In the photo on the right, a monarch mom is laying an egg on the underneath side of a leaf on a small Swamp Milkweed (Asclepias incarnata) plant. I took this photo in early May in Indianapolis, and due to this monarch's very worn appearance, it’s apparent that she was a returning migrant butterfly. Migrating monarchs which make the 2,000-3,000-mile journey south to Mexico in the fall are an unusual sight this far north in the spring. After living about eight months, monarchs typically lay their eggs in the southern U.S. and then die. It's the first generation which continue the journey north, but she kept on flying. (Indianapolis is about 2,000 miles from the overwintering monarch colonies, so imagine the thousands of miles she's flown on the way to Mexico and back!) This mom was very tired and rested frequently on the ground. I believe the many eggs she laid that day were her final gifts.
monarch butterfly eggs
From the time she emerges from her chrysalis, a monarch mom requires about five days to lay eggs. Monarch pairs mate overnight, and the female can lay eggs the next morning. It’s unknown how many eggs a female Monarch can lay in the wild, but it's thought to be 300-400, often more. In captivity they lay an average of 400-700 eggs, and sometimes hundreds more. They often lay one per milkweed plant, although there are many exceptions to that. Although, it's unknown if multiple eggs on one plant are the result of the same mom returning to the plant on multiple days, or several moms laying eggs, etc. They’ll usually lay them on the underside of a leaf, on blossoms or stems, often choosing the most tender leaves, either at the top of the plant, or on very small plants. (See my "Finding Eggs" page for more info.)
Monarch eggs are about the size of the head of a pin and come to a point at the top. They're creamy-white in color and are covered with many ridges. Each butterfly egg has a hard outer shell which protects the developing larva and it’s lined with a layer of wax which helps keep it from drying out. When a female lays an egg, it's fertilized before being attached to the milkweed plant. Monarch eggs have tiny funnel-shaped openings at one end, called micropyles. Since eggs get their hard shell before they are fertilized, this hole, which penetrates all the way through the shell, allows sperm to enter.
After an egg is laid it takes three to four days to hatch. (All stages of metamorphosis can vary in the length of time it takes because butterflies are cold-blooded or poikilothermic, meaning that their internal temperature changes depending on the environment.) The egg remains creamy-white until the monarch caterpillar’s black head capsule can be seen at the top, and within several hours the tiny caterpillar will chew its way out, only to turn and eat its own eggshell as its first meal.
The three eggs you see at the top of the page were laid by the mom you see above, and it's very possible that she could be laying them in this photo! She too was a very tired mom. I was very fortunate to be able to photograph her as she fed, rested, and shared her eggs.
Monarch moms are smart, or they have the best instincts. (I think both are true.) If you take the time to observe their egg-laying patterns throughout the spring and summer, you'll find that the location they choose will change as the plants grow. Their goal is to lay their eggs on locations on the milkweed where they will be the most protected from predators, as well to as provide the best food possible for the larva, (or caterpillars, or "cats"). They will typically lay one egg per plant, usually on the underside of a leaf. (There are always exceptions.) A friend of mine has a large Swamp milkweed plant growing right outside her front door. Monarchs love this plant. Between the two of us we found over 70 eggs on this one plant on the same day! (Had I not found them myself I'd never believe it.) My record before that was 18 eggs on one plant - on the underside of leaves, the top side, etc. ) In the spring they almost always choose the tender leaves at the top of the plant and by mid-summer when the flower buds form, they'll often lay their eggs tucked in among the buds, as seen below. And I've noticed over the years that monarch moms sometimes prefer shorter plants for their eggs instead of the largest, most mature plants. In addition, I've also found eggs on plants just three inches tall, even when there were much taller plants around, so don't ignore the little milkweed sprouts in your search. (I usually check those first.)
My photos make monarch eggs look huge, but they're actually very small, about the size of the head of a pin.
If you see an egg on a milkweed plant that's the same size but is perfectly round (no point at the top) and white, it belongs to another milkweed critter.
After the mom lays her egg it will take three to four days before the caterpillar develops and then chews it's way out of the shell. (This is temperature regulated, so if it's cooler it will take longer and if warmer it will develop faster.) Once the larva is fully developed, you'll begin to see the top of the egg turn black, and what you're seeing is the head of the caterpillar.
where did those eggs and caterpillars go?!
Eggs and baby caterpillars can disappear very quickly from a milkweed plant, which is why raising them indoors is the very best idea. They'll often get eaten by spiders, beetles, ladybugs, ants, etc. Or the babies will die in storms, etc. Only a about 10% of the eggs a monarch mom lays will survive long enough to grow into an butterfly. When raised indoors following best practices, often 90-99% survive and can be released.