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.
It would be most useful to determine whether any maize-eating insects compete with monarch caterpillars for the leaves of milkweed plants growing near maize fields in order to evaluate the argument.