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Thwarting one of cotton's nemeses.

Thwarting One of Cotton's Nemeses

There's not much good to be said for the pink bollworm, cotton's most destructive pest, except that it is being controlled to cut crop damage.

Scientists have developed strategies, such as increasing native populations of predatory insects and pest-resistant cotton varieties.

Thanks to research, growers today can also use cultural practices such as early plowdown of harvested cotton to break up stalks and bury overwintering pink bollworms. And they can disrupt normal mating by releasing sterile insects and using copies of natural compounds, called pheromones, that the pink bollworm uses to attract mates.

Such strategies, together with judicious use of insecticides, put together in various combinations, form what is called an integrated pest management system.

"Less than 20 years ago, cotton growers routinely sprayed 12 to 15 times with insecticides during each growing season. Today, they sometimes get by with five or six applications, in large part because of research at the Agricultural Research Service's Western Cotton Research Laboratory in Phoenix, Arizona, and at cooperating universities," says Thomas J. Henneberry, entomologist and laboratory director.

The pink bollworm has destroyed annually an estimated 25 percent of the cotton crop in California's Imperial Valley. In infested areas, that can represent a loss exceeding $190 per acre.

The damage is from larvae that feed on flowerbuds and bolls of cotton plants. There can be up to five generations of the pink bollworm in one 6- to 8-month cotton-growing season. The insect's world distribution, voracious appetite, and enormous populations cause tremendous damage to cotton crops.

The pink bollworm, Pectinophora gossypiella (Saunders), was first identified in 1843. It has been a destructive pest of cotton in most cotton-growing regions of the world since the early 1900's. But it wasn't until the 1950's that scientists, in an effort to control outbreaks, started looking at its biology.

"When I started genetic studies in 1967, I was looking for ways to manipulate the insect's reproduction or its behavior, hoping to make it a candidate for an autocidal or self-killing control method," says Alan C. Bartlett, ARS geneticist at the Phoenix laboratory.

Such methods include releases of millions of males that are genetically sterile. These sterile males mate with native females in cottonfields, and these mated females produce sterile eggs. This is how the pink bollworm is now controlled in the San Joaquin Valley of California.

"We discovered that there was a high degree of genetic variability in the pink bollworm, which is good because the more variability, the greater chance we'll find some genetic flaw we can use against the insect. On the downside, the greater variability makes it more difficult to manipulate the insect's genetics. It also means the insect can adapt more quickly to environmental changes and insecticides, thus surviving," Bartlett says.

As is often the case in plant breeding, plants that have natural insect resistance lack other desirable properties such as high yield. Cotton breeders get around this by crossing bollworm-resistant plants with several generations of plants that possess other desirable properties, says F. Douglas Wilson, plant geneticist at the laboratory.

In a 3-year study at two locations, one such advanced-generation hybrid, when compared to a control variety, required 41 percent less insecticide to control pink bollworm and yielded 12 percent more lint. It was also significantly earlier maturing, which reduces risk of damage caused by early fall rain and reduces the number of overwintering worms to infest next year's crop.

Parasitic and predatory insects of the pink bollworm also offer control. The newest parasite--Trichogrammatoidea bactrae Nagaraja--kills bollworm eggs before they can hatch into larvae. T. bactrae will probably adapt to the climate of the southwestern desert, which it is similar to its native Australian habitat.

"Our laboratory studies show the parasite is active at temperatures ranging from 60 [degrees]F to 90 [degrees]F. Females emerge and lay their eggs inside bollworm eggs within 24 hours. The T. Bactrae insects are nourished by eating the contents of the eggs," says William D. Hutchison, an entomologist formerly with the laboratory.

Unnatural Mating Attraction

ARS entomologist Hollis M. Flint and technician John Merkle were able to alter the response of male pink bollworms so they seek unnatural ratios of two components found in their pheromone. This is the first time an unnatural ratio of pheromone compounds has been made that males prefer over the natural ratio emitted by females.

Flint and Merkle accomplished this by releasing only one of the component isomers into cottonfields. This alters the signal received by the male's antennae. Females that produce the natural ratio of component isomers are no longer able to chemically communicate with males. Instead, males are attracted to unnatural combinations of the two isomers, which may be presented in traps or killing formulations.

ARS and university scientists determined in 1973 that the pheromone of pink bollworm is about a one-to-one mixture of the Z,Z and Z,E-isomers of 7, 11-hexadecadien-1-o1 acetate.

Known as gossyplure, it has been used to disrupt normal mating by placing numerous dispensers of the pheromone in the field to compete with females. Scientists theorize that males go on wild goose chases following the artificial pheromone dispensers and never locate a female. Also, males may become overwhelmed by massive doses of the pheromone released into the air, perhaps blocking their antennal receptor sites and central nervous systems.

The new attractive formulation contains a 9-to-1 ratio of the Z,Z- to Z,E-isomers. Males are especially attracted to this ratio of isomers in a field treated with the Z,Z-isomer, ignoring females along the way. The new system can be used to treat cottonfields and monitor populations with traps baited with the unnatural ratio. The scientists have patented the control and monitoring system and have shown the same methods work for pinkspotted bollworm, a related moth that is a problem in Australia.

PHOTO : Trap for pink bollworm moths. (K-4113-1)

PHOTO : Cotton boll sliced open to expose damage from a pink bollworm. (K-2886-13)

Scientists mentioned in this article are at the USDA-ARS Western Cotton Research Laboratory, 4135 East Broadway, Phoenix, AZ 85040. Phone (602) 379-3524.
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Title Annotation:controlling the pink bollworm
Author:Senft, Dennis
Publication:Agricultural Research
Date:Aug 1, 1991
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