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Western bean cutworm egg masses and larvae found in Ohio

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Written Thursday, July 22, 2010  

For the first time since the trapping of Western bean cutworm moths in corn began in 2006, Ohio State University Extension entomologists have identified egg masses and larvae. The find reveals that populations continue to increase and that growers will really need to monitor the pest in the future.

"The infestation of egg masses and larvae was light, but this just verifies that we won't see this pest decreasing in the coming years and growers will really have to start scouting for it each season," said Andy Michel, an OSU Extension entomologist with the Ohio Agricultural Research and Development Center.

Western bean cutworm is a common pest of Western corn-producing states that is rapidly expanding eastward and finding a niche throughout the Midwest. The number of adult moths trapped in Ohio each year has been steadily increasing.

In 2006, entomologists caught three moths in the traps. In 2007, six were caught. That number jumped to 150 in 2008 and to 550 in 2009. This year, that number has skyrocketed to 1,834 since July 20.

"We've more than tripled what we caught last year and the season is not over yet," said Michel. "We still have a least a month to go for adult moth flights."

The data suggests that Western bean cutworm will continue to spread across Ohio and increased feeding damage is likely. The larvae mainly feed on the corn ears, nibbling on the tips and boring into the middle of the ear, impacting kernel quality and affecting yields. Feeding damage can also create opportunities for disease development.

"Indiana saw damage in 2007 with economic damage following shortly thereafter. In Michigan, the first damage was found in 2007 and economic damage occurred in 2009. In Ontario, Canada, the first moth was caught in 2008, and the first damage was found last year," said Michel. "Significant damage hasn't been found yet in Ohio, but we know it's coming. All the data suggests we'll see some level of damage soon and will most likely first occur in northwest Ohio."

Western bean cutworm moths emerge in Ohio as early as June with peak flight around mid-July. The adult moths lay eggs on corn plants with the larvae hatching out in mid-August and developing through September. They mainly feed on the corn ears, drop from the plant in the fall, overwinter in the soil and emerge as adult moths again the following spring.

Entomologists have established a scouting protocol for identifying egg masses, as well as a threshold for treatment.

“The best time to scout for egg masses is before the corn has tasseled. The females prefer pre-tasseled corn and once that corn tassels, they tend to just fly around looking for a suitable host,” said Michel.

When scouting for egg masses, inspect 20 plants at five different locations in the field. The economic threshold is 5 percent of plants that have egg masses or larvae.

"Timing of egg scouting is critical," said Michel. "If you are over the threshold and need to apply chemicals, you really need to time it when the larvae are hatched but before they enter the ear. Once they enter the ear, they are protected."

Growers can best find eggs in the top one to three leaves. Females tend to prefer laying eggs on leaves with a more vertical orientation. Eggs are in randomly clumped masses of 20 to 100. They start out white, turn tan and then finally turn purple.

"Once the eggs turn purple, you have about a 24-hour window before the larvae hatch," said Michel.

Though the yield losses in Ohio cornfields have not yet been determined, research in Western states has shown that yield losses can be significant. In Nebraska, yield losses from one larva per corn plant at dent stage were estimated at 3.7 bushels per acre. In Colorado, yield losses were estimated at 30 percent to 40 percent in plants with heavily infested ears. Feeding can also contribute to secondary injury from diseases.

Though the configuration and color of the egg masses is distinct to Western bean cutworm, the larvae can be confused with other corn pests, specifically corn earworm.

"The pest has more characteristics of corn earworm than, say, corn borer. One difference is that the Western bean cutworm larva has two broad, brown stripes behind the head and corn earworm larva doesn't have that," said Michel. "But the main difference is that Western bean cutworm is not cannibalistic like corn earworm. If you find multiple larvae on a corn plant, then most likely it's Western bean cutworm."

The heaviest infestations of Western bean cutworm moths have been found throughout northwestern and northeastern Ohio counties. It has been found as far east as Trumbull, Ashtabula, Belmont and Monroe counties.

For more information on Western bean cutworm, log on to http://entomology.osu.edu/ag and click on the "corn" link.

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