Dealing with hornworms in the organic garden

Dealing with hornworms in the organic garden

I don’t know whether I hadn’t had an infestation of hornworms before, or I just hadn’t noticed them (which seems rather impossible!), but the tobacco hornworms have had a good year this year and there’s no avoiding them.  One day all your beautiful tomato and pepper plants are healthy and fine, and the next–bam! –you’re facing devastation.  My farmer–as I’ve taken to calling Lady Linda (remember-I’m an apprentice now!)–gave me the scoop on hornworms.

What is it?

tobacco hornwormThere are two types of hornworms, and they are very similar in appearance, both are a lovely shade of green and both can grow up to 4-inches in length!  The tomato hornworm (Manduca quinquemaculata) bears eight “V”s positioned down the length of its body in white and black accent colors; while the tobacco hornworm (Manduca sexta) wears seven diagonal white lines down each side.  Both possess the soft spikes that have given these species their names, and the Latin root of both caterpillars–Manduca–means “to chew” (I remember this by relating “mandible”–as in-the mouth parts of some insects–with Manduca).  Aptly named, I think!

The tomato and tobacco hornworm caterpillars are native to the United Stated and commonly are found in the northern states.  The offspring of the sphinx moth–also known as the hummingbird moth and the hawk moth–these moths are nocturnal insects, and lay their eggs in a fine powdery mass on the underside of leaves of solanaceous plants such as–tomatoes, peppers, eggplant and even potatoes.  Alternately, some solanaceous weeds serve as hosts for overwintering larvae, including horsenettle, jimsonweed, and nightshade.

Generally there are two generations a year, the initial generation, and then the generation that will overwinter to rise next year and begin the attack upon your tomatoes all over again.  The moths overwinter in the soil and will begin to emerge from the soil in early June; they may continue to do so as late as August.

Methods for control

before & after tobacco hornwormsOften, the first thing gardeners will notice is defoliation at the tops of plants, but if you look beneath the plants, on the ground, you will see the droppings left behind by the caterpillars, which is called “frass”.  The caterpillars take refuge under the leaves at the interior of the plant during the heat of the day, which makes them a challenge to find!

While you can use organic biological pesticides such as spinosad and Bacillus thuringiensis (BT), even an organic pesticide is still a toxic poison and use of such should be carefully considered before applying.

Typically, a daily regimen of hand-picking the caterpillars from the leaves of your plants will save your crop.

hornworm frass againLady Linda’s tips:  “If you pick the offenders from the plants, then sweep away the frass on the ground, the next day when you go out to hunt hornworms, you will be able to tell which plants are harboring the caterpillars just by looking for frass beneath them.  This works really well if you’re using black landscaping fabric beneath your plants, or if you’re in a hoop-house or greenhouse where greenhouse fabric is used to prevent weed-growth beneath your plants.  It’s a huge time-saver for a small greenhouse operation!”

Since the larvae of the sphinx moth overwinters in the soil, it is imperative to clear your garden of each season’s crops and weeds every fall, and to rotate your crops every spring before you begin to plant again.  Sometimes a significant distance between the emerging larvae of a harmful garden pest and the pest’s dinner is enough of a deterrent or delay to give your plants the head start they need to grow strong and healthy before they are attacked by hungry mouths.

Tilling your garden under at the end of the season will destroy any pupae attempting to overwinter there.

parasitized hornwormAny caterpillars found with a cluster of white eggs upon their backs should be left, as these larvae have been parasitized by a parasitic wasp, which will hatch and prey upon other caterpillars–a very beneficial insect to your garden!

When you pick the caterpillars, toss them in a jar and take them to your chickens–they’ll thank you for it with eggs bearing rich orange yolks!

So get over your ick factor, get out there and get those hornworms!

References & Resources

How to Control Hornworms – from Organic

Tobacco Hornworm – feature from the Entomology Department of the University of Florida

Tomato/Tobacco Hornworms – from the Colorado State Cooperative Extension



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