The monarch caterpillar, in its final stage of development (the fifth instar), must complete a mandatory journey away from its food source to prepare for its transformation. This process, known as pupation, involves shedding its larval skin one last time to form the chrysalis. The caterpillar ceases feeding and begins a restless trek to find a secure, inconspicuous location where it can safely hang, marking the transition to its metamorphosis.
The Urge to Wander
The decision to leave the host plant is driven by a strong survival instinct. Milkweed, while providing the caterpillar with its food and protective toxins, is also where most natural predators linger. By leaving the immediate vicinity of the milkweed patch, the caterpillar significantly reduces its exposure to wasps, spiders, and birds that actively hunt on the plant. This behavioral shift helps the caterpillar avoid being eaten during the vulnerable, immobile pupation period.
The wandering stage ensures the caterpillar can locate a structurally sound surface away from the milkweed plant itself. A chrysalis requires a firm, stable anchor point to withstand the physical stress of metamorphosis and eclosion. The milkweed plant is often unsuitable because it is relatively fragile and subject to wind or wilting. Finding a secure overhead anchor guarantees the chrysalis will not fall or be damaged during the 8 to 15 days of the pupal stage.
Typical Travel Distances and Range
The distance a fully-grown caterpillar travels is highly variable, but it is typically a short, focused search for the nearest appropriate anchor point. Observations in garden settings often show travel distances ranging from a few feet to about 10 feet. This short distance is usually sufficient to place the caterpillar out of the immediate danger zone of predators that patrol the milkweed.
However, far longer journeys exist when the local environment lacks suitable pupation sites. Caterpillars have been documented crawling up to 30 feet, and occasionally 70 to 85 feet, particularly if the milkweed patch is isolated in an open area with no nearby structures. The density of the milkweed patch also influences travel distance; a caterpillar in a large, dense patch may travel shorter distances simply to escape the crowded environment and find a secure perch. The caterpillar’s movement is determined by the impulse to find the closest, most structurally secure location.
Selecting the Pupation Site
The primary goal of the wandering phase is to find a site that offers both security and environmental protection. Caterpillars generally select a spot that provides shelter from direct rain and harsh sunlight, such as the underside of an overhang or a sheltered corner. The location must be slightly elevated off the ground to avoid pests and standing water. Common choices include the eaves of houses, the underside of porch railings, or thick, woody branches of nearby non-milkweed plants.
The surface chosen must be rough enough to allow the caterpillar to securely attach its silk pad. Once the site is selected, the caterpillar spins a small silk button, which serves as the anchor for the chrysalis. It attaches the last pair of prolegs to this silk button before hanging motionless in the characteristic “J” shape. This J-hang position allows hormonal processes to begin, culminating in the shedding of the final larval skin to reveal the rigid, jewel-toned chrysalis.