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ARS Home » Midwest Area » Ames, Iowa » Corn Insects and Crop Genetics Research » Research » Publications at this Location » Publication #158514
Title: EFFECTS ON MONARCH BUTTERFLY LARVAE (LEPIDOPTERA: DANAIDAE) AFTER CONTINUOUS EXPOSURE TO CRY1AB-EXPRESSING CORN DURING ANTHESIS

Author
item DIVELY, GALEN - UNIVERSITY OF MARYLAND
item ROSE, ROBYN - UNIVERSITY OF MARYLAND
item SEARS, MARK - UNIVERSITY OF GUELPH
item Hellmich Ii, Richard
item STANLEY-HORN, DIANE - UNIVERSITY OF GUELPH
item CALVIN, DENNIS - PENN STATE UNIVERSITY
item RUSSO, JOSEPH - ZEDX, INC.
item ANDERSON, PATRICIA - IOWA STATE UNIVERSITY

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2004
Publication Date: 8/1/2004
Citation: Dively, G.P., Rose, R., Sears, M.K., Hellmich II, R.L., Stanley-Horn, D.E., Calvin, D.D., Russo, J.M., Anderson, P.L. 2004. Effects on monarch butterfly larvae (Lepidoptera: Danaidae) after continuous exposure to Cry1Ab-expressing corn during anthesis. Environmental Entomology. 33:1116-1125.

Interpretive Summary: A consortium of scientists in several States and in Canada conducted a formal risk assessment of the impact of Bacillus thuringiensis (Bt) corn pollen on monarch butterfly populations. Information was sought on the toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to active amounts of Bt pollen on its host plant, the common milkweed found in and around cornfields. These studies suggested that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible. All the scientific information on acute toxicity and exposure supports the conclusion that Bt corn pollen does not pose an unreasonable risk to monarch populations. However, none of these studies considered possible chronic effects to monarch larvae through longer exposures. In the natural setting, larvae hatching at the onset of pollen shed may be exposed to biologically active Bt protein in pollen and anthers for periods of 12 days or more. These longer-term chronic effects could potentially impact the approximately 0.8% of the monarch population exposed to Bt corn pollen in cornfields. For this reason, the EPA amended registration document for Bt corn states that studies of longer-term exposure of monarch larvae to Bt pollen should be considered. Therefore, the studies reported herein address the potential longer-term chronic effects on monarch butterfly larvae feeding continuously on milkweeds with naturally deposited levels of pollen that occur within Bt cornfields. These studies analyze the hazard associated with the most extreme exposure scenario that may occur under field conditions and allow a more precise upper bound for the overall risk to monarch populations to be derived. Results of five studies conducted over two years at three locations show that monarch larvae exposed to pollen throughout their larval development were consistent for all endpoints. Results indicate that 23.7% fewer larvae exposed to Bt pollen during the first week of anthesis reached the adult stage. Exposure to Bt pollen also prolonged the development time of larvae by 1.8 days and reduced the weights of both pupae and adults by 5.5%. The sex ratio and wing length of adults were unaffected. Chronic exposure of monarch larvae throughout their development to Bt corn pollen is detrimental to only a small fraction of the breeding population because the risk of exposure is low. When this impact is considered over the entire range of the Corn Belt, the ecological outcome is very small and likely will not affect monarch populations in North America. This information will be useful for all stakeholders interested in the potential nontarget effects of transgenic plants.

Technical Abstract: Chronic effects on monarch butterfly, Danaus plexippus (L.), following long-term exposure of larvae to naturally deposited levels of Bt and non-Bt pollen on milkweed, were determined in five studies. First instars were exposed at 3-4 and 6-7 days after initial anthesis, either directly on milkweed plants in commercial cornfields or in the laboratory on leaves collected from milkweeds in corn plots. Pollen exposure levels ranging between 122 and 188 grains/cm2/day were similar to those that monarch populations experience in the general population of cornfields. Results indicate that 23.7% fewer larvae exposed to Bt pollen during the first week of anthesis reached the adult stage. Exposure to Bt pollen also prolonged the development time of larvae by 1.8 days and reduced the weights of both pupae and adults by 5.5%. The sex ratio and wing length of adults were unaffected. The ecological significance of these fitness disadvantages is discussed in light of the actual exposure risk and magnitude of differences observed. A risk assessment procedure used previously [see Sears, M.K., Hellmich, R.L., Stanley-Horn, D.E., Oberhauser, K.S., Pleasants, J.M., Mattila, H.R., Siegfried, B.D., Dively, G.P. (2001), Proc. Natl. Acad. Sci. USA 98, 11937-11942] was updated with a simulation model estimating the proportion of second generation monarchs exposed and by assigning a probability of toxic effect of 0.237. When considered over the entire range of the Corn Belt, the risk to monarch populations associated with the most extreme exposure is only 0.58%.