The tomato hornworm, Manduca quinquemaculata, is much dreaded by vegetable gardeners, because they can devastate tomatoes and other members of the nightshade (Solanaceae) family. The tomato hornworm is three to four inches long at full size (likely to be the biggest caterpillar we see in our gardens) and green in color, with a black "horn" that projects from the rear of the caterpillar.
Tomato hornworms are voracious pests, munching entire leaves, small stems and even parts of the immature fruit. While they are most commonly associated with tomatoes, hornworms are also common pests of eggplants, peppers, and potatoes. Most likely, you'll notice the damage before you notice the hornworms because their color helps them blend in so well with the plant foliage. But I have learned to ‘shine the light of science’ on this problem with great effect. When I hunt for these pesky ‘pillers, I do it in the dark of night, with a UV flashlight!
The flashlight pictured below cost me about six bucks at Walmart.
My favorite bioluminescent encounter was while making my way along a dark woodland road without a flashlight, when I spotted strange glowing patches among the trees. Something out of the Brothers Grimm? No, just foxfire, the light emitted by patches of luminous lichen on decaying tree bark. (By my guess, the name is a version of “folks’ fire,” as in fairies or “the little folk.”)
But here’s a new twist. The luminosity of some life forms appears to us only when light is altered in some way. I recently learned that tomato hornworms will glow if you shine a black light on them. Some of you might remember the black light as a gadget used to illuminate psychedelic artwork in the ’60s, by narrowing the light spectrum to the UV (ultraviolet) range. But it has broader uses in police work, including the identification of counterfeit money, which lacks a certain ultraviolet mark, and in medicine, for spotting and delineating certain pathogens that are visible in such light.
What’s interesting about the nighttime detection of the hornworm is that it’s extremely hard to see by day. This fat, four-inch-long caterpillar should be easy to find, but it has spent millennia learning how to resemble tomato stems and leaves. You don’t know it’s there until its plant-chomping damage is done. But armed with a black light, you can easily stalk your prey as it glows with a greenish color among the tomato vines. Bioluminescence is sometimes used by animals to frighten off a predator, which in this case is you. But be not unnerved. If you’re repelled by plucking huge, slightly squishy, neon-lighted worms off plants with your fingers, use tongs. But do it gently, because you have an important decision to make before you stomp on them or drop them into soapy water to end their lives.
So how does one use the blacklight to hunt down the dreaded tomato terrorist? It is easy. Just follow these simple steps:
- Take a blacklight flashlight outside when it's dark. Alternatively, put a blacklight bulb in a small desk lamp and plug it into an extension cord, if necessary, to reach the infested plants.
- Fill a cup with water and about 1 teaspoon of liquid dishwashing detergent to collect and kill the hornworms. Or just plan to drop them on the ground and stomp them. The make a very satisfying squish!
- Shine the blacklight over the plant and examine all its parts, from the ground up. Look at the undersides of the leaves, since this is a common hiding place for hornworms. They'll glow under the light.
- Pick off the hornworms from the plant that are not hosting the larvae of braconid wasps, and place them in the cup of soapy water or just squish them under foot. Dispose of the hornworms in the garbage.
Ladybugs can be purchased at most plant nurseries, and a black light that emits only UV light can be found online for about $7. After tomato season, you might find other ways to put the black light to use too. For example, you can shine it around your kitchen sink and counters, in the dark, and locate any E. coli bacteria that may be present.
Now that’s scary. Happy Gardening!!