Garden Plot: Guard your tomato plants from tomato hornworms

July 12, 2019

Keep a close eye on your tomato plants

Dan in Sterling writes: “What’s the best way to rid my garden of tomato hornworms?”

That’s a timely question, Dan, as this is the time of year to be watching out for these tomato eating fiends! You’d think it would be easy to spot the largest caterpillar in North America, but the tomato hornworm (and the closely related tobacco hornworm) are EXACTLY the same color as your tomato plants and blend in so well that you can be looking right at them and still not see them.

Instead, you should look for the damage they cause: large portions of your plants will be eaten away — generally at the top of the plants — if the hornworms are present. If it looks like a deer was browsing on your Brandywines, look just below the damaged areas for the culprits — but don’t squish them just yet!

Don’t squish a spiny hornworm!

The best way to get rid of tomato hornworms is to eyeball your plants daily. If parts of the tops are missing, run your hand just below the damage until you get to something big, green and soft.

But don’t squish that worm if it has white spines down its back. Ordinary hornworms are unadorned, but white spines on the back are a sign that this one has been parasitized by a wasp so tiny we can barely see it. Those “spines” are actually cocoons, each containing a baby wasp that is slowly eating the pest alive.

You want those cocoons to produce more of these tiny beneficials. Instead of a fast move with your thumb, prune off a couple of damaged tomato branches and take them and the parasitized pest to a shady spot nearby. The caterpillar will soon be shriveled up and within a few days, adult wasps will emerge and go right to your tomato plants looking for their next victim.

Don’t forget the frass!

We’ve now established that to rid your garden of tomato hornworms, you have to eyeball your plants daily looking for damage near the top. But you might first see the hungriest caterpillars’ frass below. Frass is a $20 word for bug poop.

These caterpillars are so big and so voracious that they deposit large amounts of frass daily. It’ll look like a lot of black specs on the leaves just below where it’s been feeding and/or on the ground below.

To find this hungry, hungry caterpillar look just above the frass-covered leaves and below the damaged areas of the plant. But remember — if it has white spines up and down its back, leave it be; wonderful wasps are eating it alive!

Bt beats hornworms and other caterpillars

If hornworms are appearing in large numbers — or you have to be away from your plants for a while (say while you’re on vacation) — spray Bt on your tomato plants.

Sold under brand names like Dipel, Thuracide and Green Step, Bt — approved for use in organic agriculture — is a naturally occurring soil organism that is deadly to caterpillars that eat the sprayed leaves but harms nothing else. It won’t hurt bees, birds, frogs, toads, fish, pets, people or even butterflies — JUST caterpillars, and JUST caterpillars that eat the sprayed leaves.

And no butterfly babies feed on tomatoes, so it will do its job without any harm.

An organic control for broadleaf weeds

Nancy in Chevy Chase writes: “My boss has a farm in Middleburg and is trying to figure out an organic way to control broadleaf weeds in a field used for bailing hay. The gentleman that does the bailing wants to use 2,4-D, but we’re afraid of the environmental impact.”

As well you should be, Nancy! Deadly to fish and amphibians and directly linked to specific cancers in male dogs and humans, 2,4-D was one of the ingredients in the infamous Agent Orange herbicide used to defoliate the jungles of Vietnam. (And for which our Vietnam-era veterans are finally being compensated.)

Luckily, an organic alternative became approved for use back in 2012 — iron. Iron-based herbicides like Fiesta and Iron-X are highly effective at killing broadleaf weeds without harming the environment — or the person doing the spraying.

We’ll dive further into this important topic next week. In the meantime, here are a couple of articles on this lifesaving magic of iron-based herbicides.

Mike McGrath was editor-in-chief of ORGANIC GARDENING magazine from 1990 through 1997. He has been the host of the nationally syndicated public radio show “You Bet Your Garden” since 1998 and WTOP Garden Editor since 1999. Send him your garden or pest control questions at

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