EFFECTS OF CRY1AB-EXPRESSING CORN ANTHERS ON MONARCH BUTTERFLY LARVAE

Authors: ANDERSON, PATRICIA - IOWA STATE UNIVERSITY; Hellmich Ii, Richard; SEARS, MARK - UNIVERSITY OF GUELPH; Sumerford, Douglas; Lewis, Leslie

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2004
Publication Date: 8/1/2004
Citation: Anderson, P.L., Hellmich II, R.L., Sears, M.K., Sumerford, D.V., Lewis, L.C. 2004. Effects of Cry1Ab-expressing corn anthers on monarch butterfly larvae. Environmental Entomology. 33:1109-1115.

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 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. During these studies a question arose concerning the possible effect of Bt anthers on monarch larvae. This paper reports studies that were designed to quantify anther distribution in space and time, measure effects of anthers in the laboratory and field, and examine effects of anthers and pollen on larvae separately and in combination. Monarch larvae exposed to an abnormally high dose of Bt anthers fed and weighed less, had delayed development, and had decreased survival compared with larvae exposed to non-Bt anthers. Effects of exposure to Bt anthers and pollen in combination were greater than effects of exposure to each separately. Anther distribution studies indicate that only a small fraction of milkweed plants in cornfields have high levels of anthers during pollen shed. Overlap of monarch larvae with corn pollen and anther shed is small. Thus, the impact of Bt corn anthers on monarch populations is likely to be small to negligible. This information will be useful for all stakeholders interested in the potential nontarget effects of transgenic plants.

Technical Abstract: Previous studies suggest that exposure to Bacillus thuringiensis (Bt) corn anthers may have adverse effects on populations of the monarch butterfly, Danaus plexippus (L.). To explore the risk of Bt-corn anthers to monarch butterflies, studies were designed to quantify anther distribution in space and time, measure effects of anthers in the laboratory and field, and examine effects of anthers and pollen separately and in combination. In the laboratory, larvae exposed to a high dose of Bt anthers fed and weighed less, had delayed development, and had decreased survival compared with larvae exposed to non-Bt anthers or no anthers, indicating that Bt anthers pose a potential hazard to monarch larvae. However, studies on dose response and anther distribution in the field showed that these doses are relatively uncommon, occurring on <5% of leaves inside cornfields during anther shed. When larvae were exposed to five Bt anthers per leaf in field-cage studies, no significant adverse effects were detected. Effects of exposure to Bt anthers and pollen in combination were greater than effects of exposure to each separately. In a cage study conducted in an environmentally controlled room with common milkweed, larvae exposed to Bt anthers and pollen continuously for 10 days had a 1 to 2-day delay in development and a 6% reduction in pupal weight. These potential effects must be put into the context of the proportion of the monarch population that is likely to come in contact with toxic levels of anthers and pollen.