corn production systems. With the corn-soybean rotation, we have selected for rootworm eggs that can survive two years, thereby hatching in cornfields following soybeans. In some situations, there is a possibility of lodging, and even a smaller chance that yield loss may occur from these extended diapausing populations of northern corn rootworms in first-year corn. Go to ISU’s ICM newsletter for more information on extended diapause http://www.ipm.iastate.edu/ipm/icm/1996/4-14-1996/diapcorn.html

To check for rootworm damage, dig a few plants and wash them off. Evaluate for feeding on the root system. To review the new corn root damage system (node-injury scale), go to the ISU Corn Rootworm Home page: http://www.ent.iastate.edu/pest/rootworm/ This scale shows how to evaluate the roots, and what to look for.

Western Bean Cutworm Some area cornfields had problems with the WBC last year. I have four pheromone traps set up in Ida and Monona counties to track the flight patterns of this pest. So far, trap catches have been light in three of the four traps. The trap at the Western Research Farm near Castana has caught some moths within the last week. As we monitor the flights, we need to be on the lookout for this pest in cornfields. Look for egg masses (5 to 200 eggs) on the upper surface of leaves. After hatching the small larvae will hang around the egg mass for a few hours before moving to the ear. This pest can be very damaging to yield and quality if left unchecked. Refer to this University of Nebraska web site for more detailed information on this potentially devastating pest of corn. http://www.ianr.unl.edu/pubs/insects/g1359.htm

http://www.ipm.iastate.edu/ipm/icm/1999/7-19-1999/midscorndis.html

http://www.ipm.iastate.edu/ipm/icm/2001/6-25-2001/controldis.html

http://www.ipm.iastate.edu/ipm/icm/2001/7-23-2001/soyscout.html

This page last updated on 02/03/04

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