Doyle Rice

USA TODAY

A swarm of butterflies, winging its way south for the winter, was "spotted" as large blue blobs on weather radar last week over southern Illinois and central Missouri.

"We think these targets are Monarch butterflies," the National Weather Service in St. Louis noted on its Facebook page, which also includes a technical explanation of how the weather service came to this conclusion.

The monarchs were flapping their way south toward their winter home in Mexico. As noted earlier this year, the colorful insects were under stress this spring because of ongoing drought, an unusually cold winter and a lack of milkweed, their primary food source.

This isn't the first time weather radar has "seen" bugs this year: Both grasshoppers in New Mexico and mayflies in Wisconsin were spotted on radar.

Scientists are finding that weather radar is proving useful to track birds, bats and insects. While this information is just clutter to the weather folks, it is just the thing biologists need to study the activities of flying creatures, a science newly christened "aeroecology."

As for the butterflies, the weather service in St. Louis wished "good luck and a safe journey to these amazing little creatures on their long journey south!"

Contributing: Associated Press