Milkweed and Monarch Migration
closely do monarchs follow after milkweed becomes available in the spring?
biologist Dr. Lincoln Brower explains why this question is important:
observations in Mexico and in the southeastern U.S. over the past few
years suggest that monarchs may be leaving their overwintering sites
in Mexico earlier than normal. The reason for this, I believe, is that
the thinned forests are resulting in drier, warmer air blowing through
the overwintering sanctuaries, activating the spring migration prematurely.
"If this is true, then monarchs may arrive in northern regions
before the danger of frost has passed. Hard frost results in the withering
and drying of the tender, newly-emerged milkweed leaves. The problem
is that, until frost danger has passed, even if monarch eggs or early
instars are not frozen, the frosted leaves can result in larval starvation.
Another problem is that monarchs that arrive very early may not be able
to find an adequate supply of sprouting milkweeds, so they may keep
migrating northwards. And of course, the further they go the worse the
"One of the reasons this study is so important is because it is
very difficult logistically to study the departure times of monarchs
from the overwintering sites. Therefore, by collecting spring migration
and milkweed data, we should definitively answer the important question
of whether monarchs are prematurely getting north of their milkweed
"In order to investigate whether this is a problem for monarchs,
participants in Journey North can do a very simple experiment. I hope
you will join us in collecting these important data."
Professor Lincoln P. Brower
Research Professor of Biology
Sweet Briar College, Sweet Briar, VA