Monarch larvae sensitivity to Bacillus thuringiensis- purified proteins and pollen
- Richard L. Hellmich*,†,
- Blair D. Siegfried‡,
- Mark K. Sears§,
- Diane E. Stanley-Horn§,
- Michael J. Daniels¶,
- Heather R. Mattila§,
- Terrence Spencer‡,
- Keith G. Bidne*, and
- Leslie C. Lewis*
- *U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS), Corn Insects and Crop Genetics Research Unit, and Department of Entomology, Iowa State University, Ames, IA 50011; ‡Department of Entomology, University of Nebraska, Lincoln, NE 68583; §Department of Environmental Biology, University of Guelph, Guelph, ON, Canada N1G 2W1; and ¶Department of Statistics, Iowa State University, Ames, IA 50011
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Edited by M. R. Berenbaum, University of Illinois at Urbana-Champaign, Urbana, IL, and approved August 17, 2001 (received for review June 12, 2001)
Abstract
Laboratory tests were conducted to establish the relative toxicity of Bacillus thuringiensis (Bt) toxins and pollen from Bt corn to monarch larvae. Toxins tested included Cry1Ab, Cry1Ac, Cry9C, and Cry1F. Three methods were used: (i) purified toxins incorporated into artificial diet, (ii) pollen collected from Bt corn hybrids applied directly to milkweed leaf discs, and (iii) Bt pollen contaminated with corn tassel material applied directly to milkweed leaf discs. Bioassays of purified Bt toxins indicate that Cry9C and Cry1F proteins are relatively nontoxic to monarch first instars, whereas first instars are sensitive to Cry1Ab and Cry1Ac proteins. Older instars were 12 to 23 times less susceptible to Cry1Ab toxin compared with first instars. Pollen bioassays suggest that pollen contaminants, an artifact of pollen processing, can dramatically influence larval survival and weight gains and produce spurious results. The only transgenic corn pollen that consistently affected monarch larvae was from Cry1Ab event 176 hybrids, currently <2% corn planted and for which re-registration has not been applied. Results from the other types of Bt corn suggest that pollen from the Cry1Ab (events Bt11 and Mon810) and Cry1F, and experimental Cry9C hybrids, will have no acute effects on monarch butterfly larvae in field settings.
Footnotes
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↵ † To whom reprint requests should be addressed at: USDA-ARS, Corn Insects and Crop Genetics Research Unit, Genetics Laboratory, c/o Insectary, Iowa State University, Ames, IA 50011. E-mail: rlhellmi{at}iastate.edu.
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This paper was submitted directly (Track II) to the PNAS office.
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↵ ‖ First instars consumed 3.3 ± 0.5 mg of leaf tissue per day (daily leaf-disc area consumed 0.19 ± 3 cm2; leaf weight 17 ± 1.6 mg/cm2). Assuming artificial diet consumption is similar, protein consumption equivalent to the dietary EC50 is 0.00209 ng of protein per day (0.64 ng protein/g × 0.0033 g/day). If pollen Cry1Ab concentration is 1.1–5.0 μg/g (reported ranges of Cry1Ab concentration in event 176 pollen), the equivalent pollen concentration leading to an EC50 is 1.9–0.42 E-06 g of pollen per day. Assuming there are ≈1.5 million pollen grains per gram pollen, the equivalent number of pollen grains leading to an EC50 is 1.7–5.7. If the daily milkweed consumption by first instars is 0.19 cm2, the number of pollen grains that must be present on the leaf equivalent to the EC50 is 7–30 pollen grains/cm2 leaf.
- Abbreviations:
- Cry,
- crystal;
- USDA-ARS,
- U.S. Department of Agriculture–Agricultural Research Service;
- EPA,
- Environmental Protection Agency
- Copyright © 2001, The National Academy of Sciences
