The last part is the most interesting...
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Date: 04 Aug 98 15:22:59 -0500
From: amy raley <amy_raley@aes.purdue.edu>
Subject: AgAnswers/BtCornFailure?/ForageSeedings/Events
To: Ag Answers mailgroup <ag_answers_mg@ecn.purdue.edu>
Reply-To: amy raley <amy_raley@aes.purdue.edu>
8/4/98
DON'T ASSUME YOUR Bt IS BROKEN BECAUSE BORERS ARE BITING
For farmers fighting corn borers, one of the most welcome
arrivals in their fields was Bt-enhanced corn.
For the same farmers, no arrival could be less welcome than corn
borers munching through their genetically enhanced crop.
However, Purdue entomologists caution farmers not to jump to the
wrong conclusions at the sight of a few caterpillars in their corn.
Although there are concerns that the corn borers may eventually
develop resistance to Bt-enhanced corn, Purdue Extension entomologist
Larry Bledsoe says some crop damage is to be expected even in the
genetically modified crops.
"No bag of Bt seed is pure. No quality control can manipulate the
amount of control in each plant," he says. "If a farmer was in a field
and found a couple of plants being chewed up by corn borers, that
would be normal."
Bt corn uses a portion of the genes of the bacterium Bacillus
thuringiensis, which produces a crystal-like protein that kills the
insect when it mixes with enzymes in the insect's gut. The crystal
protein has no effect on people, animals or even other insects that
don't belong to the Lepidopterous group of stalk-boring pests.
Scientists have moved the gene for this protein from the Bt
bacterium into corn plants, which allows the leaves and stalks of the
plants to produce it and fight a pest which has accounted for corn
losses exceeding $1 billion annually in the United States.
Typically seed lots of Bt-corn contain a small amount -- less
than 4 percent -- of plants that produce little or none of the
protein. "This means that a few plants aren't expressing the Bt gene.
That's to be expected," Bledsoe says.
Another explanation for finding corn borer caterpillars in
resistant corn may lie in where the Bt-corn is planted. "If you plant
next to a field with no resistance, some of those corn borers are
going to come into the resistant field and feed along the edges for a
while before they are killed," Bledsoe says.
A third reason for corn borers in resistant corn is that the
amount of resistance in the plants isn't consistent through the
growing season. "There's a slow loss of resistance in the plant,"
Bledsoe says. "It's very strong at the beginning of the season, but
later in the season the amount of resistance drops."
******!!!
Unfortunately, there's a fourth possibility for why a farmer
might find corn borer caterpillars in the corn, and it is that a new
strain of corn borers have evolved in that area. There have been more
than 500 examples of insects that have developed resistance to various
chemical insecticides, and widespread overuse of genetically enhanced
crops could cause the same thing to happen with those control methods.
There are no known incidents of corn borer developing widespread
resistance to Bt crops, but scientists know that it is possible,
because they've created Bt-resistant caterpillars themselves.
If a farmer has taken all of this into account and still suspects
that corn borers have developed resistance in a particular field,
Bledsoe recommends that he or she take the following steps:
* Double check field records to be sure that Bt corn was planted
in the field.
* Read the grower's guide for your seed and follow the company's
procedure for investigating suspected resistance failures.
* Contact seed company representatives and the county Extension
educator as soon as possible. "Just try to keep a cool head," Bledsoe
suggests. "You should call your seedsman, who will take samples from
the area so that the corn borers can be tested to see if any type of
resistance has developed."
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