 The study resulted in a risk assessment concluding that any risk posed to butterfly populations was negligible.A 2002 review of the scientific literature concluded that "the commercial large-scale cultivation of current Bt–maize hybrids did not pose a significant risk to the monarch population" and noted that despite large-scale planting of genetically modified crops, the butterfly's population was increasing.However, the herbicide glyphosate used to grow GMOs kills milkweed, the only food source of monarch butterflies, and by 2015 about 90% of the U.S. population has declined.