Effects of Cry1Ab-Expressing Corn Anthers on the Movement of Monarch Butterfly Larvae

Authors: PRASIFKA, PATRICIA - IOWA STATE UNIVERSITY; Hellmich Ii, Richard; Prasifka, Jarrad; Lewis, Leslie

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2006
Publication Date: 2/5/2007
Citation: Prasifka, P.L., Hellmich II, R.L., Prasifka, J.R., Lewis, L.C. 2007. Effects of Cry1Ab-Expressing Corn Anthers on the Movement of Monarch Butterfly Larvae. Environmental Entomology. 36(1):228-233.

Interpretive Summary: Previous studies have shown that anthers from Bacillus thuringiensis (Bt) corn do not pose a significant risk to the monarch butterfly. However, in these studies, young larvae (four days old) in the laboratory, without feeding on anthers, were adversely affected by the presence of a high density of Bt anthers. A possible hypothesis to explain this is that larvae exposed to Bt anthers spend more time searching or moving around and less time feeding, which results in lower weight gain. To test this hypothesis, two-day-old monarch larvae were exposed to Bt, non-Bt, or no anthers in petri-dish arenas and tracked with a video system. Larvae exposed to Bt anthers fed and weighed less than larvae exposed to non-Bt or no anthers. There was no evidence of anther feeding. There were no differences detected among treatments for measurements of distance moved or percentage of time spent moving. However, larvae exposed to Bt anthers spent more time off the milkweed leaf disk than those exposed to non-Bt or no anthers and were more likely to move off the leaf than larvae exposed to non-Bt anthers. Though the data did not support the hypothesis of increased searching behavior with exposure to Bt anthers, larvae did exhibit some degree of avoidance of Bt anthers. The behavioral changes seen are not likely to occur on milkweed plants in the field because the anther density tested is rare and natural feeding behaviors already reduce exposure to Bt anthers. However, this study suggests that ingestion is not the only way a toxin like Bt can affect non-target insects. This information will be useful for all stakeholders interested in the potential non-target effects of transgenic plants.

Technical Abstract: Studies have shown that anthers from Bacillus thuringiensis (Bt) corn, Zea mays L., do not pose a significant risk to the monarch butterfly, Danaus plexippus (L.). However, adverse effects (decreased feeding and weight) have been detected after 4 days of exposure in the laboratory to a high density of Bt anthers with little evidence of Bt ingestion. A possible hypothesis to explain this is that larvae exposed to Bt anthers exhibit increased searching behavior resulting in less feeding and lower weight gain. To test this hypothesis, 2-day-old monarch butterfly larvae exposed to Bt, non-Bt, or no anthers in petri-dish arenas were observed with a video-tracking system. Larvae exposed to Bt anthers fed and weighed less than larvae exposed to non-Bt or no anthers with no evidence of anther feeding. There were no differences detected among treatments for total distance moved, maximum displacement from release point, or percentage of time spent moving or near anthers. Larvae exposed to Bt anthers spent more time off milkweed leaf disks than those exposed to non-Bt or no anthers and were more likely to move off leaves than larvae exposed to non-Bt anthers. Though the data did not support the original hypothesis, behavioral changes were observed that suggest larvae avoid Bt anthers. The behavioral changes seen are not likely to occur on milkweeds in the field because the anther density tested is rare and natural feeding behaviors already mitigate exposure to anthers. These results suggest that ingestion may not be the only way Bt can affect non-target insects like the monarch butterfly.