Corn pollen deposition on milkweeds in and near cornfields

The density of corn pollen on leaves of milkweed plants inside and outside of cornfields was measured in several studies from different localities. Th e purpose was to obtain a representative picture of naturally occurring pol len densities to provide a perspective for laboratory and field studies of monarch larvae feeding on milkweed leaves with Bt corn pollen. Pollen densi ty was highest (average 170.6 grains per cm(2)) inside the cornfield and wa s progressively lower from the field edge outward, failing to 14.2 grains p er cm(2) at 2 m. Inside the cornfield, and for each distance from the field edge, a frequency distribution is presented showing the proportion of leaf samples with different pollen densities. Inside cornfields, 95% of leaf sa mples had pollen densities below 600 grains per cm(2) and the highest polle n density observed was 1400 grains per cm(2), which occurred in a study wit h a rainless anthesis period. All other studies had rainfall events during the anthesis period. A single rain event can remove 54-86% of the pollen on leaves. Leaves on the upper portion of milkweed plants, where young monarc h larvae tend to feed, had only 30-50% of the pollen density levels of midd le leaves.