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Floral lures may doom corn earworm.

Floral Lures May Doom Corn Earworm

For part of its life, the corn earworm, Helicoverpa zea, is a night-flying moth that sips the nectar of weed flowers, such as the gaura. Drawn by the alluring fragrance of blossoms, virgin female moths choose food

before sex: All newly emerged females spend their first night feeding and wait until their second night to mate.

This peculiar preference is the key to a strategy ARS researchers are devising to kill a most troublesome insect.

In the caterpillar stage that follows the moth form, H. zea - variously known as the corn earworm, tomato fruitworm, and cotton bollworm - costs growers about $1.5 billion in damage and control measures each year.

The brown, pink, or red H. zea (formerly Heliothis zea), about 1.25 inches long and nearly as thick as a pencil, also plagues backyard gardeners around the country. "Most home gardeners who grow tomatoes or sweet corn know this worm," says ARS enthomologist Peter D. Lingren at College Station, Texas.

Lingren, along with entomologist Jimmy R. Raulston at Weslaco, Texas, and their co-researchers, hope to concoct a deadly brew: an insecticide mixed with perhaps honey or sugar, to stimulate moths' feeding, plus chemicals that mimic the tantalizing natural scents emitted from petals of weed flowers.

"If we can use the floral essences to entice newly emerging moths to eat bait laced with insecticide," Lingren explains, "we should be able to kill them before they get a chance to mate."

In nature, night breezes carry floral aromas from petals to hungry young moths. The aromas signal nectar - fast-food for the foraging insects. "The moths' appetite for the nectar," says Lingren, "may be a crack in their armor. Using floral aromas as a lure may be a way for us to get at them, without destroying nature's equilibrium."

If unchecked, female moths will flit from the weeds over to nearby corn plants or other crops the night after mating to lay their eggs. The eggs developed into caterpillars that burrow into the corn ear, tomato fruit, or cotton boll and feed on it. "Caterpillars are extremely difficult to control once they get inside the ear, fruit, or boll because insecticides can't get at them," says Lingren.

The floral-lure approach should reduce the amount of insecticide now used to keep the caterpillars in check. That's because adult insects such as moths are typically around 10 to 100 times easier to kill with insecticides than are caterpillars.

Further, the attractants may prove less costly than insecticides. Growers may spend less money to kill female moths than to destroy offspring that hatch from the eggs. A mated female moth typically lays about 1,000 eggs; on average, at least 500 will hatch into crop-damaging caterpillars.

The scheme, in addition, might replace spraying of insecticides directly onto crops. A floral attractant, a feeding stimulant, and an insecticide might be impregnated in the kind of twist tie used to close garbage bags, for example. When twisted around a cornstalk or tomato vine, the tie need never touch an ear of corn or ripening tomato.

The lures won't threaten honey bees. "The extracts we're looking at," says Lingren, "don't attract them."

Finally, the floral essences would lure both males and females, an important advantage over single-sex lures such as today's sex pheromones.

Lingren and colleague Raulston have spent years night-stalking the moths each March through November to discover clues to the secret lives of the insects. The scientists' nocturnal forays have revealed that moths' favorite nectar-bearing plants in the South and Southwest include the night-blooming species Gaura drummondii, G. longiflora, and G. suffulta.

These species are roadside weeds, growing from 1 to 15 feet high. They produce honey suckle-like blossoms of pink, white, or reddish-pink. In bloom, a single bush of G. longiflora may boast as many as 1,000 flowers.

Each gaura flower puts out an impressive food bribe for moths - about 5 to 10 milligrams of nectar. "That's a tremendous amount for a single flower," says Lingren.

Gaura has no economic use, although Indians in Mexico once made a poultice from it to threat arthritis.

To mimic the floral essences that tantalize moths, scientists must first identify the chemicals that make these aroma-imparting compounds. That's the job of ARS chemists such as Ted Shaver at College Station and Roy Teranishi at Albany, California.

Teranishi has, for example, already pinpointed about a dozen key chemicals from G. drummondii flowers. The blooms were among the 450,000 gaura flowers that Lingren, Raulston, and others painstakingly collected last year.

Using gas chromatography and other approaches, Teranishi and colleagues at Albany pinpointed about a dozen major compounds from the floral extracts.

Although no one knows exactly what G. drummondii flowers smell like to a moth, Teranishi says our noses perceive the fragrance as sweet, very floral, and much stronger than the perfume of day-blooming weeds. "Flowers that bloom only at night have to kick out a lot of aroma so that they'll stand a good chance of being found and pollinated by night-flying moths," he explains.

One of the chemicals gives off what we would pick up as a "pleasant, greeny aroma," he says. Another compound is common in roses. Cinnamon and wintergreen scents are also part of the weed's bouquet.

The experiments at Albany are the first to identify the aromatic compounds of G. drummondii. Luckily, most of the newly identified substances can be synthesized using chemicals right off the shelf.

Now the challenge is to discover if these chemicals are indeed the ones that tempt moth appetites, and - if so - what blend of these compounds will make the most potent lure.

To find out, the Texas entomologists test the candidate chemicals in different blends. Outdoors, they monitor traps containing cotton wicks saturated with floral chemicals. Indoors, they fan scented and unscented air past the moths, then watch to see which breeze attracts the insects.

There are thousands of possible mixtures, so it may take at least a year to discover the ideal recipe.

The floral lure strategy may tomorrow protect individual crop fields and backyard gardens. The technique may offer even broader protection if it snares moths before they have a chance to migrate. Such control would be significant, because moths can travel 300 miles or more in a single night when the wind is right.

In southern Texas, for example, moths abandon maturing cornfields, unsuitable for egg-laying, and migrate north to vast cottonfields. "We want to stop the moth at its source before it has a chance to spread from one crop to the next," says James Coppedge, an ARS national adviser on insect research. If the lures turn out the way researchers hope, growers and gardeners alike will have a new, safe, and powerful weapon for fighting the versatile moth.

PHOTO : Chemist Roy Teranishi analyzes the volatile chemicals from night-blooming flowers that attract the corn earworm moth. (K-4052-1)

PHOTO : Corn earworm moth feeding on gaura, a night-blooming weed. (K-4054-14)
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Title Annotation:includes related article on hunting moths in the dark
Author:Lingren, Peter D.
Publication:Agricultural Research
Date:Jun 1, 1991
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