Commentary on the Nature article: "Transgenic plants: Insectidal toxin in root exudates from Bt corn, by Saxena, Flores and Stoksky, NYU and Venezuelean Science Institute"

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 affect 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 over time, 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 US Environment Protection Authority 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 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. 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.

Charles Benbrook, Benbrook Consulting Services, 5085 Upper Pack River Road, Sandpoint, Idaho 83864 208-263-5236 (Voice), 208-263-7342 (Fax)