Monarch Butterfly Eggs
A Conversation With Dr. Lincoln Brower

Several years ago I was doing research in Florida and I discovered that, if you look carefully at the eggs on the milkweed plants, some had been eaten.
Then one day I discovered that they had actually been eaten by monarch caterpillars.

How do they get out of their own eggshells? They have to eat their way out. And, for a few minutes after they have managed to eat their way out of their own eggshell, if they're walking around and they happen to find another monarch egg, they will start eating it. So basically, they young larvae are cannibalistic. They will eat other individuals of their own species.

Then we discovered in this field that there were two species of monarch-like butterflies--the other is called the queen--and it also occurs along the Gulf Coast states. Both monarchs and queens lay their eggs on the same milkweed. We found that not only were they eating their own eggs but they also eat each other's eggs. We found by setting up an experiment that the queen monarchs ate more monarch larvae than the monarch eggs than they ate of their own. So one of the hypotheses that we had many years ago was that the origin of the migration was to get away from the queen because of this egg cannibalism.

If you watch monarch females that are coming in to lay eggs on milkweeds--anywhere in the country--they usually only lay one egg at a time. There's good reason for that, and I think you know what it is now.

If you find a whole bunch of monarch eggs together that have been laid on a milkweed it means that something is wrong with the female. She's either sick or very, very old and can't hold back. Or, she has been flying along for a long time and several eggs have matured and she lays all of them at once. We really don't know all of the details, so there is a place that one could do a very interesting study.

But normally, after they lay one egg, there's a refractory period of several seconds or maybe one minute. They fly up and they may come back down again and they may lay an egg on the same plant but usually not in the same place. If that refractory period is for a minute or two they'll fly around and by then they'll have found another milkweed plant. So they really do spread their eggs all over the place, and one of the reasons they do that is they are cannibalistic. This is all speculation, but one of the reasons monarchs may just keep going is so they spread those eggs out over all of those plants. So, if you're going to fly, keep flying northeastwards and you'll cross over milkweeds all the way up to the Great Lakes--and that new generation does that.

So when a female monarch is coming back from Mexico and is looking for milkweed plants, she has to be really, really careful not to lay more than one egg on the same leaf--or even better on the same milkweed plant--because if she does lay more than one egg and they're close together the first that hatches out is very likely to eat the other.

Dr. Lincoln Brower
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Monarch butterfly caterpillar eating its shell after hatching

Egg loading
Photos by Jim and Teresa Gallion

These pictures show tiny, new milkweed plants loaded with eggs. "Egg loading" is a sign that milkweed is in short supply. If milkweed were plentiful, monarchs would not lay so many eggs on a single plant. Migrating monarchs often move north in the spring just as milkweed emerges.