> 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.
His remarks here are somewhat misleading. The few percent of plants in
the Bt corn that are susceptible to ECB, result, in my experience, from
seeds of the wrong variety of corn mixed in by mistake. The Bt gene is
often placed only in the male parent in hybrid seed production. This
reduces the time needed to blow up parent seed for the Bt version of a
hybrid. In seed production in the Midwest, it is almost never possible
to obtain as much isolation from other corn fields as one would like.
The foreign pollen that blows into the seed production field often
results in a few percent contamination of the seed by genetic off-types.
Generally speaking, these do not carry the Bt gene. Usually, if one
looks closely at these plants during flowering, they can be seen to be
off-types, differing in things like height, glume color, or tassel
morphology. This QC problem is going to be embarassing for many seed
companies when they market herbicide resistant corn!
> 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."
The first Bt corns introduced in 1996 by Ciba Seeds, (Maximizer)
employed a promoter that led to expression only in green tissue. Second
generation ECB sometimes gets into the stalk by boring through just
above the leaf collar, and into the ear through the silk channel. As
the plant turns brown, susceptibility increases dramatically, but it is
too late to make much difference in practical terms. In walking fields
of this stuff under heavy ECB pressure, I've only seen about one in a
thousand plants with stalk breakage (in fields with high genetic
Monsanto's main construct, "YieldGard", is expressed throughout the
plant, and provides more complete and longer-lasting protection. But
probably not much better in an economic sense, for ECB anyway.
> 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.
So far this doesn't seem to be the case, but it is probably a matter of
time. The doomsayers predict that widespread resistance will arise in a
few years. The industry predictions are much longer. Since Bt corn has
been out there for three years, and large acreages are now planted, I
guess we will know in a few years if the doomsayers are right!
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