The Dreaded Tomato Hornworm

Now that my tomato plants have been planted, it won’t be long before the dreaded tomato hornworms begin to show up and start busily munching away. Tomato hornworms can also be a problem for plants related to tomatoes such as eggplant, peppers and potatoes. One day in the not too distant future, I’ll trek out to my veggie garden and quickly notice the tell-tale signs of these hungry invaders – the munched foliage at the ends of the branches, and not far below the missing leaves will be the droppings (caterpillar poo).

I'll start looking for the nasty green worms, which takes a while because they blend in perfectly with the stems and foliage. I try to catch them early on so there is minimal damage to the plants. Last year, I found four hornworms, a couple of smallish ones and two rather large ones, early in the summer. The small worms were only about an inch long, which means the eggs they came from had just recently hatched. They have ravenous appetites and eat so much very quickly that they grow fast. If I had taken a couple days longer to detect them, they could have easily been the size of a finger, and perhaps devoured much of the plant they were on. Later in the summer, I found many many more – it was an especially bad year for them. I did find most them while they were on the small side, so damage to my plants was minimal.

How do you recognize a tomato hornworm? How do you manage them? Here’s a little information from UC Davis’s Web site (http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/hornworm.html):

Tomato hornworms of all sizes have a distinctive horn at the rear end. If you find them early on they are small, but mature caterpillars are very large – up to 4 inches long. Tomato hornworms are likely to be the largest caterpillars you will see in the vegetable garden. Their striping pattern makes them hard to spot despite their size. Large, black droppings on the ground beneath tomato plants or on the leaves below where they are feeding usually indicate the presence of hornworms. The adult moths that are responsible for these worms have a wingspan up to five inches.

Damage by hornworm larvae (the worm/caterpillar) is usually most common in midsummer, but there may also be a small population peak in the late summer. Infestations tend to be more severe in warm inland areas such as the High Desert. The larval form is the only stage that causes damage. The eggs are laid singly on leaves. The larvae feed and grow, and then burrow into the soil to pupate. The adult/moth develops over the winter and emerges and the cycle continues. Entire leaves and small stems may be consumed by the larvae. Large pieces from green or ripe fruit may also be chewed.

To manage hornworms, handpick or snip hornworms with shears. Hornworm eggs can be attacked by Trichogramma and the larvae by Hyposoter exigua – two types of parasitic wasps – which can be purchased through various catalogs and released into the garden. There are also several general predators such as praying mantids to help keep populations under control. Bacillus thuringiensis (Bt) or spinosad are effective against smaller larvae. Discing or rototilling after harvest destroys the pupae in soil and helps prevent adults from developing.

I tend to handpick the hornworms, and then feed them to my chickens. It is important to keep looking and not assume there are only one or two. It is best to look for them early in the morning, because they tend to hide out during the heat of the day. Look along the stems and under the leaves, especially near the ends of the branches and near where damage has been found. If you notice the droppings, look above that area. Tomato hornworms used to scare me so I would snip them with shears, but the disgusting green goo they leave behind is not very appealing. I eventually learned to put on a pair of gardening gloves and simply pick the worms off the plant. Chickens enjoy feasting on tomato hornworms, so if you happen to have chickens, it’s a great way to dispose of the worms once you have removed them from your plants. If you notice the eggs or pupa (cocoon), destroy them before they develop into the dreaded hornworm.