During the last several days, the Red Admiral population in central Iowa has rapidly risen as the new summer generation butterflies have emerged. The larger, darker summer butterflies are quickly displacing the smaller spring-generation butterflies from the territories that Red Admirals occupy in the evenings. We have also been seeing some of these fresh individuals flying northward during the day, perhaps migrating, so observers to our north might want to watch for migrants from the south. Also look for the local summer generation butterflies farther north to start emerging during the next few weeks.
Also, be aware that there may be especially large concentrations of Red Admirals emerging within a few miles or kilometers of rivers, lakes, or wetlands where stinging nettles, the larval foodplant, grow abundantly. In late June and early July 2015, huge concentrations of Red Admirals appeared in at least a dozen locations in Iowa; most or all of these sites were near rivers or streams. We will be watching for similar outbreaks this summer. Observers typically report hundreds to thousands of Red Admirals swarming along roadways on either side of a bridge, or when canoeing or kayaking on a river. So, if you see one of these outbreaks, please note the date, location, and its approximate size and report it to us.
--Royce Bitzer, June 10, 2016