Survival of monarch butterfly, Danaus plexippus (Nymphalidae), larvae on milkweed near Bt cornfields

Pollen from corn plants genetically modified to express endotoxins from Bacillus thuringiensis (Berliner) has been identified as a potential hazard to the monarch butterfly, Danaus plexippus (L.), developing in and near cornfields. We conducted two field experiments to examine the effect of Bt corn on larval survival. A two-parameter Weibull model was used to perform detailed comparative survivorship analyses. Survival on milkweed plants near Bt corn and non-Bt corn was similar. Larval mortality rates were lower on milkweed plants located 0, 1, and 2 m from Bt corn compared with larvae 8 m from the corn. Cardinal direction from Bt corn did not influence larval survival. Multiple rainfall events likely resulted in the relatively low Bt corn pollen densities on milkweed leaves. We present evidence that late instar movement may bias estimates of survivorship in field studies.