Dr. Brower Finds Frozen Milkweed in Florida

April 9 , 2009

I am just back from North Florida where I was helping the BBC Natural History Unit that is working on an insect TV series. Currently, I have two wonderful University of Florida entomology graduate student collaborators, Kelly Sims and James Dunford, who have monitored our spring milkweed site for about a month so far.

We visited the research area where University of Florida students and I have been running a census since 1981. We have a wonderful research site that has a very large number of the Sandhill Milkweed plants thriving. This is a beautiful milkweed species (Asclepias humistrata), and also a major food plant on which the monarchs that are returning from Mexico establish their new spring generation.

We know from our census work that these milkweeds begin emerging from the ground in March. The earliest ones to emerge are often frosted back, and the leaves immediately wither and dry. Hence, any eggs laid by monarchs that return too early from Mexico and lay their eggs on the sprouting milkweeds that freeze will will end up starving to death.

Normally, the weather in late March and early April no longer has frost. However, on the early morning of 8 April 2009, there was a very light frost that severely damaged many of the milkweeds. Over the two preceding days we had counted the old females and males that had flown back from having overwintered in Mexico and were establishing the new spring monarch generation. We also saw and counted several dozen fresh eggs on the leaves. That the butterflies are just reaching this area was indicated by the fact that no larvae had yet hatched from the eggs.

The occurrence of frost as late as this is very unusual in North Florida. However, the apparently severe damage to the plants (yesterday morning through afternoon, 8 April, many individual plants were wilting and the leaves beginning to shrivel) is an example of one of the hazards that monarchs face. My concern is that the forest degradation that is occurring in Mexico is resulting in the surviving butterflies leaving the overwintering sites earlier each year and therefore getting to the milkweed habitats along the Gulf Coast States before the danger of frost has passed. Kelly and Jim will shortly be reporting on the state of these milkweeds. The lesson learned is that the timing of the spring remigration from Mexico is a critical aspect in the life history success of this marvellous migratory insect.