This report is not likely to generate the attention received by the
much-debated Losey et al. Nature article on Bt-pollen and Monarchs,
although it certainly should.
In a nutshell, the new report finds that a common current Bt-transgenic
corn variety exudes Bt toxin through root exudates -- actually, this
finding comes as no great surprise, just as the Losey et al. finding that
Bt corn pollen can kill Monarchs was not a surprise. But the new report
and research documents that the activated Bt toxin is exuded through roots,
binds with soil particles, becomes very stable -- persisting in the soil
for 243 days, and that it remains active (i.e. toxic) to soil insects for
very long periods. Hence, Bt toxin from root exudates will be augmented by
toxin in residues from corn plant residues later in the fall and winter.
The implications of the findings reported in the new article are largely
unknown but could be enormous. Bt comes from a common soil bacterium; to
the extent that Bt-transgenic crops significantly enhance the quantity of a
particular toxin in soil, there will be impacts on other soil
microorganisms and soil microbial communities. These impacts will, in turn
effect nutrient cycling and uptake, microbial biocontrol of soil pathogens,
and perhaps the development and triggering of the corn plant's immune
system. As the authors point out, some of the effects are likely to prove
positive and some negative.
These impacts may be transitory and insignificant; it is likely they will
be major in some soils and systems and not in others; they may prove
short-lived and highly cyclic, or they may build overtime, reaching a point
in some fields where major and sustained shifts occur in soil microbial
communities. They may prove modest compared to the impact of tillage; they
may reinforce some of the adverse impacts of tillage, or actually prove a
bigger problem in no-till systems.
The second reason that the paper is so important is that the next major
EPA review of a GMO corn is about to get underway, with commercial
introduction planned for crop season 2002, if regulatory approvals are
received in time. The new variety is being developed by Dow AgroSciences
in cooperation with Pioneer and has been engineered to express a Bt toxin
in corn root systems and exudates at levels high enough in order to control
the corn rootworm complex, by far the toughest, nastiest set of insect
pests corn farmers have to manage.
Resistance management will again be a major concern, and again the high
dose strategy will almost certainly be a cornerstone of the strategy
deployed and the evidence advanced by the company in support of approval.
There has been no public discussion or scientific community appraisal of
what a Bt-toxin "high dose" for corn rootwrom management will be, but rest
assured it will be two or more orders of magnitude higher than the levels
analyzed in the just published Nature piece.
Proponents of the new Bt-corn technology will correctly point out that
corn rootworms are damaging pests in second year corn fields (i.e. a field
planted to corn two or more years in a row). In most parts of the corn
belt, rotation with soybeans remains an effective cultural practice that
lies at the heart of corn rootworm IPM systems, and has for 30 years. But
the plot has thickened in recent years -- a new strain or subspecies of the
western corn rootworm has adapted around rotation and is causing economic
damage in some first year corn fields in parts of the corn belt. Its range
is expanding every year and insecticide applications for corn rootwrom
control are clearly rising. (For a detailed discussion of this adaptation
and overview/references/links to key University of Illinois research on
this new problem, see Section D, "Evolving Insect Pest Challenges," page 17
in the January 1999 paper "World Food System Challenges and Opportunities:
GMOs, Biodiversity, and Lessons from America's Heartland," accessible at
<http://www.biotech-info.net/IWFS.pdf>).
Until recently, most of the soil insecticides used to control corn
rootworms have met everyone's definition of nasty. Highly toxic carbamate
and organophosphate insecticides accounted for the lion's share of acres
treated and pounds applied. Most farmers hate handling these
insecticides; they pose significant risks to birds, fish, pets, and a range
of beneficial organisms. In the last two years, two much safer new
insecticides have come on the market. A synthetic pyrethroid product
marketed by Zeneca called Force (active ingredient, tefluthrin) is getting
rave reviews by farmers and may soon emerge as the product of choice. From
an environmental perspective, it is far, far less damaging than the OP and
carbamate insecticides it is replacing.
EPA's review and approval decision on the new Bt-corn for rootwrom control
is going to really put the agency to the test. Without a doubt, there will
remain major unresolved issues regarding resistance management and soil
microbial community and plant health impacts. In addition, there will be
major debates about the actual "benefits" of the technology in light of the
availability of cost-effective alternatives.
In the meantime and hopefully prior to approval, information is needed on
the exact toxin expressed in roots; the level of expression and the
temporal dynamics of expression, along with levels in plant tissue and
residues; its fate in soil ecosystems under different tillage and planting
systems; the impact of the Bt toxins on various beneficial and pathogenic
soil microorganisms and arthropods/decomposers. This information will be
among that needed in order to determine whether this technology might lead,
on balance, to sustained and significant adverse impacts on soil quality
and plant health.
As the authors state, what goes on underground in a field planted to
today's Bt-corn varieties is largely a mystery. Enhance the toxin levels
100- to 1,000-fold and it becomes a mystery of some consequence and
immediacy.
chuck benbrook
Charles Benbrook CU FQPA site www.ecologic-ipm.com
Benbrook Consulting Services Ag BioTech InfoNet www.biotech-info.net
5085 Upper Pack River Road IPM site www.pmac.net
Sandpoint, Idaho 83864
208-263-5236 (Voice) 208-263-7342 (Fax)
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