The tomato hornworm, the larval stage of the five-spotted hawk moth (Manduca quinquemaculata) or tobacco hornworm (Manduca sexta), represents one of the most destructive pests a tomato grower can face. These large, voracious caterpillars can defoliate entire plants overnight, yet their brilliant green color offers nearly perfect camouflage against the leaves. Finding these pests is challenging, but successfully locating them is the only way to prevent crop loss. Growers must focus on systematically searching for physical evidence of their presence and using specialized tools to overcome their natural defense.
Recognizing the Tell-Tale Signs of Infestation
The most reliable sign of a hornworm infestation is the presence of frass, the caterpillar’s dark, pellet-like droppings. As these larvae grow, the frass becomes noticeably large, resembling small black or dark-green spheres. These droppings are often the first observable clue, accumulating on the leaves directly below where the worm is feeding or scattered on the soil at the base of the plant. Following this trail of frass upward through the plant often leads directly to the hidden pest.
Hornworms also leave a distinct pattern of damage on the foliage. They rapidly defoliate a plant, often consuming leaves completely down to the bare stem in a process called “skeletalization.” This feeding damage typically starts high on the plant, as adult moths prefer to lay eggs on younger, upper leaves. Locating a section of the plant stripped bare of leaves indicates the pest is nearby, even if not immediately visible.
Daytime Visual Search Strategies
While the hornworm’s camouflage is highly effective, careful, systematic searching during daylight hours can still yield results. The best time to conduct a thorough visual search is during the cooler parts of the day, such as early morning or late evening, when the caterpillars are often more active and less likely to be hiding deep within the plant. A successful strategy relies on understanding where the large caterpillars prefer to conceal themselves.
Shadow and Wiggle Techniques
One effective tactic is the “shadow technique,” which involves looking for the pests on the underside of leaves or stems. Because the caterpillars are so large, they cast a noticeable shadow or create a subtle bulge that can be spotted from a distance, even if their green bodies blend into the background. A second method is the “wiggle technique,” where the gardener gently shakes the tomato vines or branches. Hornworms will sometimes react to this disturbance by momentarily moving conspicuously or by losing their grip and falling, revealing their location.
It is most efficient to concentrate the search on areas where fresh damage or frass has been observed, moving outward from the site of the evidence. Hornworms, with their large size and high metabolism, must feed constantly, meaning they will always be close to the most recent destruction. By systematically checking the stems and leaf undersides near recent defoliation, growers can significantly narrow the search area.
Specialized Detection: Using UV Light
When daytime searching proves difficult, the most specialized and effective method involves using an ultraviolet (UV) light source, often called a blacklight, after dark. This technique exploits a fascinating biological property of the caterpillar’s cuticle, where hornworms possess substances that absorb UV light and then re-emit it as a visible, bright fluorescent glow. When a long-wave UV flashlight is used in the dark, the hornworm will fluoresce a vivid blue-green color, making it stand out dramatically against the non-fluorescing green foliage. The process is most successful when performed after nightfall, as the contrast between the glowing caterpillar and the dark garden is maximized. A simple, handheld UV-A flashlight is sufficient for this purpose and provides an easy way to locate even small larvae that were missed during the day; once found, they should be removed immediately and disposed of, typically by dropping them into a container of soapy water.