ITHACA, N.Y. -- An increasingly popular commercial corn, geneticallyengineered to produce a bacterial toxin to protect against corn pests, hasan unwanted side effect: Its pollen kills monarch butterfly larvae inlaboratory tests, according to a report by Cornell University researchers.
Writing in the latest issue (May 20) of the journal "Nature", the Cornellresearchers note that this hybrid crop, known as" Bt"-corn, has genes fromthe bacterium" Bacillus thuringiensis" ("Bt") spliced into the plant genes.These hybrids are very effective against the ravenous European corn borer,a major corn pest that is destroyed by the plant's toxic tissue. Theengineered corn is safe for human consumption.
Unlike many pesticides, the "Bt"-corn has been shown to have no effect onmany "nontarget" organisms -- pollinators such as honeybees or beneficialpredators of pests like ladybugs. But the "Bt-"modified corn producespollen containing crystalline endotoxin from the bacterium genes. Whenthis corn pollen is dispersed by the wind, it lands on other plants,including milkweed, the exclusive food of monarch caterpillars and commonlyfound around cornfields.
Says John E. Losey, Cornell assistant professor of entomology and theprimary investigator on the study: "We need to look at the big picturehere. Pollen from "Bt"-corn could represent a serious risk to populationsof monarchs and other butterflies, but we can't predict how serious therisk is until we have a lot more data. And we can't forget that Bt-cornand other transgenic crops have a huge potential for reducing pesticide useand increasing yields. This study is just the first step, we need to domore research and then objectively weigh the risks versus the benefits ofthis new technology."
Like all grasses, corn is wind-pollinated, and the pollen can be blownmore than 60 yards from the edge of cornfields. "Pollen is that yellowdusting your car gets on spring and summer days; pollen is everywhere,"Losey explains. "That's why we are concerned about this problem."
Other researchers on the study were Linda S. Rayor, Cornell instructor inentomology, and Maureen E. Carter, Cornell research aide.
"Monarchs are considered to be a flagship species for conservation. This isa warning bell," says Rayor. "Monarchs themselves are not an endangeredspecies right now, but as their habitat is disrupted or destroyed, theirmigratory phenomena is becoming endangered."
In the laboratory tests, monarchs fed milkweed leaves dusted with so-calledtransformed pollen from a "Bt"-corn hybrid ate less, grew more slowly andsuffered a higher mortality rate, the researchers report. Nearly half ofthese larvae died, while all of the monarch caterpillars fed leaves dustedwith nontransformed corn pollen or fed leaves without corn pollen survivedthe study.
The toxin in the transformed pollen, the researchers say, goes into the gutof the caterpillar, where it binds to specific sites. When the toxinbinds, the gut wall changes from a protective layer to an open sieve sothat pathogens usually kept within the gut and excreted are released intothe insect's body. As a result, the caterpillar quickly sickens and dies.
"Bt"-engineered corn is among the first major commercial successes foragricultural biotechnology. Last year, more than 7 million acres of thehybrid crop were planted by U.S. farmers primarily to control the Europeancorn borer. Before the advent of "Bt"-corn, this pest was extremelydifficult to control because it bores into the stalk, where it is protectedfrom pesticides. It produces several generations a year. Because it was sodifficult to control effectively with pesticides, annual losses averaged$1.2 billion. In contrast,"Bt"-corn provides essentially total season-longcontrol at a reasonable cost without the use of pesticides. At least 18different Bt-engineered crops have been approved for field testing in theUnited States. As of last year, transformed corn, potatoes and cotton hadbeen approved by the U.S. Department of Agriculture for commercial use.
Several factors make monarch caterpillars particularly likely to makecontact with corn pollen, Losey says. Monarch larvae feed exclusively onmilkweed because it provides protection against predators. The plantcontains cardenolides, which are toxic, bitter chemicals that the monarchcaterpillar incorporates into its body tissues, rendering it unpalatable topredators. Milkweed grows best in "disturbed" habitats, like the edges ofcornfields, Losey notes.
The butterflies overwinter in Mexico and by the spring begin migratingnorth. The first generation of the year crosses into Texas, other GulfCoast states and Florida, seeking milkweed on which to lay their eggs andfeed. By late May or early June, the second generation of adults hasemerged and heads north to areas including the Midwest Corn Belt. Monarchcaterpillars are feeding on milkweed during the period when corn isshedding pollen, Losey says. Thus "they may be in the right place at theright time to be exposed to "Bt"-corn pollen."
The web version of this release, including any accompanying photos, may befound at http://www.news.cornell.edu/releases/May99/Butterflies.bpf.html
Materials provided by Cornell University. Note: Content may be edited for style and length.
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