Roots and Shoots: Bad Bug, Interesting Story

By Pamela Doan

I watered the plants in the morning and noticed a few top stems on a tomato looked as if they had been bitten off. Deer don’t normally nibble tomato plants, and the other plants weren’t bothered, but I was busy, and after glancing over the rest of the plant and not noticing anything amiss, I didn’t think about it again. When I came back to water the next day, it was completely defoliated.

The culprit, too full of tomato leaves to make a quick getaway, was lying in the bottom of the plant’s container. A thick hornworm lay there, Manduca spp., lolling around in the dirt.

As far as caterpillars go, hornworms are as yucky as they sound. They can grow up to 4 inches long and are green and smooth except for the namesake horn at the tail. It has arrow-shaped, white and black markings on its sides.

I can’t describe it any better than to say that it’s soft and squishy looking. It definitely falls into the category of insects that most people don’t want to handpick off plants and dispose of. Unfortunately, that’s the best solution to get rid of them. Soapy water will suffice for a quick exit, all squishiness aside.

A hornworm made quick work of a tomato plant's foliage when I wasn't paying attention. (Photo P Doan)

A hornworm made quick work of a tomato plant’s foliage when I wasn’t paying attention. (Photo P Doan)

When I looked it up, I realized that, of course, this is a transitional body. As an adult, it becomes what is commonly referred to as a “hawk moth.” I’ve seen these before, and they are cool. I felt bad for disposing of it then, even though I had to do something or else it would have laid eggs in the soil and eaten my other plants, too. I’m not getting many tomatoes off these plants this year, and each one is valued. Sorry, moth!

Maybe next year I plant a few tomatoes for the hornworms/moths? Probably not.

Here’s where the paper wasps come in handy as a security patrol. They are predators, as are lady beetles and green lacewings. They can keep an eye on the tomato plants and serve as a biological control for the pests. Wasps feast on soft-bodied insects, and the hornworms are a great lunch. Consider that before spraying a wasps’ nest if it’s in a location that is out of the way and they don’t pose a threat. Living with them nearby can be a great benefit for the tomato harvest.

I have focused on tomatoes, but hornworms survive on all nightshade plants, including eggplant and potatoes. My caterpillars are the second round of this insect species for the season, too. Their parents emerged from the soil in the spring and then laid eggs that hatched in midsummer. My hornworm would have completed its life cycle by going into a larval stage, overwintering in the ground, then returning in the spring as a moth to start all over again. Nature is amazing.

One other fascinating tidbit I learned about the hornworm from an online fact sheet maintained by the University of Minnesota Extension program is that a parasitic wasp uses the hornworm as an incubator and food source. It lays eggs inside the hornworm, and the tiny cocoons project from its body.

If you see these on a hornworm, the site advises that it be left in the garden until the wasps complete their pupate stage. The adults will eventually emerge from the hornworm, killing it in the process. Then the good bugs can go find new bad bug hosts for the next generation. It’s a beautiful cycle.

Other pests that can bother tomatoes are aphids, flea beetles, cutworms and whiteflies. Each is primarily controlled by observation and handpicking or spraying with water, in the case of aphids. Although there are pesticides available, this is the best method. Tilling the soil should be sufficient to get rid of any burrowers or eggs.

Since I’ve got tomatoes planted in containers, I’ll be sure to rotate the soil in the containers, too, just like I would in the garden beds. By removing the host plant from the same soil year after year, I can disrupt the chain of events.

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