Did Harsh Winter Kill Crop Insects?

Iowa had an extremely cold winter, but there was a lot of snow cover to help some types of crop insects survive.

Iowa just came through a very harsh winter with the coldest weather in the last five years and much more snow than normal - double the normal amount of snow in some areas of the state. How will those months in the freezer affect insect populations in 2008?

"Extremely cold weather such as we just experienced can have a real impact on some insects like bean leaf beetle, which doesn't survive very well," says Matt O'Neal, an Iowa State University soybean entomologist. "This insect has a greatly increased mortality when there are a lot of days and nights with below-zero temperatures."

So you might think you're off the hook for early season management of this pest in 2008. But one complicating factor is the amount of snow cover. Snow insulates the soil extremely well, allowing overwintering beetles to survive the cold temperatures, says O'Neal.

Some of bugs can survive hard winter

Especially if you are planting soybeans earlier to try to optimize yield, you should give some thought to how cold it was in your area and also consider the amount of snow cover, says O'Neal. Some farmers, if planting early, use an insecticide seed treatment to protect their beans early from defoliation by bean leaf beetles.

Although seed treatments can reduce bean leaf beetle populations, ISU research suggests that a liquid insecticide applied during the late June-early July population is often necessary to protect yields. A seed treatment by itself did not consistently protect yields in ISU studies.

O'Neal says you should scout for bean leaf beetle as soon as beans are emerging, to be sure you don't have beetle populations above the threshold. Thresholds vary based on soybean growth stage, price of insecticide application and price of soybeans. Visit www.ipm.iastate.edu/ipm/icm/ and read the article in May 2, 2005 ICM newsletter. Scouting during the warmest part of the day is critical during soybean emergence.

What about soybean aphid this year?

Another insect that's shown up in recent years is soybean aphid. It overwinters as an egg and can survive very cold temperatures, even harsh conditions in Iowa and Minnesota.

"What aphid outbreaks we'll see this summer depend on how many eggs were laid last year," says O'Neal. "That is, how many aphids were able to go from soybeans to buckthorn in the wooded areas in the fall to lay their eggs."

This past fall ISU specialists monitored aphids with a series of traps. "We saw very few aphids flying back from soybean fields to buckthorn," he says. "We think for the 2008 growing season we'll have a lot fewer aphids, nowhere near the large number Iowa had in 2007."

This spring's delayed planting due to rainstorms and wet soil may increase the odds for black cutworm problems on corn. The storm fronts are likely carrying adult black cutworm moths into the Midwest. As the females land in the fields, they seek attractive sites for laying their eggs, and they will have no trouble finding plenty of fields in which weeds are growing.

Watch for black cutworm on corn

Also this spring, you should watch for black cutworm feeding on corn seedlings. Black cutworm moths fly into Iowa each spring from southern states and lay eggs.

Egg-laying females are attracted to spring weed growth and fields of soybean stubble. The eggs hatch and small larvae feed on weeds or corn leaf tissue. Once they reach the length of a dime, then they can start cutting corn from emergence up to the five-leaf stage.

Extension has traps across Iowa to monitor incoming moth flights. Based on captures and weather information, ISU entomologists issue predictions of dates when the first cutting of seedling corn will likely occur. "If you scout a few days before the first cutting is predicted for your area you may find 'hot spots' based on leaf feeding," says ISU Extension entomologist Marlin Rice. "That can give you a head start on making decisions."

Scouting of seedling corn near the first cutting date is the only reliable method to determine if a cutworm problem exits. Then an insecticide can be applied if needed. Corn hybrids with Herculex traits have significant protection from black cutworm, but fields should still be monitored because minor damage can still occur.

Look for leaf injury on young corn

Black cutworm is a concern if you see leaf injury. Don't worry about dingy cutworms - they also feed on young corn leaves but rarely cut off plants. If you see leaf feeding, try to find the cutworms to determine if they are black or dingy. There's no need to spray dingy cutworm.

If you find leaf feeding and only black cutworms, Rice suggests you mark off 100 plants in a row with flags and scout these same plants for cutting over a period of several days at a few places across the field. That helps monitor cutworm activity and determine if they're cutting plants, and helps you figure the percentage of cut plants.

The economic threshold is when cutworms average less than ¾ inches long, says Rice. Consider applying insecticide if 2% or 3% of the plants are wilted or cut. If cutworms are longer than 1 inch, apply an insecticide if 5% of the plants are cut. If a field has a poor plant population (20,000 or less) these thresholds should be lowered. Stop scouting when the field is sprayed or when corn plants have five fully-developed leaves.

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