I spotted my first monarch caterpillar (Danaus plexippus) of the season last week. I almost overlooked its tiny black, white and yellow striped body, which was about 1/4 inch, or 7mm long. It crawled slowly on a milkweed plant (Asclepias syriaca) where I saw my first adult monarchs of the year on June 3.
This tiny caterpillar faces many challenges on its journey to adulthood. In fact, fewer than 10 percent of all monarch eggs metamorphosize into butterflies, according to Monarch Joint Venture. Many garden critters prey on monarch caterpillars, especially parasitic wasps, tachnid flies, jumping spiders, the larvae of Asian ladybugs, and lacewing larvae. Most eat the caterpillars. Others, such as tachnid flies, lay their eggs on them. The eggs hatch and burrow into the caterpillar, which they use for nutrition and protection. It’s easy to dislike insects that prey on monarchs, but these predators also destroy many garden pests and are vital to the health of gardens and woodlands.
Weather conditions, food availability, pesticide use, damage from being stepped on or run over, and infection from bacteria and viruses also reduce monarch caterpillar numbers. The 10 percent that survive grow quickly on their milkweed diet and will molt five times over 9-to-14 days. Each stage between molts is known as an instar. The 5th instar usually leaves the milkweed plants in search of a safe place to form its chrysalis, where it completes the change to adulthood. The entire process from egg to adult lasts about one month.¹
Monarch caterpillars can be very elusive, but I’ll keep watching for them. In spite of predators, poor weather and other challenges, it’s likely that at least one or two will complete their journey to adulthood in our garden. They’ll pollinate many plants, lay eggs for the next generation of monarchs, and add beauty to our world.