The Dreaded Tomato Hornworms Have Arrived

Here we go again! It seems to be that time of the year when the tomato hornworms make their annual appearance and start busily munching away on my tomato plants. So far, in the past couple of weeks, I have found four of them. They can also be a problem for other related plants like eggplant, peppers and potatoes – so far so good on those.

It doesn’t take too long to notice the tell-tale signs of these hungry invaders. The foliage at the ends of the branches on the affected plants will eaten, and not far below the missing leaves is where you’ll find the droppings (caterpillar poo). If left very long, entire branches, and even entire plants, can be devoured.

With a lifetime of gardening experience, I know to start looking for the nasty green worms around mid to late June and throughout the growing season. At first they are tough to spot because they blend in perfectly with the stems and foliage. It’s best to catch them early on when there is only minimal damage to the plants – only a couple of bare stems. So far this year, as I said above, I have found a total of four hornworms, a couple of smallish ones and two rather large ones. 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.

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.

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.