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Studying Milkweed and Monarch Migration

How closely do monarchs follow after milkweed becomes available in the spring? Monarch biologist Dr. Lincoln Brower explains why this question is important:

"My 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 problem gets.

"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 food source.

"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

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