The researchers who monitor monarch butterfly populations in Mexico each winter have noticed a drastic decline in their numbers over the last 20 years, but citizen scientists who conduct monarch counts during the summer have not noticed such a dip.

Because of this discrepancy, there have been challenges to the belief that a decrease in milkweed has driven the decline of the monarch butterfly. New research from Iowa State University theorizes that milkweed loss is still the main cause of the decreasing population.

Milkweed used to be more common in agricultural areas, but widespread use of glyphosate has removed it from fields. As a result, monarchs who would summer on agricultural land have been forced to congregate elsewhere: the milkweed-populated areas where citizen scientists conduct monarch censuses.

The study looked at historical data on milkweed populations in both agricultural and non-agricultural area, and, using that, adjusted monarch census data accordingly to show that the decline in milkweed is indeed connected to loss of monarch butterflies. And, the decline in the U.S. population is the same as the decline of the overwintering population. To read the full study, visit https://tinyurl.com/y8n72qo5.