2024 Spring Outlook

The historically strong El Nino brought mild temperatures to much of the Midwest (see Wisconsin for example) throughout the winter. In some places, February and early March saw some of the warmest monthly temperatures on record. Most also saw a severe lack of snow compared to the average. You may be wondering what this means for the butterflies. 

There is much nuance surrounding what effect this odd winter has on different species. Swallowtails (Papilionidae) generally benefit from a milder winter as they overwinter as pupae. Pupae are sensitive to severe cold temperature shifts. Whites and Sulphurs (Pieridae) are similar. According to reports from Wisconsinbutterflies.org, both Cabbage Whites and Clouded Sulphurs have already made an unseasonably early appearance. These species rarely emerge prior to April in northern latitudes. This year, however, a few emerged in early March. The Anglewings have had a strong year so far. Many reports were submitted in February and March. Most of the individuals reported have been in remarkably good shape after overwintering. The earliest ever Wisconsin Green Comma was submitted last week. 

 Species like Compton Tortoiseshell, the Oeneis Arctics, and the northern Boloria Fritillaries all require cooler temperatures to thrive. It is too early to determine how the weather has impacted these species. It will take a whole spring of monitoring and comparing data to see the complete picture. It is likely that species need freezing temperatures at night during the winter more than anything else. The first freeze is an important game changer every fall. It ends most of the leftover adults from the summer and sends a wakeup call to the species going into estivation to prepare. It seems unlikely that there is much of a difference for Lepidoptera in overnight lows of the teens-20s compared to the low 30s. The cold threshold for most Anglewing adults is often estimated to be around -20- -30. But even this is complicated because it seems like every time we get a cold winter, everyone is shocked to see butterflies alive and well in the following spring. 

All we know is there have been an unusually high number of late winter sightings this year which bodes well for the upcoming season.