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Corn pest logs thousands of frequent flyer miles.

Corn growers, beware! The moths that are parents of the familiar black cutworm appear to be fly-by-night insects that originate in the South but migrate to midwestern cornfields.

The insects may travel at 60 to 70 miles per hour some 1,000 to 2,000 feet up, riding strong southerly winds ahead of cold fronts.

Black cutworm larvae are an intermittent anxiety for corn growers. In some years, the roving parents produce few offspring; the cutworms cause little damage and go unnoticed. But in other years, crops may be decimated by plant-chewing larvae.

Therein lies a job for scientists: to learn enough about biology and insect migration patterns to predict when trouble is likely to occur and to devise economical control strategies.

ARS entomologist William B. Showers at Ankeny, Iowa, and his colleagues have inferred details about the cutworm's travel itinerary after taking a close look at the pollen the moths carried.

After examining moths of nightflying black cutworm, Agrotis ipsilon, retrieved from sex-pheromone-baited traps in late April, it was clear the insects had flown a long distance. Still sticking to the moths' mouth parts were pollen grains from purple-flowering woody legumes such as Calliandra and Texas ebony (Pithecellobium) that grow not less than a 1,000 miles to the south.

That discovery proved for the first time that in the wild, black cutworm moths have a taste for travel.

From studies of laboratory-reared insects, the scientists had already determined that mother moths could migrate from the Texas and Louisiana Gulf Coast to Missouri, Kansas, and Iowa. The lab moths--also caught in pheromone traps--were marked by a telltale reddish cast caused by a fat-soluble red dye that had been added to their diet.

The dyed moths made the trip from either Crowley, Louisiana, or College Station, Texas, to Iowa within a matter of 3 or 4 nights.

As more is learned about the insect and its life cycle, scientists may become able to not only predict outbreaks but also prevent them at the source--somewhere farther south. But first, a lot of questions about the migratory cycle need to be answered, says Thomas W. Sappington, an Iowa State University entomologist who works with Showers.

Could there be genes in some individuals within black cutworm populations that cause them not to migrate? Do moths that fly south from the Midwest in the fall stop in Texas to produce another generation that flies to Mexico? How do environmental influences such as day length, temperature, and crowding affect migration ?

Sappington is studying the migratory potential of moths reared in outdoor cages or in indoor insectaries and then tethered by glue to a flight mill in an environmental chamber where they get a workout. When light and temperatures simulate evenings of an April or perhaps October day, 1-to-5-day-old moths speed around the flight mill's 2-meter circumference past an electronic eye while a computer counts the revolutions. Sappington finds that 3-day-olds begin long-duration flights earlier--about 2 hours after sunset--than the other moths and that they fly about 6-1/2 hours.

In another flight-mill study, Sappington observed that female moths laden with fertilized eggs were almost as likely to make migratory-length flights and to fly almost as far as unmated females.

The observation, which supports a belief that most migrating females arrive in the Midwest already mated, was enlightening but not surprising, he says. In field studies in Ohio and Iowa, newly arrived mated females had been caught in light traps, but scientists were unsure when the mating took place. Had the matings occurred before a local flight to the trap or before or during migration?

The tentative answer--before or during migration--is being used to refine predictions of how soon cutworm larvae develop after the moths' arrival in an area.

In one set of experiments, Sappington set up wire-mesh "honeymoon" cages where he placed pairs of the male and female moths. Them, for two nights, they dined at a cotton ball soaked in honey-water and beer, and most moths successfully mated.

On the third night, it was time for the moths to take spins on the flight mill. Mated and unmated moths of both sexes had about the same tendency to engage in long-term flight.

Mated females began their long flights about 3 hours later in the evening than unmated females, but Sappington concluded that matedness doesn't seriously affect moths' ability to make long flights. The mated females flew just as fast and for only 2 hours less.

The experiments showed for the first time that in the springtime migratory period, young moths' hormones did not suppress reproductive maturation or sexual activity as migration approached, as happens in some other insects. However, in autumn this lack of suppression might not be true. To find out, scientists are conducting further laboratory and field studies.

Seemingly, the reproductive systems of black cutworm grandchildren of the spring migrators shut down during southward autumn migration time, thus preventing egg laying until the trip is completed. A possible explanation is that in autumn, northerly winds are usually at lower altitudes and have less velocity than the southerly low-level jets of spring. The flights south might take several days longer.

Computer Versus Cutworm

The researchers are combining this knowledge of black cutworms' migration from over wintering sites in the South with other discoveries to help crop consultants and farmers better time their field scouting to coincide with likely damaging outbreaks.

The Iowa Extension Service is using a computerized forecasting model developed by a team of meteorologists, climatologists, economists, and entomologists. The scientists are refining the model in a 3-year pilot research program, now in its second year.

An earlier model put together by meteorologist Michael D. McCorcle, formerly of Iowa State University, described the migration of adult moths and forecast where, when, and in what concentrations the moths would be found in the Midwest. Then, climatologist S. Elwynn Taylor of the university and ARS' Showers developed a model to predict the development of larvae, when to expect first damage, and the likely extent of damage to a crop in areas of infestation. The model requires information relating to the moth's arrival, soil temperatures, and hatching of larvae from the moth's eggs.

To refine data input to the black cutworm forecasting model, entomology graduate student James J. McNutt, along with other research team members including economist Ame E. Hallem, began checking on the model's accuracy in 1991. The refinements may help improve prediction accuracy and let farmers make the most cost-effective management decisions.

For example, a historical analysis of weather patterns and predictions of weather that consistently cause late planting of a crop could justify applying an insecticide to a field before an expected black cutworm outbreak.

As McNutt monitored tilled and untilled fields with different planting dates within a 50-mile radius of Ames, he found that the model best predicted cutworm damage early in the season. In four fields, immigrant moths arriving in late April produced progeny that predictably destroyed about 5 percent of one-to-two-leaf seedlings of corn planted in early May. No-till fields provided a better environment for egg laying, larval survival, and seedling destruction.

Weed management is an important part of minimizing black cutworm damage. Showers says destroying weeds at least 8 days before corn planting may be a good idea. Moths lay their eggs in fields that are weedy before corn is planted. But removing weeds soon after corn planting could be disadvantageous: Larvae with no weeds to feed on will turn to devouring corn seedlings.

When a black cutworm hatch is imminent, the research team has found that timely applications of insecticides best prevent outbreaks if the insecticides are not tilled into the soil. Insecticides used in the study included fenvalerate and permethrin, a synthetic chemical related to naturally occurring chemicals in chrysanthemum plants.- By Ben Hardin, ARS.

William B. Showers is in the USDA-ARS Corn Insects Research Unit, Ankeny Research Farm, Box 45B, Ankenv, IA 50021. Phone (515) 964-6664.
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Title Annotation:research on black cutworm moths by William Showers and Thomas W. Sappington
Author:Hardin, Ben
Publication:Agricultural Research
Date:Jun 1, 1992
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