题目选项分析

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. Therefore, use of the modified maize inadvertently imperils monarch butterflies.

Which of the following, if true, most seriously weakens the argument?

【选项】Per unit of volume, the natural insecticide produced by the genetically modified maize plants is less toxic to insects than are many commercial insecticides commonly used on maize plants.