At the request of the Union of Concerned Scientists, Chuck Benbrook prepared an analysis of the benefits assessments developed by EPA for review by the Scientific Advisory Committee (SAP) during its October 18-20, 2000 meeting on the reregistration of Bt crops (for information on the meeting and the full text of some of the presentations, go to http://www.biotech-info.net/bt-transgenics.html#ECB). Benbrook's review of the EPA benefits assessment is accessible at http://www.biotech-info.net/UCS_benefits_text.pdf; its availability was announced on the Internet.

On October 31, 2000, Leonard Gianessi circulated via the Internet a critique of Benbrook's review. The full text of the Gianessi review has been posted on Ag BioTech InfoNet at http://www.biotech-info.net/bt_benefits_gianessi.pdf. It, and other papers by Gianessi, including a lengthy report on the benefits of Bt crops, are also available via the National Center for Food and Agricultural Policy's website (http://www.ncfap.org/selected.htm). Here, Benbrook responds to some of the major points raised in Gianessi's critique.

My initial review and Gianessi's comments focus on the assessment of the benefits of Bt corn. My comments were directed to the EPA's benefits assessment - its conclusions and their analytical foundation. Gianessi's comments raise questions with some aspects of my analysis, but mostly make the generic case that Bt corn delivers significant benefits, especially in high ECB pressure years. This point is also made in EPA's assessment and is not disputed in my comments; in years of high ECB pressure, Bt corn provides a good return to the grower's investment.

There are two key issues in dispute: Does the available data and EPA's analysis support the agency's conclusion that Bt corn has reduced insecticide applications targeting the ECB about 30 percent since 1995? Second, does the annual planting of Bt corn deliver net benefits, given that ECB population levels are highly episodic?

Has Bt Corn Reduced ECB Insecticide Use 30 Percent as EPA Claims? I argued in my original paper that the EPA's data and analysis does not support that conclusion. In order to reach its conclusion, EPA had to determine what portion of all corn insecticide acre-treatments targeted the ECB. I reviewed EPA's method, adopted those aspects of it which I felt are supported by reliable information, and altered assumptions that do not conform with well-known information about corn insecticide use.

The key problem I highlighted with EPA's analysis was its assumption that one-half of corn acres treated with chlorpyrifos and methyl parathion target the ECB, the other half, the corn rootworm/cutworm complex of insects (or just other insects). Instead, I cited industry, EPA, academic and a report by Gianessi showing that the majority of acres treated with these compounds target rootworms/cutworms, not the ECB. Accordingly, I assumed just one-quarter, not 50 percent, of the acres treated with these two materials targeted the ECB - still probably an over-estimate.

Gianessi's critique is silent on this core issue, neither disagreeing with my 25-75 ECB versus rootworm split for chlorpyrifos and methyl parathion, nor supporting EPA's 50-50 split.

Gianessi points out that some of the minor corn insecticides I counted as targeting the ECB are also used for other insects (i.e., dimethoate, esfenvalerate). I agree. No one knows for sure what portion of the acres treated with these materials were for the ECB in contrast to other insects; the information I reviewed suggests that most applications target the ECB either as the major or a secondary pest. Still, I have moved these insecticides to the "ECB and Other Pests" group (see below), in effect assuming that only 25 percent of the acres treated with them target the ECB. This change, or even assuming that none of the acres treated targeted the ECB, does not substantially change the bottom line.

He states that fipronil is not used for ECB control, while I assumed one-quarter of fipronil use targeted the ECB. Again, I accept this change and have incorporated it in the Revised Table 1.

Gianessi criticizes me for not including foliar Bt in the acres treated for ECB control, asserting that I conveniently chose not to include it because foliar Bt is the one insecticide whose use has clearly declined with the adoption of Bt corn. Gianessi is an expert in the design and use of USDA pesticide data, and hence his criticisms on this count are both disingenuous and unfounded. I excluded Bt because USDA did not report Bt insecticide use consistently in the years covered in the table. Gianessi notes accurately that USDA reported 1 percent of the surveyed corn acres were treated with foliar Bts in 1995, based on data collected on Bt use in just 3 of the 17 states surveyed. Actual Bt use in 1995 was no doubt higher nationally.

My Table reported insecticide use in 1995, 1998, and 1999. In both 1998 and 1999, USDA did not collect nor report foliar Bt use at the national level. Bt use was surveyed in just 2 of 15 states in 1999 and in 1 state in 1998, so no one knows whether use remained at about 1 percent, went up or down.

Gianessi is correct in his assertion than the planting of Bt-hybrids no doubt ended the use of foliar Bt sprays on those conventional farms that applied Bt in 1995 but switched to Bt corn varieties thereafter. But other corn growers may have used or switched to foliar Bts, including some not wanting to plant GMO crops. Perhaps foliar Bt use has declined something like one-half of one percent; it matters little in the analysis I presented.

I have recalculated the data in Table 1 based on Gianessi's criticisms. Instead of breaking out corn insecticide applications across the "ECB," "Rootworm," and "ECB + Rootworm" groups, in Revised Table 1 I use "ECB," "Rootworm," and "ECB and Other Pests." For the present purposes, there is no need to be more specific. Gianessi is no doubt correct that some of the acres treated with various insecticides are for control of mites, grubs, and other insects not in the soil borne complex nor ECBs.

I moved fipronil into the predominantly corn rootworm category as suggested, and dimethoate, esfenvalerate, and fenvalerate to the "ECB and other pests" category.

Gianessi also questions whether most acres treated with lambda-cyhalothrin target the ECB. He presents no information to challenge EPA's judgment that it is used mostly for ECB control. Accordingly, I did not change how acres treated with this compound are classified in the Revised Table 1.

With the changes suggested by Gianessi, and the conservative assumption that only one-quarter of the acres treated with "ECB and Other Pest" insecticides targeted the ECB, the data still do not support EPA's conclusion that ECB acres treated have declined 30 percent. The Revised Table 1 shows a 19 percent increase between 1995 and 1999, modestly less than the original table.

The only way to reach EPA's conclusion is to assume that a significant portion of chlorpyrifos and methyl parathion acre treatments target the ECB. This is not the case, as pointed out in my comments.

My original comments, and the revision presented above, use a set of assumptions at least as plausible as EPA's and reach a different conclusion. Perhaps Gianessi agrees that available data on target pests does not support a firm estimate of changes in ECB insecticide use.

Gianessi comments point to an important generic research task - developing more definitive information on the target pest triggering pesticide applications. In most cases, there is no dispute over the major insects triggering corn insecticide applications, but in a few cases there is disagreement, or annual variability, in turn driven by changes in insect populations. If Bt corn varieties for management of the corn rootworm are approved by EPA and marketed, such information will become even more essential. The same point applies in assessing pesticide use impacts of both Bt sweet corn and Bt cotton.

Does it Pay to Plant Bt Corn Annually?

A second issue in dispute is whether it makes sense for farmers to plant Bt corn every year, recognizing that in some years, population levels will not justify the added expense. Gianessi believes Bt corn pays even in low-pressure years; several land grant studies do not support his conclusion.

Sorting out net benefits over time is complex and must take into account many factors, including other potential benefits, costs, and adverse impacts that are hard to translate into dollars and cents. Gianessi believes that Bt corn works well enough and poses so few risks that it should be used routinely; I think that a strong case can be made to rely more heavily on well-documented preventive practices, and to target planting of Bt corn to just those acres where scouting data and historical records project damaging ECB populations.

More targeted use of this technology will surely increase grower benefits per acre. It would also serve other useful purposes, such as enhancing the chances of resistance management and minimizing Monarch butterfly, soil health, and other potential problems.

No doubt Gianessi and I will return to these issues annually as new data are released by USDA on insecticide use in major Bt crops. The same issues and uncertainties will arise next year in the absence of better information on the target pests triggering insecticide treatments in Bt fields. Accordingly, I will be compiling as much target pest data as possible between now and the release of the year 2000 chemical use data next May. Any assistance from experts in the field would be much appreciated. If you have target pest data or know where I can access it, please contact me via e-mail at benbrook@hillnet.com. I'll be sure to share any information gained with Gianessi.

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