Have you noticed enormous swarms of Red Admirals in your area? Observers throughout Iowa have been reporting huge outbreaks of Red Admirals along particular stretches of roadway, most of which are near the floodplains of streams and rivers. One of the first of these outbreaks to be reported was discovered on the road west of Luther, Iowa, which crosses the Des Moines River. The number of flying butterflies started increasing at the western edge of Luther, and seemed to grow exponentially for the next several miles until they peaked near the river, where perhaps several thousand butterflies were swarming at any one time. They seem to be concentrated in a belt along the river and extending two to three miles on either side of it.
These butterflies had nearly all emerged recently, and had most likely fed as larvae on stinging and wood nettles that are especially abundant in floodplains. If the kind of outbreaks now happening in Iowa are also occurring elsewhere in the Midwest or in other parts of North America, the Red Admiral summer generation this year could be enormous, perhaps one of the largest ever observed.
Royce Bitzer, June 30, 2015