Populations of insects can fluctuate from year to year based on a variety of factors. This year it appears everything lined just right allowing the painted lady butterfly (Vanessa cardui) the opportunity to have very high numbers across most of Iowa this month.
There were many factors that would have played into making these numbers possible. First, the painted lady butterflies’ spring migration went very well which allowed for good numbers to push up into the northern part into their range last spring. Weather in Iowa this summer was favorable for butterflies allowing the multiple generations that occur in the summer to magnify their numbers. Parasitoids and general predation of the caterpillars may have been lower.
Now, all those painted lady butterflies that built up large numbers over the summer need to migrate south for the winter. Winters in Iowa are generally too cold for painted ladies to survive so they fly south to subtropical regions with warmer temperatures. The migration makes for quite a show as people are seeing large clusters of individuals feeding on fall blooming perennials and annual plants that will hold their flowers till the first frost. While the largest populations have moved on for most part, numbers will continue to be higher than normal till the first frost.
For another discussion of the painted lady migration and to see several excellent photographs see The Roused Bear blog by Harlan Ratcliff at https://therousedbear.wordpress.com/
Reiman Garden runs a citizen scientist program called the Iowa Butterfly Survey Network. In the program we train individuals across the state to go out and survey butterfly populations. To give you an idea of just how much larger this year’s number were, last year our largest recorded number of painted ladies on a survey in September was 21 individuals; Last week when we conducted our survey 747 individuals were recorded.
Even though the masses have moved on there are still plenty of painted lady butterflies zipping around outside so take a moment, step outside and see just how many you can find.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 15, 2017. The information contained within may not be the most current and accurate depending on when it is accessed.