Previous studies suggest that exposure to anthers from corn, Zea mays L., expressing Bacillus thuringiensis (Bt)-derived protein may have adverse effects on larvae of the monarch butterfly, Danaus plexippus (L.). The objectives of this research were to: (1) measure the effects of short-term and long-term exposure to anthers in the laboratory, (2) examine anther distribution in space and time, (3) measure the effects of long-term exposure to anthers in the field, (4) examine how exposure to Bt anthers and pollen affects larval fitness and behavior, and (5) investigate how larvae were affected by Bt anthers without detectable ingestion of Bt anther tissue. Laboratory studies revealed that monarch butterfly larvae will feed on anthers on milkweed leaves and showed that Bt anthers are a potential hazard to monarch butterflies. However, toxic anther densities are rare on milkweeds in and near corn fields. Field-cage studies detected no adverse effects on development or survival after larvae were exposed to five Bt anthers per leaf for 11 days. Based on a low probability of exposure to toxic densities, Bt anthers alone are not likely to pose a significant risk to monarch butterflies in Iowa. Data from petri dish and cage studies supported the hypothesis that exposure to Bt anthers and pollen have additive effects on larvae and a possible behavior mechanism for additive effects was revealed. A video-tracking system was used to investigate effects seen on larval feeding and weight after 4 days of exposure to Bt with little detectable anther-tissue feeding. A possible hypothesis to explain these effects is that exposure to Bt anthers results in increased searching behavior which in turn results in less feeding and reduced weight. Larvae did not exhibit increased searching behavior when exposed to Bt anthers, but they did exhibit some degree of avoiDance; Although the behavioral changes seen in this study are not likely to occur in the field because the anther density tested is rare and natural feeding behaviors already mitigate exposure to Bt anthers, this study shows that direct toxicity is not the only means by which a toxin like Bt can affect non-target insects.