When a gardener discovers unexplained damage to their tomato, pepper, or tobacco plants, the first sign of the culprit is rarely the worm itself. Instead, the presence of small, dark pellets scattered on the foliage or the ground signals a hornworm infestation. These pellets are technically known as “frass,” the entomological term for insect droppings, and they are the most reliable indicator that a large, voracious caterpillar is actively feeding nearby. Understanding the characteristics of this frass provides the necessary visual details to confirm the pest’s identity and begin the search.
The Distinct Appearance of Hornworm Frass
Hornworm frass is noticeably large compared to the droppings of most other garden pests. The shape is typically blocky or cylindrical, resembling a small pellet or barrel that is firm and dry to the touch, not smeared or sticky like some other insect excrement. This distinct, sculpted shape is a result of the caterpillar’s digestive process and its high-volume consumption of plant matter.
The color of the frass provides additional clues about its freshness and the host plant it was produced on. When freshly expelled, the pellets are often dark green, closely matching the color of the tomato or tobacco leaves the hornworm has recently consumed. As the frass dries out, the pellets cure to a matte or shiny black color, making them highly visible against the green foliage or brown soil below. A thorough inspection of this unique appearance can quickly confirm the presence of either the tomato hornworm or the tobacco hornworm.
Frass Size and Placement Clues
The dimensions of the frass pellets are directly proportional to the size and age of the larva that produced them, offering a way to estimate the threat level. Hornworms pass through five larval stages, or instars, and the frass from a newly hatched worm is minuscule and easy to overlook. However, the most destructive, late-stage larvae produce frass that can reach the size of a pea, often measuring up to an eighth of an inch long.
Spotting these larger, pea-sized pellets is a warning that a mature hornworm is present and feeding at its maximum capacity. The frass rarely stays attached to the upper leaves where the worm is eating; instead, it tends to fall straight down. Consequently, the pellets accumulate on the lower leaves, branches, or on the soil directly beneath the caterpillar’s current feeding location. This vertical placement provides an immediate tracking system for the hidden pest.
Using Frass to Locate the Pest
The presence and placement of the frass serve as an essential tracking tool, allowing gardeners to pinpoint the location of the camouflaged hornworm. These caterpillars are masters of disguise, blending seamlessly with the green stems and foliage of the host plant, making them difficult to spot. Once an accumulation of frass is located, the search should begin immediately above the droppings, tracing the path upward along the stems and branches. If the worm is not immediately visible during a daytime search, looking for areas of stripped leaves or branches with only the central vein remaining will narrow the search area.
For particularly elusive worms, the frass can confirm the need for a nighttime search, as hornworms fluoresce a bright green under a simple ultraviolet or black light, making them easy to spot against the dark foliage. Once located, the most direct method of control is handpicking the worm and removing it from the plant.