Milkweed is the only host plant the female monarch will lay its eggs. Monarch females usually lay a single egg on a milkweed plant, often on the bottom of a leaf near the top of the plant. However, they also lay eggs on the buds, on top of the leaves, and new leaves near the top of the milkweed plant. As females lay their eggs, they secrete a small amount of glue to attach the eggs directly to the plant. Female monarch butterflies only lay eggs over a 2–5 week period. During this time, she probably lays an average of 300-500 eggs in the wild.

Each egg is formed inside the female prior to fertilization, including the hard outer shell, called the chorion, which protects the developing larva inside. The shell is lined with a layer of wax, which helps keep the egg from drying out. Monarch eggs are about the size of a pinhead or pencil tip. The eggs are creamy white and the shape of a football. Monarch eggs will hatch within 3-5 days, depending on temperature. Just before a monarch egg hatches, the dark pigmentation of the monarch’s head capsule develops. This is visible through the translucent egg shell and gives the egg its characteristic black dot (see photo #6 below).