Absence of toxicity of Bacillus thuringiensis pollen to black swallowtails under field conditions

Abstract

A single laboratory study on monarch butterflies has prompted widespread concern that corn pollen, engineered to express Bacillus thuringiensis (Bt) endotoxin, might travel beyond corn fields and cause mortality in nontarget lepidopterans. Among the lepidopterans at high potential risk from this technology is the black swallowtail butterfly, Papilio polyxenes, whose host plants in the midwestern U. S. are located principally in narrow strips between roads and crop fields. A field study was performed to assess whether mortality of early instar black swallowtails was associated either with proximity to a field of Bt corn or by levels of Bt pollen deposition on host plants. Potted host plants were infested with first instar black swallowtails and placed at intervals from the edge of a field of Bt corn (Pioneer 34R07 containing Monsanto event 810) at the beginning of anthesis. We confirmed by ELISA that pollen from these plants contained Cry1Ab endotoxin (2.125 +/- 0.289 ng/g). Although many of the larvae died during the 7 days that the experiments were run, there was no relationship between mortality and proximity to the field or pollen deposition on host plants. Moreover, pollen from these same plants failed to cause mortality in the laboratory at the highest pollen dose tested (10,000 grains/cm(2)), a level that far exceeded the highest pollen density observed in the field (200 grains/cm(2)). We conclude that Bt pollen of the variety tested is unlikely to affect wild populations of black swallowtails. Thus, our results suggest that at least some potential nontarget effects of the use of transgenic plants may be manageable.

Figure 1

Average wind speeds (km/h) and wind directions during the times that each array was monitored in the field. Direction refers to the direction that the wind traveled. Data were obtained from the Illinois State Water Survey station located 6 km southwest of the field site.

Figure 2

Pollen loads, larval mass, and survivorship of early instar black swallowtails as a function of distance from the edge of field of Bt corn. (a) First array. Plotted data are the mean and SD of pollen counts associated with the five plants in each row. The regression was based on average total pollen counts at each distance from the edge of the field (F = 32.3, df = 1,3, P = 0.0108, r² = 91.5). (b) Second array. The regression for pollen count as a function of distance from the field was based on averaged pollen counts at each distance (F = 285, df = 1,3, P = 0.0005, r² = 98.9). Total pollen counts were based on the summed counts for the two sequentially placed traps in each pot.

Figure 3

Survivorship of black swallowtails on Bt-, control-, and non-Bt pollen-treated leaf disks of wild parsnip. At the end of the third day, only the Max 454 pollen caused significant mortality compared with the acetone control (X² = 31.4, P < 0.0001) and non-Bt Pioneer variety (X² = 38.9, P < 0.0001).