The hornworm undergoes metamorphosis, a process involving several distinct stages, with pupation transitioning the larva (caterpillar) into the adult moth. The completion of the larval phase is a gradual shift marked by specific, observable changes. Recognizing these signs is important for anyone managing a hornworm, as the window for successful pupation is brief and requires specific environmental adjustments. This article outlines the signs indicating the larval stage is complete and pupation is imminent.
Visible Shifts in Appearance
The first indications that a hornworm is ready to pupate are physical, occurring after the larva has reached its maximum size in the fifth and final instar. While a fully grown hornworm can measure over three inches, size alone is not the indicator of readiness. The most noticeable change is a shift in coloration once the worm stops growing.
The larva’s vibrant feeding color, typically a rich green, begins to lighten and lose intensity. The resulting color is often a duller, paler green, sometimes taking on a slightly yellowish, pinkish, or brownish tint across its body. As the color dulls, the worm’s skin may also appear smoother or tighter than during the active feeding phase.
A more specific anatomical sign is the appearance of the dorsal aorta, the insect’s heart. This dark, pulsating line becomes distinctly visible along the center of the hornworm’s back. The visibility of this vessel confirms that the internal physiological changes necessary for pupation are underway.
The Pre-Pupation Behaviors
Changes in behavior provide the most reliable confirmation that the larval stage is complete. The primary behavioral signal is the abrupt cessation of feeding. A hornworm that previously consumed large amounts of foliage will suddenly stop eating, even when fresh food is available.
Following the end of feeding, the larva enters the “wandering phase.” This involves the worm climbing out of its food source and actively searching or pacing the edges of its habitat. This movement reflects the worm’s instinctual need to find a suitable location for burrowing, typically soft soil away from the host plant.
Wandering can last for a day or two as the worm searches for a spot to settle. During this period, the process of gut clearing is completed. The larva expels the last of its waste, which is necessary because the pupa is a non-feeding, immobile stage that must be empty of waste materials.
Managing the Transition Phase
Once physical and behavioral cues are observed, action is required to ensure a successful transition to the pupal stage. If the hornworm is reared in an enclosure, it must be provided with a deep, loose substrate for burrowing. Suitable materials include fine, slightly moist soil, sand, peat moss, or coconut coir.
The depth of the substrate is important, as the worm needs to create a subterranean chamber. A recommended depth of four to six inches allows the larva ample room to burrow and form its chamber without disturbance. Once the wandering worm is placed on the substrate, it will begin to push its way down into the material.
Handling must be minimized once the wandering phase begins and the worm is transferred. Any significant disturbance can interrupt chamber formation and hinder successful pupation. Once the worm burrows out of sight, it should be left completely alone as the larval skin hardens and darkens underground, transitioning into the immobile, brown pupa.