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Monarch butterflies lay their eggs exclusively on plants in the milkweed family (Asclepiadaceae), usually on the underside of a leaf.
Rather than make assumptions about where milkweed occurs, researchers conducted a census. Milkweed densities generally are much higher in non-agricultural habitats than in cornfields, and densities are higher along field edges than within fields (Oberhauser et al., 2001), as shown in Figure 4.
This is an important finding because milkweeds in nonagricultural habitats would not have harmful levels of Bt pollen drifting onto their leaves, because levels of pollen deposition fall off sharply just a few feet from cornfields. Therefore, monarch caterpillars in field margins are less likely than those in cornfields to encounter Bt corn pollen. However, a high percentage of monarchs are likely to be found in and around cornfields or other agricultural habitat simply due to the prevalence of agricultural land in some states.
Figure 4 Milkweed density in study sites Source: Oberhauser et al., 2001
Within cornfields Other agricultural land Edges of cornfields Nonagricultural
0.004/m2 0.003/m2 0.039/m2 0.027/m2
Within cornfields Edges of cornfields Nonagricultural
0.285/m2 0.525/m2 1.052/m2
Within cornfields Nonagricultural
Monarch caterpillars in field margins are less likely than those in cornfields to encounter Bt corn pollen.