JoAnne Skelly: Green monsters eating tomato plants

I was harvesting tomatoes on my friend’s plants, with her permission of course, when I noticed there were many stems with no leaves. I immediately thought “Hornworms!” Sure enough, as I started inspecting the plant closely, I found a three-inch bright green worm clinging to a defoliated stem. It was stuck like it was glued on.

It was really difficult to spot because it blended into the surrounding foliage and stems. I examined other plants and found five more. I also found a parent, a sphinx moth, quite possibly laying eggs on a tomato leaf.

One day tomato plants look healthy, green and lush. The next, the leaves are gone and only stems remain. People ask, “how can something be eating the leaves – the plants are hanging high out of reach of critters?” “What are all these peppercorn-size pellets on the ground?” Hornworms are well camouflaged and hard to see. However, once you spot this bright green 2 to 4-inch caterpillar, you will never forget it. The pellets are fecal droppings.

Hornworms are some of the largest caterpillars in the west. Their parents are called sphinx moths, hawk moths or hummingbird moths. These large moths resemble a hummingbird in flight as they feed from deep-throated flowers, such as honeysuckle. They are in abundance this year. They are strong fliers and can travel long distances.

The female lays smooth, round, pale green eggs singly on the upper surfaces of leaves. The caterpillars hatch and feed for a month or so on the leaves. Then they migrate from the plant to the soil where they pupate. The “horn” at the hind end of the larval or caterpillar stage is actually a flexible spine.

Extra vigilance will help you control hornworms. They are easier to discover at dusk and dawn. Look carefully to find them on the stems; pick them off and destroy them. They cut easily with shears. As I pulled off the first worm, it exuded a green slime all over my fingers. YUCK! I was more careful with the others, cutting off the stem rather than touching them. I then cut them in half with my shears. Messy, but effective.

Bacillus thuringiensis is a bacterial disease you can purchase and apply. It will infect the hornworms and kill them. It is non-toxic to humans. Hand picking solves the problem more quickly though. Tilling or digging the soil after harvest will destroy the overwintering pupae to reduce next year’s infestation.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu.