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How to Handle Hornworms

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

Mike: What can be done to prevent those ugly, green, caterpillar-like horned worms that destroy tomato plants? They almost wiped me out a few years ago. Thank You.

    ---Barb in Maple City, Michigan

Answer: As with many similarly named pests, the hornworm is not a worm but a caterpillar—a BIG one; at four inches long, it's one of the biggest out there. The 'horn' is a scary looking—but fake—appendage on its hinder to scare predators.

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.

(Here's a link to a great extension bulletin from Colorado showing what the egg and pupae stages look like; the adult moths and nasty caterpillars too: http://www.ext.colostate.edu/pubs/insect/05517.html.)

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.)


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