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Monarchs migrate north and east from their overwintering grounds in the fir forests of the Sierra Madre Mountains near Mexico City and establish populations in the southern United States east of the Rocky Mountains. This first generation of monarchs produces adults that move into the northern United States and southern Ontario, including Corn Belt states such as Nebraska, Iowa, Illinois and Minnesota, where the majority of Bt corn is grown. Two to three generations of monarchs are then produced in these areas. Adults from the last generation migrate to Mexico to overwinter.
Monarchs undergo complete metamorphosis with four distinct stages: Egg, larva (caterpillar), pupa (chrysalis) and adult (see Figure 1). Caterpillars go through five “instars,” meaning they shed their skin five times as they grow progressively larger. Monarchs go from egg to adult in about a month. The adults live another two to six weeks in summer, and those that overwinter in Mexico can live more than six months.
The larval stage, when caterpillars feed exclusively on milkweeds, is of greatest interest to researchers, because milkweeds are found within and around Bt cornfields throughout the Corn Belt.
Considering the vast amount of land on which corn is grown, it is not surprising that data suggest that a significant proportion of monarchs produced in the Midwest come from agricultural areas. To evaluate the potential impact of Bt corn on monarch populations, researchers studied the spatial and temporal overlap between the monarch populations and corn pollen production (Oberhauser et al., 2001).
Researchers found in northern areas of the Corn Belt (Ontario, Minnesota and Wisconsin), the peak numbers of caterpillars of the final monarch generation did coincide with pollen shed in the summer of 2000. This meant there was a large overlap of first and second instars with pollen shed, as shown in Figure 2. The overlap of the two windows was much less in the south (Iowa) and east (Maryland), where pollination occurred before peak population buildup. Although Ontario had the greatest overlap, it also had the greatest proportion of non-agricultural monarch habitats. Milkweed in Ontario cornfields was found not to be an important contributor to monarch production.
Figure 1 Monarch life history
Egg: 3-4 days
Larva: 12-16 days
Pupa: 9-12 days
Adult: 2-6 weeks in summer
Adult: 7-9 months over winter
Figure 2 Overlap of small monarch larvae & corn pollen shed Source: Oberhauser et al., 2001