Certain genetically modified strains of maize produce a powerful natural insecticide. The insecticide occurs throughout the plant, including its pollen. Maize pollen is dispersed by the wind and frequently blows onto milkweed plants that grow near maize fields. Caterpillars of monarch butterflies feed exclusively on milkweed leaves. When these caterpillars are fed milkweed leaves dusted with pollen from modified maize plants, they die. Ttherefore, by using genetically modified maize, farmers put monarch butterflies at risk.
Determining whether monarch butterfly caterpillars are actively feeding during the part of the growing season when maize is releasing pollen would be most useful to evaluate the argument.