Many stop to mate and lay their eggs on milkweed plants. The eggs hatch into caterpillars within a few days. After the caterpillars pupate and become butterflies a month later, they also join the flight northward. During the flight, the new adult butterflies may also stop to breed. Up to five generations of monarch butterflies may migrate in one season.
All monarch butterflies congregate at specific winter roosting sites in California and Mexico.
Number varies and are white to off-white in color and about the size of a small head of a pin. You can find eggs on the bottom of the milkweed leaves, while others are accidentally placed on the top of the leaves if the monarch butterfly is disturbed while laying her eggs. Caterpillars, when first hatched, are about as thin as a thread and almost as long as the tip of a ball-point pen. During this time they shed their skin about once every couple days or so. The shedding process is a form of growth.