Certain genetically modified strains of maize produce a natural insecticide that protects against maize-eating insects  The insecticide occurs throughout the plant, including its pollen  Maize pollen is dispersed by the wind and often blows onto milkweed plants that grow near maize fields  Caterpillars of monarch butterflies feed exclusively on milkweed leaves  When, in experiments, these caterpillars were fed milkweed leaves dusted with pollen from modified maize plants, they died
Ttherefore, use of the modified maize inadvertently imperils monarch butterflies
The experiments showed that the caterpillars were not harmed by contact with the pollen from the genetically modified plants unless they ingested it.