Submitted to: Environmental Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/11/2005
Publication Date: 8/1/2005
Citation: Anderson, P.L., Hellmich II, R.L., Prasifka, J.R., Lewis, L.C. 2005. Effects on fitness and behavior of monarch butterfly larvae exposed to a combination of Cry1ab-expressing corn anthers and pollen. Environmental Entomology. 34:944-952. Interpretive Summary: Anthers and pollen from Bacillus thuringiensis (Bt) corn frequently fall onto milkweeds growing in and near corn fields. Previous studies have shown that exposures to Bt pollen or Bt anthers alone do not pose a threat to monarch butterflies. However, pollen and anthers do not occur alone on milkweed leaves in corn fields, they occur together. This study examines how exposure to a combination of Bt anthers and pollen affects monarch fitness and behavior. When monarch larvae were exposed to a combination of Bt anthers and pollen, they were more adversely affected than when they were exposed to Bt pollen and anthers separately. Laboratory experiments showed that adding pollen to the leaves increased the likelihood that larvae would consume anthers, and larvae may be able to detect and avoid Bt tissues. In cage experiments with common milkweed, larvae exposed to a combination of Bt anthers and pollen took 1.9-2.6 days longer to develop, and pupae weighed 6.4% less than those exposed to non-Bt anthers and pollen. Despite these adverse effects, the overall risk to monarch butterflies in North America is minimal because exposure to hazardous levels of Bt anthers and pollen remains low. This information will be useful for all stakeholders interested in the potential non-target effects of transgenic plants.
Technical Abstract: Anthers and pollen from corn, Zea mays L., expressing Bacillus thuringiensis (Bt) derived protein frequently fall onto common milkweed, Asclepias syriaca L., growing in and near corn fields. Previous studies have shown that alone, Bt anthers do not pose a significant risk to the monarch butterfly, Danaus plexippus (L.). To examine how exposure to a combination of Bt anthers and pollen affects larval fitness and behavior, three studies were conducted: a laboratory study using severed leaves in petri dishes and two studies with caged milkweed plants (tropical milkweed, Asclepias curassavica L., and common milkweed, respectively) in an environmentally-controlled rearing room. In the laboratory, additive effects of Bt anthers and pollen were detected. During the first 4 days of exposure, larvae more frequently consumed anthers when pollen was present. When Bt toxin was present, larvae fed on anthers less frequently, suggesting some degree of avoidance or non-preference for Bt tissues. An additive effect also was seen in both cage studies. In the common milkweed cage study, larvae exposed to Bt anthers and pollen took 1.9-2.6 days longer to develop, and pupae weighed 6.4% less than those exposed to non-Bt anthers and pollen. The anther and pollen levels tested are common on milkweed leaves in corn fields during anthesis. However, when put into the context of exposure and risk to the monarch butterfly population in North America, Bt corn will not pose a significant risk.