How to Handle Hornworms
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Question: Mike: I have converted my friend Steve to organic; he's seen the light and does not want pesticides on his food. But he has been decimated by tomato hornworms the past two years. I suggested plantings to attract beneficial insects, but that isn't quick enough for him. He doesn't mind hand picking, but says he keeps finding them too late—when they're fully grown and destroying his plants overnight. I haven't found anything useful on the Internet; what is your wisdom?
- ---Jack in Cincinnati, Ohio
- ---Barb in Maple City, Michigan
The tobacco hornworm has a red horn; the tomato hornworm has a dark green/black one. But both consume tomato plants—and become large dramatic moths that are important pollinators of night blooming flowers. But you probably don't care about their good habits, because their big babies really can defoliate tomato plants almost overnight.
One way to cut their numbers right now is to use a long handled hoe to cultivate the top couple inches of soil around where your tomato plants grew last year in the hope of exposing the big brown pupa—like a butterfly chrysalis, but with a distinctive little handle—where they've spent the winter down in your soil. Moths will emerge from these massive cocoons over the next month or so, mate and lay pearl-like eggs on your tomato leaves, from which the monsters you know so well will hatch and start to eat.
Once the eater emerges, one of the best controls, as Jack in Ohio so wisely notes, is to use 'companion plants' to attract miniature parasitic wasps—so small we can barely see them—that lay their eggs in or on the hungry caterpillars. After the eggs hatch, the developing wasp larvae spin cocoons on the back of their prey for protection as they slowly consume the pest to fuel their growth to adulthood.
(That's why you should never squish a hornworm that has what looks like grains of rice stuck to its back. Pick the pest off the plant, put it in a jar with some tomato leaves for food and cover the jar with large-holed screening. That will allow the baby wasps that emerge from those little cocoons to escape and go lay eggs in more hornworms.)
Back to the Plants of Attraction: These highly beneficial little waspies like a little nectar and pollen to cleanse their palates in between caterpillar meals, especially from tiny flowers that grow in umbrella-like clusters, like dill, fennel, Queen Anne's lace and biennials like carrots and parsley left in the garden for a second year. They also like compound, daisy-like flowers, tansy, spearmint, clover, sweet alyssum and many others. The wasps are native almost everywhere—so ""if you plant it, they will come"".
If not, you can buy them from suppliers of beneficial insects. Typically, they arrive in the egg or cocoon stage in a little paper cup. You pin the cup to a tree or a stake near your tomatoes and the wasps come out to play. Obviously, you have to avoid chemical pesticides if you want these highly beneficial creatures to survive and rent rooms inside your hornworms.
Oh and birds also love big tasty caterpillars—so having a birdbath near your plants is also nice insurance. And you get to watch the birds.
One reason Jack's friend doesn't see the beasts until his plants are almost gone is that they feed mostly at night. So hornworm-hounding is a lot like slug control; go out after dark with a flashlight and search through the leaves. Despite their massive size, their camouflage is close to perfect, however, so you have to look closely to see that big green monster hiding in the middle of lots of big green leaves . Most people see their 'frass'—a twenty-dollar word for bug poop—long before they see the beasts themselves. So keep an eye out for big black poopy-pellets.
When you see them, spray the leaves of your tomato plants with the old original form of Bt, Bacillus thuringiensis kurstaki. Don't worry—you don't have to remember that panic-inducing name; most people just call this decades old organic caterpillar killer ""Bt"", even though there are other Bts that target other pests, like Colorado potato beetles. The one that stops caterpillars—ALL caterpillars, by the way, not just these big poopers—is the original Bt, the first one discovered: The kurstaki strain, often abbreviated ""BTK"".
Brand names under which this Bt is sold include "Dipel", "Thuricide" and "Green Step". (The package will specify that the product controls caterpillars.) Spray your tomato plants—in the evening—as soon as you spot the pests (or their poop). Or if you're worried you'll notice them too late, spray every couple of weeks when we get into hornworm season. Those hungry hornworms will lose the ability to feed as soon as they munch the sprayed leaves, and then they'll die a few days later.
Bt is one of the safest, most environmentally sound pesticides you can use. It affects ONLY caterpillars, and ONLY caterpillars that eat the sprayed parts of the plant. So good insects like ladybugs, lacewings, and bees can land on your plants without being harmed. And birds and toads can eat a caterpillar that just got a tummy full of Bt without any harm to them. It doesn't even harm nice butterflies because they aren't chewing your tomato leaves. But it does work against all caterpillars, so if other pre-moth pests are chowing down on your plants, Bt is still the answer.