Toxicity and exposure studies show that Bt-expressing corn pollen poses negligible risk to populations of the monarch butterfly, but the potential of Bt-corn anthers to harm monarch butterfly larvae is unclear. Laboratory studies showed adverse effects when larvae consumed small anther pieces produced as an artifact of pollen processing. However, only whole anthers are found deposited on milkweed plants in corn fields; it is unknown whether monarch butterfly larvae feed on whole Bt-corn anthers and if such feeding has adverse effects. To assess if Bt-corn anthers pose a significant risk to monarch butterflies in Iowa, we determined what densities of anthers are toxicologically significant, quantified anther densities on milkweed leaves in and near cornfields, and tested for effects of relevant anther densities in the field. Laboratory studies showed that monarch butterfly larvae will feed on whole corn anthers on milkweed leaves, but such feeding was sporadic. During the first four days of development, larvae were being affected indirectly, without ingestion, by the presence of Bt anthers. Behavioral studies were conducted to examine this phenomenon. Although Bt anthers were shown to pose a potential hazard to monarch butterfly larvae when tested in the laboratory, further studies on dose response and anther distribution showed that toxicologically-relevant densities are uncommon on milkweed leaves in cornfields during anthesis. When monarch butterfly larvae were exposed to Bt anthers in the field, no adverse effects were detected.