It's Tomato Hornworm Time!

It's Tomato Hornworm Time!

I spotted my first tomato hornworm the other day – oh joy of joys. Well, actually, I only spotted it after Dan pointed it out to me; he's got the eagle eyes in the family and you sure need sharp visual acuity to find those little devils. Not so “little” really – at up to 4” (10 cm), tomato hornworms are one of the largest insect pests you'll deal with if you live in southern Ontario and Quebec. 

Even though they're big and scary looking, I always feel somewhat conflicted about taking them out. You see, the tomato hornworm caterpillar is the larva of the hummingbird moth (Manduca quinquemaculata), which I happen to really like. I really like my tomato plants alive and bearing fruit, too, so I apologize to the caterpillars as I eliminate them because you can't have both.

The moths emerge from the soil in spring after they've overwintered. They lay their eggs on the leaves of host plants and, after a few days, the larvae emerge and start eating voraciously for the next four weeks. They then bury themselves in the soil to start the cycle again. The larvae that hatch from the second generation of moths eat into the fall and then burrow into the soil to overwinter in cocoons.

Although they're most commonly called tomato hornworms, these caterpillars actually feed on any of the members of the solanaceae family, including peppers, potatoes, eggplants and nightshade. Tomatoes are their favourites – they eat the leaves and stems, as well as both the green and ripe fruit. Left alone, they can destroy a plant in a number of days. This is why they're considered so destructive and why we need to be constantly checking for them starting around now. You know you have hornworms if you notice whole branches of your tomato plants have been stripped bare of leaves, or if you see their telltale clusters of brown/black droppings on lower leaves. The caterpillars themselves are green (I have also found black ones) with white v-shaped lines on their sides and a black horn on their tail end. This horn looks dangerous but it doesn't sting. They blend in well with the plants, often hanging upside down along a stem, and I have to train my eyes to find them each season. They really are quite frightening the first time you encounter them, and these guys have attitude. Often, when I try to pick them up, they'll rise up and threaten to attack me, making a clicking sound.

Because hornworms dislike the heat of mid-day and will rest in the lower levels of the plants, the best time to look for them is at dawn or dusk, when they're feeding on the upper stems. Hand-picking them, although tedious, is the best way to get rid of them. Disposing of them can be done by squishing them with your (booted) foot (Dan's preferred method), cutting them in half (ugh!) or dropping them in a container of soapy water (my choice). Parasitic wasps can help you out: they lay their eggs on the larvae and their young feed on the host hornworm and then pupate in white cocoons on its back. So, if you find a hornworm with a lot of what look like pieces of rice on it, leave it alone. Those wasps, when they hatch, will also feed on other hornworms in the vicinity.

A couple of prevention techniques include tilling the garden well in the late fall and spring to unearth the pupae and planting marigolds, borage, basil or dill as companion plants. We now till our gardens every fall and have noticed a significant decline in the number of hornworms munching on our tomatoes in the last few years. They can still cause a fair amount of damage, though, so it's off to the garden every day to do our check.