Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


July 14, 2006

July 14, 2006


Western Bean Cutworm

Western bean cutworm moth flights have been recorded in the area and egg masses are being found on corn leaves.   Scouting for the pest needs to be done now. Check 20 consecutive corn plants in 5 places in the field. Egg masses are usually laid on the upper surface of the leaf. Individual eggs are about the size of a pinhead and are white when first laid, but turn tan and then purple before hatching. The University of Nebraska recommends considering an insecticide application if 8% of the plants have egg masses or if young larva are found in the tassel. Young larva are tan with a darker faint diamond pattern on the back. The timing of the insecticide is critical. If an insecticide is needed, apply when tassel emergence is 90-95% complete. If the tassels have all emerged, the application should be made when 70-75% of the larva have hatched. For pictures and more information, see the last ICM Newsletter at



Spider Mites

Hopefully the recent rains will reduce problems with this pest, but some infestations have already been found in the area. Under drought conditions, the two-spotted spider mites can thrive. Insecticide treatments should be considered if spider mite damage is present, mites are still found, and the forecast is for continued dry weather. Initial damage appears as little yellow spots or “stipples” on the leaves and progresses until leaves die and drop. Lorsban and dimethoate are labeled for spider mites. The synthetic pyrethoids are not effective on spider mites and can actually cause spider mite problems by killing the predator mites that keep spider mite populations in check. For more information see the last ICM Newsletter at

Japanese Beetles

Japanese beetles are back in the areas that have had problems with the pest in the past, especially within about 20 miles of Cedar Rapids. There are no specific economic thresholds for Japanese beetles, but general thresholds for any insect pest that causes defoliation of soybeans is 40% defoliation in the vegetative stage and 20% defoliation during pod fill. People tend to overestimate defoliation. 20% defoliation looks much worse than it sounds. The pictures in the following article can help in estimating percent defoliation: Corn should also be watched closely now for the pest since the beetles will clip silks. For more information and insecticides labeled for Japanese beetles see the July 28, 2003 ICM Newsletter at

Bean Leaf Beetles


The first generation bean leaf beetles are showing up now.  When they first emerge from the soil they are grey in color and have a soft shell.  As the shell hardens, they may change to other colors (red, yellow, brown). As this generation populates, we can monitor for threshold levels with a sweep net.  It is best to sweep in the afternoon when they tend to be at peak activity.  The general threshold for reproductive stage soybeans is 4 to 5 per sweep in row beans and 3 to 4 per sweep in drilled beans.  It is usually the second generation that emerges in August that causes the most damage. More detailed threshold information can be found at:


Soybean Aphids


I have yet to find a single soybean aphid in fields I’ve been monitoring in the area. It’s looking like soybean aphids are not likely to be a big problem this year. Brian Lang is reporting very low numbers in NE Iowa.

Soybean Rust

So far soybean rust has been reported in Texas, Louisiana, Alabama, Georgia, and Florida. Most of the finds have been in kudzu. More info can be found at




Potato Leafhoppers


High numbers of potato leafhoppers can be found in some hay fields. For more information on managing potato leafhopper, see pages 107 - 110 of the June 21, 1999 Iowa State University Integrated Crop Management Newsletter or  Remember, waiting to see hopperburn is waiting too long as substantial losses have already occurred by that time.


If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: July 29, 2006
Contact: Jim Fawcett

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