First things first: I don’t have tomato hornworms (Manduca quinquemaculata) in my own garden. What you see above is a photo that I took a few weekends ago of a Tobacco Hornworm (Manduca sexta) fatting itself up on my tomatoes. The caterpillars of these two distinct species of moth look very much alike and are easily confused. For reference, the tomato hornworm has v-shaped markings down its sides and a dark black/brown “horn” protruding out the back, while the tobacco hornworm [seen above] has diagonal strikes down its sides and a bright red “horn.”
Despite this distinction I decided to post under the name tomato hornworm because it is the most commonly known of the two.
Needless to say, they are both gigantic caterpillars that devour tomato plants and just about anything in the Solanaceae family (tomato family such as peppers, eggplant, nicotiana, etc) so it really doesn’t matter either way. They are both beautiful and awe-inspiring creatures that grow into equally fascinating sphinx moths. Incidentally, I should have known what I was in for when we started catching glimpses of the moths hovering around weeks ago. Still, they may be glorious to behold, but they are a menace. I do not want these super eaters/plant destroyers in my garden!
The Damage They Do
Prior to encountering these critters in my garden, my knowledge of hornworms was primarily based on anecdotes from other gardeners and literature. Everyone mentioned that they consume plant leaves and stems with gusto, but nobody mentioned that they also eat the fruit. I found several half-eaten, green tomatoes still on the vine well before I finally spotted the culprit. I knew it was a new predator because they lacked the bite markings typically left by mammals and many of the half-eaten fruit were located underneath the foliage — mammals such as squirrels tend to go for the fruit that is right out front and easily within reach.
Handpicking is the way to go here, especially in a small, urban garden. They look ferocious and I will admit that for my first encounter I picked them off while wearing gloves. I have since learned that they are harmless. If you have chickens, they’ll make a delicious treat!
The key to keeping them under control is diligence. Check plants daily and really look all around because these guys have a remarkable ability to blend into their surroundings. The two I found on my plants got to be so big because I simply wasn’t looking. Also look out for eggs on the underside of leaves. Since finding them I have checked all over all of my plants (including peppers and other tomato family crops) and continue to survey for damage each morning. So far no new damage has occurred. You can also look out for their poop (aka frass), which shows up as dark green/black blobs on foliage and fruit.
As always, I also recommend interplanting around or nearby your tomatoes with nectar-rich flowers (and umbels such as lovage, dill, etc) that will attract parasitic wasps. These wasps are an amazing ally to have in the garden in general, and one of the pests they parasitize are hornworms. The adult wasp lays its eggs inside the hornworm, feeding and eventually killing it when they mature. If you happen to find a hornworm with what looks like small, white grains of rice all over its body, leave it alone as these are the pupae of the wasp doing their thing.
Finally, while I am not a tiller, I do plan to do some work in the soil around my tomatoes at the end of the season this fall. Hornworms drop into the soil and pupate there through the winter and chances are good that I missed in few before I caught onto their presence in my garden. Destroying the darkly coloured pupae in the fall is one way to stop their lifecycle and prevent their return in the spring.
- As always I recommend my favourite book on organic pest control, “The Organic Gardener’s Handbook of Natural Insect and Disease Control” edited by Barbara W. Ellis and Fern Marshall Bradley. Note that there is now an updated version of this indispensable guidebook.
- Bees, Wasps, and Ants: The Indispensable Role of Hymenoptera in Gardens by Eric Grissell – Thanks to Paul Riddell who recommended this one via Facebook.
- Peterson Field Guide to Moths of Northeastern North America
- Butterflies and Moths of North America