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Swallowing the Tale of the Swallowtails: No "Absence of toxicity" of Bt Pollen The paper which claims "absence of toxicity" of Bt-pollen under field conditions is faulty in experimental design and actually demonstrates toxicity of Bt-pollen in the laboratory. A study in Cornell University last year (1) prompted widespread concern that pollen from Bt-corn may be harmful to the Monarch butterfly. Researchers from the University of Illinois now claims that a field study on the black swallowtail, Papilio polyxenes, shows that Bt-pollen is not toxic to this species (2). The black swallowtail feeds on host plants found in narrow strips between roads and crop fields in midwestern USA. A day after the start of Bt-pollen release, researchers set up five rows of five potted host-plant beside a field of Bt-corn (Pioneer variety 34R07 expressing the CrylAB gene in its pollen), at various distances from the edge of the field. Pollen traps consisting of a microscope slide coated with vaseline was placed with each plant to measure total pollen deposited. A second set of potted plants were placed behind the first set three days later. Ten first instar larvae were put on each plant, and the number of live larvae on each plant recorded daily for 7 days. However, no control experiments were set up. A proper control experiment would have consisted of a replicate set of potted host plants and larvae placed next to a non-GM corn field. It rained during the 5th and 7th day of the first experiment, and during the 2nd, 4th and 5th day of the second experiment. Would that not have washed away the pollen from the surface of the leaves. If so, what relevance would the pollen counts - on greasy pollen traps - have on actual pollen ingested by the larvae? Pollen counts decreased sharply with distance from the field as expected; but there was no correlation between pollen counts and mortality. Even though the larvae were counted everyday for seven days, the detailed counts were not given. Instead, the aggregate percentage mortality was presented. Not only were the mortalities high, they were also highly variable. The means ranged from 45 to 82%, and in many cases, the standard deviation in each direction was almost as large as the mean. It was obviously impossible to draw any conclusion from such an experiment. But they stated, "No significant relationships between larval survivorship or mass were detected either as a function of distance from the edge of the field or as a function of pollen deposition." That was true, but the main reason may be that it was a bad experiment. They suggested that the high mortalities might be due to predation. If so, would mortality not be correlated with "larval mass"? Yet no such correlation was reported. Back in the laboratory, they deposited different amounts of Bt and non Bt pollen on leaf-discs and fed each in a single dose to a first instar larva which was observed over the next three days. They found no effect with the Bt-pollen collected from the field, even at the highest dosage. But exactly how much Bt toxin did each larva consume? From the figures presented, it can be calculated that at the highest dose used - 10 000 pollen grains – the larva would have consumed only 1 picogram of Bt protein, ie, 1/1 000 000 000 000 or one trillionth of a gram, over the three days. With another Bt-corn pollen - Novartis Max 454 - which expresses 40 times as much Bt protein, ie, 40 picograms, a highly significant increase in mortality was found on the third day: 80% compared with about 10% for the rest. As the laboratory experiments involved feeding a single dose over three days, it gave no information as to the effects on mortality of cumulative doses over the entire life-cycle of the butterfly, such as it may experience in the field. The claim of "absence of toxicity" in the title of this paper is thus misleading to say the least. It will be an abuse of science if this report were to be accepted as evidence that Bt-pollen is safe for black swallowtails. References and Notes 1. Losey, J.E., Rayor, L.S. and Carter, M.E. (1999). Transgenic pollen harms monarch larvae. Nature 399, 214. 2. Wraight, C.L., Zangeri, A.R., Carroll, M.H. and Berenbaum, M.R. (2000). Absence of toxicity of Bacillus thuringiensis pollen to black swallowtails under field conditions. PNAS early Ed.
Mae-Wan Ho
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