The local news for most of the eastern US and Canada has been aflutter (ha) recently with reports of the irruption of Vanessa atalanta – the Red Admiral butterfly. While this is a common occurrence every spring for these butterflies to migrate north from their overwintering grounds in the southern US, the sheer numbers this year are staggering. There are quite literally thousands of admirals in our back yards.
So what’s different this year?
There is much speculation about the warm spring weather (warmest March on record for many places) and often lots of misinformation to go along with some armchair entomology. Most of the news sources I’ve come across say the warm spring has allowed these butterflies to flourish and reproduce in abnormal numbers. That isn’t quite possible however, V. atalanta overwinters as an adult. The southern states provide temps just warm enough for adult Vanessa butterflies to hide in the fall and be the very first to awaken in the spring to get a jump start on mating. Even if the butterflies were awake in February the host plants were not yet up (thistles); the butterflies in our backyards are from last year.
But what if weather did play a role in this boom cycle? Last year was a La Niña year with our beautiful and mild winter. The year before was an El Niño, most of the eastern US was assaulted with winter and we suffered at the hands of the epic Chicago “snowpocalypse”. Perhaps this combination depressed population numbers sufficiently in 2010/2011 which then decreased parasitoid load, allowing for greater overall butterfly fecundity in the summer of 2011. Those overwintering butterflies were then granted a warm winter that could have allowed for a lower winter mortality. As the butterflies moved north this spring there were no frosty nights to cut into populations – just lots of hungry birds. The result would be an abnormal influx of migrating butterflies.
But then again…
Despite butterflies being so popular and well studied there doesn’t seem to be a perfect grip on what conditions each of the Vanessa species prefer. The variables of host plants, population range, weather and parasitoids all play important roles in abundance and distribution. Did the weather cycles of the last few years variably effect one species over another? Who wants that PhD project (from hell)?
One of the best posts I’ve read is by Doug Taron over on Gossamer Tapestry – he is skeptical of this warm weather theory. Critically this irruption is effecting only the population of the Red Admiral and any theory involving weather would also likely effect other migrant butterflies like the Vanessa virginiensis – the American Lady. He also points out that an abundant butterfly like the admiral has classic cyclical population booms and busts in sync with parasite populations. I recall another summer in the late 90’s of staggering Admiral populations. Parasites really do better explain what we see in our yards this spring. While the weather might not have caused the abundance of V. atalanta, there can be no denying that butterflies of all species have woken up much earlier this year.