Monarch larvae sensitivity to Bacillus thuringiensis- purified proteins and pollen

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.

Figure 1

Mean ± SE 96-h weight (mg) of monarch larvae after exposure to milkweed leaves with no pollen and leaves with various levels of pollen from Bt11 and Mon810 events (Cry1Ab), Dbt418 event (Cry1Ac), Cbh351 event (Cry9C), and Tc1507 event (Cry1F) hybrids, and near-isoline hybrids. Number of trials combined for each event were (Bt11: 7), (Mon810: 3), (Dbt418: 2), (Cbh351: 2), and (Tc1507: 1).

Figure 2

Mean ± SE weight (mg) of monarch larvae after feeding on treated milkweed discs for 96 h. Treatments included: (a) pollen from event Cbh351 (Cry9C) hybrid G8539Bt that was processed with 250-μm sieve (beginning), pollen from same hybrid processed with 90-μm sieve (finely sifted), and siftings remaining after the initial sample was finely sifted; and (b) no pollen, pollen from event Bt11 (Cry1Ab) hybrid N7070Bt, and pollen from hybrid N7070 (non-Bt) that were processed in the same manner as pollen in a. Mean pollen levels for a and b were ≈600 and ≈900 pollen grains/cm², respectively.