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Tales from a Plant Addict

Fun (& a few serious) facts, tips and tricks for every gardener, new and old.

Tomato and Tobacco Hornworms

I've been growing a couple of tomato plants in containers on my patio this year, and have been pretty pleased with the results. The plants look good, and I'm starting to get a few ripe cherry tomatoes here and there.

Growing tomatoes in containers is not without its challenges. For example, tomato plants in containers use A LOT of water, and it can be difficult to keep up with watering, even with the rains we've had this year. But this week I discovered a new challenge which I never expected to see with tomatoes in containers.

While watering one night this week, I saw some black droppings on the patio around my tomatoes. That could only mean one thing: somehow a hornworm had taken up residence on my patio tomatoes. I sprang into action and started scanning the tomato foliage. I spotted the telltale stubs of what used to be leaves on one of my tomato plants.

I quickly found the offender, a large green caterpillar about four inches long and approximately the diameter of my thumb. Thankfully I didn't find any more besides this one large specimen—but that doesn't mean I won't find more in the coming days.

My attention turned to the three plants in our vegetable garden. I haven't found any yet on these plants, but I've had some issues with hornworms in my garden in the past. I won't be surprised if they show up sometime soon.

It turns out there are actually two different species of hornworms likely to be found on tomato plants. The tobacco hornworm, Manduca sexta, is a large green caterpillar with seven diagonal white lines on each side, and a curved red horn at the hind end. This is the culprit tormenting my poor tomato plants. The tomato hornworm, Manduca quinquemaculata, is also a large green caterpillar, but there are eight V-shaped marks on each side, and the horn is straight and bluish-black in color.

Both species are voracious feeders on tomato plants, and any plant related to tomato (solanaceous, or nightshade family), including potatoes, eggplants, peppers, or weeds such as nightshade or jimsonweed. I typically don't see hornworms on anything but my tomato plants. The exception was a few years ago when the hornworm population seemed to explode overnight. That year I found hornworms on my pepper plants for the first time ever.

Both species of hornworm have evolved to escape detection quite well. Their color is a perfect match to a tomato plant. Their bodies resemble the underside of a tomato leaf. Plus they tend to hide deep in the foliage to escape the direct sun during the day, venturing onto outer stems in the early morning and evening. For most of their life, they easily go unnoticed.

After about three or four weeks after hatching from their egg, a hornworm is mature, usually measuring about four inches long. This mature hornworm can easily eat several tomato leaves and even fruit in a single day.

Hornworm damage is fairly easy to spot if you know what to look for. Leaves will be devoured, leaving only the petiole sticking out. You might see large green or black droppings at the base of the plant. Typically feeding begins on the upper part of the plant.

Many people are shocked to discover that hornworms actually do become something other than a tomato plant eating machine. When mature, the hornworms drop off the tomato plant, burrow into the ground and form a pupa. When they emerge, they are a large moth with a wingspan or four or five inches, commonly referred to as a hawk, sphinx, or hummingbird moth.

The first hornworms of the year eat, pupate and hatch out into moths during the summer, but the second generation overwinters as a pupa underground. In the spring or fall, you may find the pupas when working in the garden. They resemble brown cigars or jugs with a curved brown handle.

One great way to prevent a hornworm problem is to destroy these pupas, whether by tilling a large garden or individually picking by hand in a small garden. This is another reason to rotate where you plant different crops in the garden.

I was shocked to find the hornworm on my patio tomato plant since all the soil I used was fresh. Where did they come from? Everything I've read talks about the adult moth emerging in the spring and laying eggs on nearby tomato plants.

I doubt that the potting mix I used had any hornworm pupae in it. There is a flower bed right next to the patio, and I had tomatoes on the patio last year. I don't remember seeing any hornworms on my plants last year, but in theory at least a hornworm could pupate in the flower bed next to the patio and lay eggs on the tomatoes the following year.

My vegetable garden is not all that far from the patio, so it wouldn't be too difficult for an adult moth to find my patio tomatoes and lay eggs on them. This seems to be the most likely explanation since I don't remember seeing any hornworms on my patio plants last year.

While it is possible to use insecticides such as carbaryl, permethrin, spinosad or Bt to control hornworms, most sources suggest hand-picking and destroying them is the most effective way to control them. It's also very satisfying to the psyche!

Mother Nature has her own way of controlling hornworms. There are multiple species of parasitic wasps that will lay their eggs in a hornworm. The eggs hatch, feed on the hornworm from the inside, then emerge from the hornworm and form a cocoon. These will look like little white tic-tacs on the outside of a hornworm. If you find such a hornworm, leave this one alone. It will die when the new wasps emerge from the cocoons, and the wasps will move on to find a new hornworm to lay eggs in and the cycle continues.

It looks like I have won the battle of the hornworms, at least for now. I will continue my patrols for hornworms every time I water. My plants are recovering, and soon I should have an overabundance of ripe tomatoes. It just goes to show that you cannot escape Mother Nature. Pests will find your plants; it just may take them awhile.



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