Questions about geneticly engineered plants with the gene to produce
bt so they will not be eaten by Colorado Potato Beetles (CPB) and other
herbivorous insects have come up on sanet-mg, sustag-l, and other
ag lists and usenet groups. I asked a friend who is a potato pathologist
about the possibility of CPB developing resistance to bt if the
Monsanto potato clones are widely used. Here is the gist of his
answer, perhaps people from Monsanto could correct any points that I
have gotten wrong:
Lab tests have shown that CPB can be selected to tolerate higher levels
of bt in their diet than the general population of CPB now shows. If
the food provided to a lab population contains just enough bt to kill
most of the population, the descendents of the survivors will tolerate
higher levels of bt, perhaps several times higher. However the
Monsanto potato clones have bt levels hundreds of times higher than
the minimum needed to kill most of the population. So even unusually
resistant CPB will be killed if they munch on these plants. If all
the insects are killed (no survivors), there is no opportunity for
selection for an increasingly resistant population, rather selection
would be for CPB who avoid bt potatoes. Anyway perhaps someone from
Monsanto could comment further on these points.
Concern has been expressed that if CPB or other insects develop
tolerance to bt, organic growers (who are now applying bt extracts to
their crops as a biological control) could lose an important control
tool. If the reasoning and information in the above paragraph is
correct, then application of bt extracts at low levels or unevenly
would be more likely to cause selection for resistance than would
widespread use of the geneticly engineered plants.
By the way, another use of the bt clones might be as a barrier around
standard plant varieties. If the barrier is wide enough, very few of
the CPB would get across it, and those few could be controlled with
spot application of bt extract or other control methods. Since the
barrier strip could be harvested and marketed separately, this might
be a most economical organic control measure.
Tom Hodges thodges@beta.tricity.wsu.edu