Battling Tomato Pests: Identify and Control Hornworms

They’re green, they’re hungry, and they’re gunning for your tomatoes. 

We’re talking about hornworms, the caterpillars of hawk moths, specifically the five-spotted hawk moth (Manduca quinquemaculata).

During their larvae cycle, these hornworms gorge themselves on plants in the nightshade family. This can include eggplants, peppers, and potatoes, but their food of choice is the tomato plant. Given the chance, these annoying little pests can decimate your tomato crops.

That’s why it’s important to be able to spot hornworms and — more importantly — do what it takes to protect your tomatoes from them.

Hornworm havoc

Why are hornworms a particularly troublesome pest? The problem is, not only do they feed on the leaves of tomato plants, but they also like to munch on stems, flowers, and the fruit. 

With their voracious appetite, hornworms can strip a tomato plant of its leaves within a few days, weakening the plant and hampering its ability to photosynthesize (make energy using sunlight and carbon dioxide). This, in turn, can cause your tomato plants to become stunted and produce fewer tomatoes. And it’s entirely possible that the tomatoes they do grow will have holes in them, also courtesy of the snacking hornworms.

To add insult to injury, the weakened tomato plants are more susceptible to other pests and diseases.

Recognizing hornworms

So, how can you tell if hornworms are in your garden before they start messing with your plants? 

First of all, you should know what to look for. Mature hornworms can grow up to 4-inches long, making them one of the largest caterpillars you’ll find in your garden. Their bright-green color can make them tricky to spot since they blend right in with tomato foliage. 

But hornworms do have markings that will give them away, including white or yellowish stripes running along their sides. Tomato hornworms have eight V-shaped markings.

You can also keep an eye open for signs of infestation, including chewed leaves, defoliation, and the presence of black droppings. The best time to check is early morning or late afternoon, when hornworms are most active and easier to spot.

Hornworm lifecycles

And where are hornworms most likely to appear in the garden? Generally, adult hawk moths lay their eggs on the undersides of leaves in the late spring. About a week later, larvae hatch and begin their feeding frenzy. After three to four weeks, plenty of time to do some real damage to your tomato plants, the larvae burrow into the ground and form pupae. They stay in that stage through the winter and emerge as adults in the spring, ready to being the growth cycle all over again.

Effective hornworm control methods 

Hornworms in the garden is never good news, but there are measures you can take to prevent infestation — and to deal with them if they’re already present.

Let’s start with preventative measures. One fairly simple step you can take is to practice crop rotation: Don’t plant tomatoes or other nightshade family plants in the same spot each year. By moving things around, you’ll disrupt the hornworm life cycle and reduce the chances of infestation.

Augment those efforts by tilling your garden soil in the fall and spring to expose and kill overwintering pupae.

Even with these steps, you should still make it a habit to inspect your tomato plants for hornworms and their eggs (which are small, round, and yellowish-green). If you find any, pick them off and destroy them.

Another strategy is companion planting. Some plants, including basil, marigolds, borage, and dill, actually repel hornworms. Plant one or more of them near your tomatoes, you might be able to dodge the hornworm bullet.

You can take a similar approach with beneficial insects like ladybugs, lacewings, and parasitic wasps (especially braconid wasps), which prey on hornworms or their worms. You can attract the insects you want by adding plants they like, possibly alyssum, dill, or parsley, to your garden near your tomato plants.

What if you already have hornworms in your garden? Try natural control methods. Bacillus thuringiensis (Bt), for example, is a naturally occurring bacteria that can be used as a safe and effective insecticide for hornworms. Spray it on your plants when the caterpillars are small.

Neem oil is a natural insecticide, too, that can control hornworms as well as other pests. Apply it by spraying or wiping it directly on the leaves of affected plants."

As a last resort, you also have the option of using chemical insecticides labeled for hornworms. Follow the instructions carefully to avoid harming beneficial insects in your garden.

If you think you have a severe infestation, you might have to turn to a professional pest control service for help.