When a garden plant suddenly shows severe damage, the culprit is often a large caterpillar with a distinctive spike on its tail. These pests, known as horned caterpillars, are the larval stage of Sphinx or Hawk moths and can strip a plant of its foliage in a matter of days. Addressing an infestation requires a multi-pronged approach. The following methods provide practical solutions to remove these insects and manage their presence in your garden.
Confirming the Pest and Damage
Identifying the specific horned caterpillar is important for effective management. The most common species are the Tobacco Hornworm (Manduca sexta) and the Tomato Hornworm (Manduca quinquemaculata). They grow up to four inches long and are bright green, providing excellent camouflage. A distinguishing feature is the horn, or tail spine, located on the rear end, which is usually red on the Tobacco Hornworm and black on the Tomato Hornworm.
The most telling sign of an infestation is the rapid, severe defoliation, often starting at the top of the plant. These feeders consume entire leaves, leaving only the central leaf veins untouched. Another indicator is the presence of large, dark, pellet-like droppings, known as frass, found on leaves or on the soil beneath the feeding site. Observing these signs confirms the presence of the pest and indicates that removal strategies should be initiated immediately.
Immediate Non-Chemical Removal Strategies
Physical removal is the primary method for controlling a localized infestation. Hornworms are masters of camouflage, but focusing on signs of damage helps locate them. Begin by following the trail of defoliation to the newest damage, then search the leaves and stems above the most recent frass.
Finding the green caterpillars against green foliage can be challenging. A highly effective technique is to search after dark using a UV blacklight, as the caterpillars fluoresce brightly, glowing an iridescent blue-green. Once located, grasp the caterpillar firmly and remove it from the plant.
Safe disposal ensures the pest cannot return to the garden. Dropping the removed caterpillars into a container of soapy water is an effective way to kill them. For plants with heavy damage concentrated on a single branch, pruning that section immediately reduces the caterpillar population and removes any eggs.
Utilizing Biological Control Agents
Targeted biological controls offer an effective solution, especially for widespread or recurring issues. One common application is Bacillus thuringiensis var. kurstaki (Bt-k), a naturally occurring soil bacterium. This strain works as a stomach poison, meaning the caterpillar must ingest it to be effective.
When a caterpillar consumes foliage treated with Bt-k, the bacterial toxin paralyzes its digestive system. This causes it to stop feeding within hours and die within a few days. The product is specific to caterpillars and is harmless to humans, pets, and beneficial insects like bees and parasitic wasps. For best results, Bt-k should be applied in the late afternoon or evening, as the bacterium degrades quickly in direct sunlight.
Another biological agent is the tiny parasitic wasp, Cotesia congregata, a natural enemy of the hornworm. The female wasp injects her eggs into the caterpillar, where the larvae develop internally for about two weeks. The larvae then chew out of the host and spin small, white, cottony cocoons on the caterpillar’s exterior.
Do not destroy any hornworm found covered in these white structures. These are not the caterpillar’s eggs but the developing cocoons of the beneficial wasps. Leaving the parasitized caterpillar in place allows the adult wasps to emerge, mate, and seek out more hornworms to parasitize, providing ongoing natural pest control.
Long-Term Prevention and Habitat Management
Stopping the caterpillar population requires disrupting the life cycle of the adult moth. Hornworms overwinter as pupae buried several inches deep in the soil, usually near the base of the plants they fed on. Tilling the garden soil in late fall or early spring exposes these pupae to cold temperatures, predators, or physical damage, reducing the population that emerges in the summer.
A physical barrier can prevent the large Sphinx moths from laying eggs on host plants. Lightweight row covers or fine netting placed over the plants and secured at the edges will physically block adult access. This exclusion method is effective for protecting susceptible plants during the peak egg-laying period.
Companion planting can deter the adult moth or attract natural enemies. Strongly aromatic herbs like basil and flowers like marigolds are thought to confuse the female moth, making it harder to locate host plants. Other plants, such as dill or borage, can act as a nectar source for beneficial parasitic wasps, encouraging them to remain in the garden.