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Attack of the tomato hornworms!

August heat brings out spike in population of these giant caterpillars

Green caterpillar on tomato plant branch
The tomato hornworm's coloring makes it hard to see among tomato leaves. Look
for its poop on leaves below or, at night, search using a UV flashlight. (Photo: Debbie Arrington)

They look like something out of a sci-fi movie – if you can spot them. And what they do to tomato plants is absolutely horrifying.

Recent hot, dry weather brought out a late-summer surge of one of the biggest bugs in our vegetable gardens: The tomato hornworm.

They love these last days of summer – and munching away on ripe tomatoes. While many critters retreat in high heat, these invaders actually spike in numbers and activity as the mercury rises.

Natural enemies usually keep their populations under control. But hornworm numbers tend to go up in August along with the temperature.

I saw this firsthand in my own Sacramento garden. I've picked off five hornworms from my tomato vines in five days.

Fat as a finger and just as long, hornworms rank as Sacramento’s largest caterpillars. They eat big bites out of their favorite food: Juicy tomatoes. They’ll also eat hard green tomatoes, leaves and stems.

Their stripes let them hide in plain sight. They blend in so well with their surroundings, they can seem impossible to spot.

What’s easy to see are leaves that have been stripped bare to the stem and damaged tomatoes. Other pests and critters may take bites out of tomatoes, but those stripped leaves – especially if the damage seems to appear overnight – usually indicate hornworms.

If you suspect hornworms, look for their poop. They leave large black or green droppings on or around the plant. If you see those droppings, carefully inspect the plant’s nearby leaves and stems. When you find it, pick off the hornworm and dispose of it. (Folks who raise chickens say their hens love them as a snack.)

If the hornworm escapes capture, it burrows into the soil and pupates into a moth of equally gigantic proportion: The hawk moth. Emerging in spring, this brown and gray moth has a 5-inch wingspan.

Rototilling the tomato bed after harvest prevents those moths from ever developing – and laying eggs next spring.

For more on tomato hornworms: http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/hornworm.html


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Garden Checklist for week of July 7

Take care of garden chores early in the morning, concentrating on watering. We’re still in survival mode until this heat wave breaks.

* Keep your vegetable garden watered, mulched and weeded. Water before 8 a.m. to conserve moisture.

* Prevent sunburn; provide temporary shade for tomatoes, peppers, eggplant, melons, squash and other crops with “sensitive” skin.

* Hold off on feeding plants until temperatures cool back down to “normal” range. That means daytime highs in the low to mid 90s.

* Don’t let tomatoes wilt or dry out completely. Give tomatoes a deep watering two to three times a week. Harvest vegetables promptly to encourage plants to produce more.

* Squash especially tends to grow rapidly in hot weather. Keep an eye on zucchini.

* Some weeds thrive in hot weather. Whack them before they go to seed.

* Pinch back chrysanthemums for bushy plants and more flowers in September.

* Harvest tomatoes, squash, peppers and eggplant. Prompt picking will help keep plants producing.

* Remove spent flowers from roses, daylilies and other bloomers as they finish flowering.

* Pinch off blooms from basil so the plant will grow more leaves.

* Cut back lavender after flowering to promote a second bloom.

* One good thing about hot days: Most lawns stop growing when temperatures top 95 degrees. Keep mower blades set on high.

* Once the weather cools down a little, it’s not too late to add a splash of color. Plant petunias, snapdragons, zinnias and marigolds.

* After the heat wave, plant corn, pumpkins, radishes, winter squash and sunflowers. Make sure the seeds stay hydrated.

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