The dilemma of an oversized hornworm stems from its rapid growth and substantial size, creating problems for two different audiences. Hornworms are the larval stage of Manduca species, such as the tomato hornworm and tobacco hornworm. As voracious feeders, they quickly defoliate host plants, becoming exponentially destructive garden pests when they reach their mature length of up to four inches. Alternatively, reptile enthusiasts who use them as feeder insects find their rapid growth means they quickly become too large for smaller pets, such as juvenile chameleons or bearded dragons. Whether a gardener or a pet owner, the sheer size of the final instar hornworm necessitates a clear course of action.
Managing Hornworms as Garden Pests
When a hornworm is discovered in the garden, its large size makes it highly visible but also signifies extensive plant damage has already occurred. The most immediate and effective control method is manual removal, which involves picking the caterpillar off the plant. Although the horn-like projection on its rear is harmless, gardeners may choose to wear gloves for comfort during removal.
Once removed, the hornworm must be humanely disposed of to prevent it from completing its life cycle. A common and recommended method is to drop the worm into a container of soapy water, which quickly drowns the insect. For a more certain termination, placing the hornworm in a sealed container and freezing it for 24 to 48 hours is considered a humane approach before disposal.
Relocating the hornworm elsewhere is generally ineffective, as it will likely seek out and consume other susceptible nightshade plants, such as peppers, eggplants, or potatoes. Gardeners should also check for hornworms parasitized by the beneficial Cotesia congregata braconid wasp. If the worm is covered in small, white, rice-like cocoons, it should be left alone, as the emerging wasps will help control the local hornworm population.
Repurposing Oversized Feeder Insects
When a hornworm purchased as a feeder insect grows too large for a reptile pet, the first strategy is to slow its growth to keep it a manageable size for longer. This is achieved by lowering the enclosure temperature, as growth is directly tied to warmth. Keeping the worms at cooler temperatures, such as 55°F, significantly reduces the speed of their development. For a temporary stop to growth, the worms can be placed in a refrigerator at approximately 45°F for a couple of days, which effectively halts development.
If the hornworm is already too large, it can still be repurposed for a larger animal. Many owners of bigger exotic pets, such as large monitor lizards, adult chameleons, or certain species of birds and amphibians, actively seek out these oversized, high-protein feeders. Local pet stores or online forums for reptile enthusiasts are excellent places to trade or sell the excess large worms to owners who can use them. The worms can also be humanely euthanized and then cut into smaller, manageable pieces for smaller pets, though this method is messier due to the insect’s high water content.
Transitioning Hornworms to the Adult Stage
A humane alternative to disposal is allowing the oversized hornworm to complete its natural life cycle. When the caterpillar reaches its final stage, it stops feeding and begins to wander, becoming a pre-pupa. This wandering behavior signals that the worm is ready to burrow and pupate.
To facilitate this transition, the worm needs a suitable substrate in a dedicated container, such as shredded paper, sawdust, or moist vermiculite or potting soil. The worm will create a pupation chamber, and its outer layer will harden into a mahogany-colored pupa. This stage lasts approximately two to three weeks, during which the insect undergoes metamorphosis.
The adult insect that emerges is a large, heavy-bodied sphinx or hawk moth, which is a powerful nocturnal pollinator. These moths are often mistaken for hummingbirds due to their size and characteristic hovering while feeding. If the resulting moth is a native species, it can be released outside, contributing to local pollination efforts.