I think a previous message explained the meaning of an "incompletely
dominant autosomal" gene. The significance of this finding is that all the
strategies for Bt resistance management are based on the assumption that
resistance to Bt will be recessive. I gather from the paper in Science that
it is still possible that the resistance might be functionally recessive at
high doses. Part of the rationale for high doses with high expression
through as much of the plant as possible was to increase the likelihood that
resistance genes would be rare and functionally recessive. I assume the
authors of this paper are working on determining the level of resistance
this laboratory-selected strain has to the various Bt-transformed products
and how that resistance (if it even is apparent at that high dose) is
inherited.
The assumption that the resistance would be recessive is the basis for
specifying refugia. The recessive gene would have no effect on survival of
the corn borer unless it is present in two copies. The refugia would
produce susceptible corn borers (because there is no selection for
resistance to Bt there) to mate with any individual survivor with two copies
of the gene for resistance. If the gene for resistance is recessive, those
offspring of the matings between susceptible and resistant individuals would
be susceptible and would not survive on a Bt-transformed plant, and would
thus have no advantage in fitness. If the gene for resistance is completely
dominant, the offspring of matings between susceptible and resistant
individuals would be just as resistant as the resistant parent and would
survive just as well. If the gene for resistance is incompletely dominant,
the offspring between a parent with 2 genes for resistance and another
parent with two genes for susceptibility would have some resistance to Bt,
but not as much as if it had two genes for resistance. So, having refugia
would not help much with resistance management.
It is not clear what *would* help with resistance management in this
situation. For the answer to that, I would turn to someone like Fred Gould
(entomologist at North Carolina State University), who knows how to make
models to answer complex questions of population genetics. I bet he and
other modelers are already working on it.
To turn to the more entomological questions in your post: Insects (in this
case, moths, since they are the only insects affected by this kind of Bt) of
corn that attack fruit and vegetable crops -- there several of them, and
they attack lots of crops. European corn borers are major pests of sweet
corn, of course, and there have been excellent efforts to use Bt for
managing them in sweet corn. University of Massachusetts (Dr. David Ferro
and Ruth Hazzard) has done good work on this. ECB is also an important pest
of peppers and snap beans, and a minor pest of potato and apple. But,
because it feeds on such a wide range of crop plants (the book in front of
me -- European Corn Borer, Development and Namagement, North Central
Regional Extension Publication No. 327 from May 1989-- mentions cotton, lima
bean, soybean, small grains, sorghum, tomato, and onion), you have to be
concerned about the possibility of exchange of resistant individuals among
crops. I don't think the movement of ECB is well understood -- and neither
is diversity within the species. There are different "strains" of ECB -- in
terms of which pheromone blends they respond to, whether they have one or
two generations per year, and in terms of host plant preferences. But the
relationship, if any, between these characteristics identifying strains is
unclear.
The corn earworm also feeds on a very wide range of host plants. It
overwinters in the south and migrates north each year, so there is clearly
movement on a regional scale, and within a region there are massive
movements from one crop to another. At North Carolina State there is a
model of corn earworm population dynamics and movement between crops,
developed by Dr. Ron Stinner, which also provides the basis for Fred Gould's
models. Corn earworm is a major pest of tomatoes, cotton, and soybean.
In addition, there are numerous cutworms and armyworms that occur in corn,
but are generalist feeders. Any of these could upset pest management plans
in vegetables based on the use of Bt.
Hope this is helpful.
Kimberly A. Stoner
CT Agricultural Experiment Station
P.O. Box 1106
New Haven, CT 06504
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