Fresh Painted Ladies have been proliferating in Nebraska since about July 18, suggesting a recent mass emergence. This outbreak has been most closely monitored in the eastern part of the state, but large numbers have been observed throughout Nebraska. Meanwhile, here in central Iowa, 150 miles/240 km to the east, only a few have occasionally been spotted in the past several weeks. Will they eventually arrive here, and if so, when? I keep watching the skies for migrants and the flowers for many sudden arrivals, but so far, I've seen almost none.
Almost none, because this afternoon I spotted a single Painted Lady, the first I had seen in just over three weeks. It was nectaring - or trying to nectar - in our small prairie patch on the Iowa State University campus, but wasn't having much luck because a particularly aggressive and persistent Monarch chased it several times and drove it away. So I have an observation, but no photo to accompany it.
Shortly after seeing the butterfly, I realized that the weather was turning warmer and more humid, and the wind was increasing from the south. Could this type of weather change suggest that Painted Ladies might arrive soon, riding the wind from some source region somewhere to the south? Or will Painted Ladies arrive here simply by expanding eastward from what may be a source region in Nebraska?
Because migrating butterflies often take advantage of winds blowing toward the direction to which they are inclined to go, wind flow patterns often reveal the course of a migration. The mid-day wind flow patterns from EarthWindMap on July 17 and 18th showed widespread southerly wind flow across western Iowa and eastern Nebraska on the 17th, and across eastern Nebraska through all of Iowa on the 18th. If the Painted Ladies had come from a widespread area somewhere to the south or south-southwest, one would have expected a strong front of arrivals in both states at about the same time. The current pattern of abundance suggests possibly a more localized irruption or a wind-borne migration from a relatively small source region. Observing flight and wind directions, looking at relative butterfly abundance in different locations over time, and examining patterns of daytime ground clutter on radar maps may provide helpful information about how this irruption develops and spreads.
-- Royce Bitzer, July 24, 2020