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Natural enemies gang up on corn pests.

Natural Enemies Gang Up on Corn Pests

A half century ago in the Midwest, cornfields in terrible condition were a common sight. Strewn and littered with broken stalks, these unproductive fields had fallen victim to the combined ravages of an insect, the European corn borer, and microorganisms that cause stalk rot. While farmers applied chemical insecticides to cope with the problem, microbiologists searched for antibiotic remedies and plant geneticists redoubled efforts to breed resistant corn.

Despite progress, European corn borers still cause an estimated $400 million loss each year. The nation's farmers spend another $50 million to control the pest, says Leslie C. Lewis, an ARS entomologist at Ankeny, Iowa. Estimates on losses from stalk rot generally range even higher.

To reduce corn losses and the prospect of pests developing resistance to chemical controls, scientists hope to marshal several natural forces. Among these are borer-killing insects and microbes such as Beauveria bassiana and Nosema pyrausta that kill borers. Another beneficial microbe, Gliocladium roseum, feeds on stalk rot fungi.

"We're exploring the interactions in nature to find ways of improving control," says Lewis. "We can't depend on one method by itself to control a pest indefinitely."

Traditionally, the front line of defense against lodging (stalks flattened to the ground) has been breeding corn with stalk strength and resistance to stalk rots and corn borers. Scientists are always looking for these traits among newly acquired strains of corn and related plants, says agronomist Mark J. Millard who is with Iowa State University at Ames. Millard is curator for more than 9,000 strains of corn at the ARS North Central Regional Plant Introduction Station on the university campus. About a thousand new accessions are added to the collection each year.

Leaves of some modern corn hybrids contain enough of a compound called DIMBOA to keep each year's first generation of corn borers from doing much damage, says Lewis. It's the second generation hatching in midsummer that poses the greatest threat as larvae eat tunnels in the stalks. The concentration of DIMBOA in corn that has nearly reached its mature size is not great enough to do much good.

Another line of defense besides breeding is gaining popular appeal--applying a bacterial insecticide called Bt (Bacillus thuringiensis). ARS scientists were first to show that Bt does a good job killing corn borers when it is applied to corn just as larvae of the second generation begin to hatch. Bt produces crystals that are toxic to corn borers but harmless to helpful insects, earthworms, birds, and mammals.

A major seed corn company, trying to ensure the safety of its detasseling crews, depends heavily on Bt, avoiding exposure of detasseling crews to chemical pesticides. Detasselers are workers who are transported through fields to strip the male organs from corn plants, preventing self-fertilization of the female portion of the plants. Farm workers apply granules of Bt spores and crystals over the whorl--curled top center leaves--of the plant. Usually, toxicity to the corn borer begins to break down within 24 hours.

Experimentally, at least one biotech company has transformed another bacterium to produce a longer-lasting Bt toxin. And another firm has transformed a bacterium to produce the toxin while living in the seed and the borer's home--the plant's vascular system.

Although genetic diversity from among thousands of Bt strains might be tapped for this corn borer control strategy, Lewis says, "We've yet to learn how quickly borers may develop a tolerance to the toxin if Bt is in every corn plant."

Through research to improve alternative control strategies, Lewis says, corn growers won't be tempted to "put all their eggs in one basket" and Bt resistance might develop more slowly. His research is now focused mainly on the fungus Beauveria bassiana that is entomopathogenic--it makes corn borers sick. Like the bacteria that were genetically engineered to produce Bt toxin, B. bassiana is an endophyte; it lives inside the corn plant.

"B. bassiana has probably been around since dirt," Lewis jokes. The endophyte occurs naturally and has a history of partial effectiveness in keeping corn borers in check. Once scientists find ways to improve how B. bassiana works in an ecosystem with other natural controls, it's expected the pest will develop only a little resistance, if any.

As with other entomopathogens, B. bassiana often suppresses pest populations, most noticeably after a crop may have already become severely damaged. But in a recent 2-year study, Lewis, along with co-worker Lori Anderson Bing, put corn grits sprayed with a suspension of B. bassiana conidia--spores--into the corn whorls before tasseling. By early August, the fungi penetrated the plant's vascular system and reduced season-long corn borer tunneling in stalks about one-third.

Now in the ARS teacher fellowship program, Bing and graduate student Bruce L. Wagner hope to gain insights on how the fungus moves into corn's vascular system. The study may help scientists find ways to enhance corn borer control.

The researchers would like to learn what roles plant genetics, allelochemicals, and other naturally occurring microbes may have in fostering or hindering the populations of B. bassiana and corn borers.

Some light has already been shed on interactions of corn genetics, stalk rots, and corn borers in a 2-year study on six corn hybrids by plant pathologist Charles C. Block of Iowa State and three ARS scientists recently retired or reassigned from Ames and Ankeny.

Two weeks after the hybrids were in full bloom, the scientists inoculated corn-borer-infested stalks and healthy stalks with a mixture of spores from fungi that cause Iowa's three most prevalent stalk rots. Then, in early fall, they found that corn borers increased severity of the stalk rots. But which species of pathogenic microbes had become most prevalent depended on which corn hybrid was involved.

Although the study was focused on yield-robbing pathogenic fungi, Block notes that breeding corn to resist borers could help prevent wounds to the plant. Wounds provide passageways for nonpathogenic microbes to enter the plant and weaken stalks as they digest cellulose.

Corn genetics may also have a great impact on how well several fungi that are mycoparasitic--eat other fungi--thrive as endophytes in plant tissue, says ARS plant pathologist Nader G. Vakili. In a field experiment at Ames, he found that one corn line produced a 65 percent greater yield when protected by the mycoparasite Melanospora damnosa. In a 4-year study he found that populations of M. damnosa steadily built up in a field of continuous corn.

In Central America, disease development is suppressed in indigenous corn intercropped with pole beans, says Vakili. Within inbred corn lines in the ancestry of Corn Belt hybrids, there seems to be a mechanism for control that can be expressed in varying degrees. And as the desired corn genes are identified, they may become valuable to breeders. State agricultural experiment stations have developed 2 inbreds that most often seem to favor beneficial microbes: B73 and M017.

In an experiment intended for studying mycoparasites, Vakili planted B73 in a field highly infested with the entomopathogen B. bassiana. In the fall, he found one-fourth of the stalks supporting B. bassiana. Vakili says, "To me that suggested the seeds I was using had some heterozygous recessive gene that made the stalks amenable to the fungus."

Vakili found that B. bassiana does more than kill corn borers as he looked through his microscope at a newly hatched saprophytic nematode--a nematode that scavenges bacteria and spores of fungi inside plants. Crawling across an agar plate, the nematode backed off from a colony of B. bassiana "like it burned its snout."

With a glass needle, Vakili placed some nematodes in the middle of B. bassiana colonies. A day or two later the nematodes "just stuck straight up like telephone poles," Vakili says. "They were dead."

In research at the National Soil Tilth Laboratory, Ames, Vakili is now trying to learn what effect B. bassiana in the soil may have on earthworms.

Studies by Vakili and Lewis have shown that the vascular systems of some corn hybrids abound with B. bassiana as long as the plants remain green.

Two more nemesis of corn borer, the funguslike microsporidium Nosema pyrausta and a tiny wasp called Macrocentrus grandii are being researched by Lewis and Iowa State entomologist David Orr. The wasp deposits a single egg into a borer larva before it bores into a corn plant. The single egg is destined to become 25 wasp larvae. If the borer larva happens to be diseased with N. pyrausta, the new wasp larvae also become diseased.

There is concern, however, that the two beneficial insects may work against one another. Diseased corn borer larvae parasitized by M. grandii fail to spin cocoons that are needed by the wasp larvae to provide scaffolding for their own cocoons. Without the security of a cocoon, the wasp larvae may become nervous, wander from their source of food, and die.

Sick corn borers suffer from a wanderlust of their own, as Orr learned by training a videocamera on the insects. Larvae that were infected or parasitized became irritable, moving 10 times as much as healthier borers. Larvae that were both infected and parasitized moved 50 times as much.

Orr and Lewis also found that the wasp could transmit disease from infected to healthy borers. They hope to test whether they can release laboratory-raised wasps that will be able to transmit the disease and thereby double the impact of the wasps or the disease alone.

To contact scientists mentioned in this article, write or telephone Ben Hardin, 1815 North University St., Peoria, IL 61604. Phone (309) 685-4011.

PHOTO : Entomologist Leslie Lewis checks for dead European corn borers in corn stalks that contain a biological control fungus. (K-4130-12)

PHOTO : Corrugated paper rings, such as this one held by entomologist Leslie Lewis, form pupation sites for corn borer larvae during laboratory rearing. (K-4131-5)

PHOTO : European corn borer larvae. (K-4126-11)

PHOTO : Iowa State University entomologist David Orr examines a corn stalk. (K-4125-5)

PHOTO : Plant pathologist Nader Vakili checks fungal samples. When used in combination, Beauveria bassiana and Sphaeronaemella helvelli, sharply reduce damage caused by second-generation European corn borers. (K-4133-8)
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Title Annotation:natural method of controlling the European corn borers
Author:Hardin, Ben
Publication:Agricultural Research
Date:Aug 1, 1991
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