August 20, 2016

1 - Tomato horn worms are common in New Mexico and seem to appear overnight.

Yard and Garden August 20, 2016

Q.

Yesterday afternoon I thought my tomato plants looked a little strange, then this morning I noticed that almost every leaf on some of the plants were gone. I saw a few big, fat, green worms with a weird-looking spikes sticking out from their ends. They looked dangerous. What they and what can I do about them?

A.

You have described the tomato horn worm. This is the caterpillar, or juvenile phase of a hawk moth or sphinx moth. There are almost 1500 species in this group of moths; fortunately, most are not present in New Mexico. Larvae of some hawk moth species feed on weeds, so not all hawk moths are bad. The adult moths are interesting as dusk and dawn pollinators when they hover near white, or pale colored flowers. They are often thought to be hummingbirds since they are similar in size and hover over the flowers while they are consuming the nectar of the flowers. Some of the adult moths are pretty, others are not as attractive, but all are interesting (if you like insects).

While the large adult moths are interesting, the larvae can appear frightening and can devastate a tomato plant in a single evening. The "spike" you described, or the horn that gives the horn worm its name is not dangerous. It will bend if you touch it. The caterpillar may be picked from the plant by hand, if you can muster the courage. You can then dispose of it. You will notice that it regurgitates a dark colored material to spit on your hand when you pick it up. Several species of the horn worm feed on plants that contain toxic materials that do not harm the horn worm caterpillars, but can be used to discourage ants and parasitoid wasps that would otherwise feed on the caterpillar. The tomato horn worm and its relative, the tobacco horn worm, which also feeds on tomatoes, egg plants, and other plants in the tomato family (Solanaceae) collect toxic materials from the tomato (tobacco and other solanaceous plants).

These pests of some of our garden plants are able to camouflage themselves within the foliage of the plant so that they can become quite large and damaging to the plants before gardeners can find them. They can also travel from plant to plant to conceal the extent of their damage. They pupate in the soil or under mulch and debris, so you may find their pupae as you are digging your garden. If you find these large pupae in the spring, you can anticipate pests in the summer.

They may be treated with a number of pesticides, but as larvae of moths they may be treated with Bacillus thuringensis based pesticides and some other organic pesticides. However, manual removal is the quickest way to end the damage they do to your garden. However, remember there are some "good" horn worms that feed on weeds.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

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