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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:


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Dig In: Garden Checklist

For week of March 19:

Spring will start a bit soggy, but there’s still plenty to do between showers:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Watch out for aphids. Wash off plants with strong blast from the hose.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to fight blossom blight.

* Feed citrus trees as they start to blossom.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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