Corn earworm moths get a lift from the wind

Most corn earworms cannot survive the cold of a Northeastern winter, but each summer this sweet corn pest arrives back in the cornfields of the northeastern United States more quickly than most people believe is possible. Now, a team of Penn State meteorologists thinks it knows how the small moths travel long distances so quickly, and perhaps can predict where and when they will appear next.

“For years, researchers have assumed that the moths travel in parcels of air,” says Matthew Welshans, undergraduate in meteorology and undergraduate research assistant at Penn State’s Environment Institute. “Few had actually tested this assumption, and no one tried to predict where or when the moths would land and earworms would appear in the Northeastern states.”

Working with Dr. Shelby Fleischer, professor of entomology; Paul Knight, Penn State meteorologist; and Dr. Douglas A. Miller, assistant professor of geography, Welshans investigated the potential paths of corn earworm moths and other pests such as armyworm if they rode the wind as they spread northward during the spring and summer.

“We found a discernible trend that the corn earworm travels at some height and is impacted by the direction of the air currents,” he told attendees of the 84th annual meeting of the American Meteorological Conference today (Jan. 14). “Depending on the time of day, the travel height could be from 500 meters (1640 feet) to a kilometer (3274 feet).”

The corn earworm larva is a major pest of sweet corn that destroys the top of the corn cob. While they will eat both field and sweet corn, a little damage to tops of field corn is irrelevant, while a chewed-up ear of sweet corn is not marketable.

“The northeastern United States accounts for more than 100,000 acres of sweet corn, or about a third of the total crop in the U.S.,” Welshans said. “The crop was valued at more than $147 million in 2000.”

The corn earworm moth lays its eggs on the corn silk but will lay them on other parts of the plant if the silk is not available. The eggs hatch in 2 to 10 days into small larvae that eat down the corn silk into the kernels at the tip of the ear. Because corn earworms are cannibalistic, usually only one or two larvae make it to the tender kernels. Eventually they drop to the ground and burrow in to pupate and emerge from the ground as moths. In the south, the earworms can have three generations per season with the last pupae wintering over before emerging to restart the cycle. In the north, assuming ground temperatures are normally cold, the winter-pupated insects freeze in the ground.

“Each year, in Pennsylvania, new corn earworms must fly into the area and repopulate,” says Welshans. “But, the population grows much faster and greater than a slow move northward.”

The researchers used a real-time tracking program called PestWatch that already exists in the Northeast. PestWatch uses blacklight traps that capture male and female moths and pheromone traps that capture male corn earworm moths. Individual volunteers count the insects in the traps once a week and report back to the PestWatch researchers at Penn State. The annual spread of the pest is put online so that farmers can see where insects are showing up (www.pestwatch.psu.edu).

Using this real pattern of insect population, along with a model developed by the National Oceanic and Atmospheric Administration to show wind patterns and a weather forecasting model to predict weather patterns, the researchers compared the actual pattern of insect appearances to that predicted by the wind and weather models.

“We want to be able to forecast when and were the moths, and subsequent larvae, will show up so that we can target the insects,” says Welshans. “Then farmers can tailor the insecticides to reduce the amounts used or change their harvest or culling efforts.”

With the two models and up-to-date information on where moths are, the Penn State researchers can not only track the insects but can also look backward at their paths to see where they are coming from and forward, to see where they will hit next.

“We are currently working on a flash program to animate the trajectories and integrate the PestWatch data so we can see the paths forward or backward,” says Welshans. “Hopefully, this will be on-line for summer 2004.”

One problem with the model is that volunteers only check the traps once a week so information tends to lag behind actual movement of the moths. However, because the various volunteers check their traps on different days of the week, using that to fill the time lag might be possible.