Bug Profile: Tomato Hornworm
Manduca quinquemaculata or Tomato Hornworms are one of the most devastating pests known to garden tomatoes (and, on occasion, potatoes, peppers, eggplants, tobacco, and moonflowers too). They are huge - compared to other common garden bugs - and with an appetite to match. In their arsenal of fright tactics is a single, sharp horn on their rear and a head-thrash when disturbed, accompanied by a clicking sound. They are also not worms at all, but rather the caterpillar stage of the Five-spotted Hawkmoth, a hawk moth in the family Sphingidae.
Description: Hornworms have heavy, light-green bodies that can grow up to four inches in length. Tomato Hornworms have an intricate pattern on their backs, consisting of eight white Vs and culminating in a dark horn or spike. They are closely related to Tobacco Hornworms (that behave very similarly), which have a red horn and seven diagonal white lines on each side. In their adult stage, the grey-brown moths fly around at night and feed on nectar.
Hornworms are found in all 48 contiguous U.S. states. Although they prefer tomatoes, but they will eat any plants in the Solanaceous family, devouring large sections of leaves and stems (and later – fruit) in a matter of a few hours. Damage from hornworms can look similar to deer: branches will be completely defoliated and stems chopped off.
Life cycle: After overwintering in the soil in their pupae stage, they emerge as adult moths in late spring, and post mating, leave spherical green eggs on the undersides of leaves. Larva hatches in five days – this is the hornworm phase – and they spend the next three to four weeks feeding and growing, before burrowing back into the soil to pupate. The creature completely transforms once more: in the pupae phase, they are one to two inches long, dark brown, and with a curved handle-like encasement around what will later develop into their proboscis (mouth). In a particularly warm season, hornworms may go through two life cycles, emerging once again two to four weeks after burrowing back into the soil.
Locating: Although hornworms camouflage extremely well to the plants they are eating, they leave a tell-tale pile of poop under their feeding area. If you find a pile of dark-green droppings, known as frass, (don’t worry – we’ve provided a visual) that look like the tiniest hand-grenades, hornworms are near. Spray the foliage above the droppings with water – this will make the caterpillars thrash their heads and make a clicking noise and they will be easy to spot.
Control: The most effective time to control hornworm is when they emerge in their caterpillar stage in July and August. Picking them off by hand is effective, although they can have such a strong grip on the stem they are attached to that it’s easier to just snip the branch right above where the pest is sitting. To dispose of them, there are two common solutions: dropping them into a bucket of soapy water or… squishing them.
Hornworms do have once extremely effective natural enemy: non-stinging, parasitic wasps. The braconoid and tichogramma wasps use the caterpillars as hosts for their young by depositing eggs into the caterpillar’s body. Their larva feeds on the hornworm and then emerges to spin cocoons right on the outside of the caterpillar. If you see a hornworm with what looks like a row of rice kernels on its back, it’s been parasitized and has already stopped eating. It’s best to leave them be and let the wasps hatch. The wasps are attracted by the nectar of marigolds, carrots, mints, dill, fennel, cilantro, sage, oregano, thyme, and basil, so interplanting these crops with tomatoes can help with control. Adding flowers to attract other beneficial insects, such as lacewings and ladybugs, which attack hornworm eggs, is also be helpful. Our Good Bug Blooms mix is great for that.
Planting Nicotiana, or fragrant tobacco, as a trap crop is another technique to lessen their population. Plant this flower as far from your tomatoes (and other Solanaceous crops) as possible. After the first batch of flowers dies back, cut the plants back to half their size and dispose of the plant material – and, hopefully, numerous hornworms along with it. The flowers will bounce back and continue blooming!
Following a serious outbreak, cultivate the soil after pulling tomatoes, peppers, potatoes, and eggplants up in the fall. This will expose the pupae, who need to be underground to survive. Another option is to simply sick chickens onto this area – they will happily peck through the soil to find the bugs/snacks.
For more information and images, visit Cornell’s entomology profile on Tomato Hornworms.