From moth to monster: Hornworms return

A few weeks ago you were sitting back admiring your freshly planted garden. Neat little rows of tomato, pepper, squash, and cucumber plants accompanied by flowers and herbs were all planted in view from your back deck. As you sat there basking in the evening sun, relishing in your hard work, a little moth fluttered from flower to flower sipping nectar.

hornworm moth
A hawkmoth, the Carolina Sphinx, is the adult form of a tobacco hornworm. Photo: Mike Raupp, University of Maryland, Department of Entomology

With her hummingbird like flutters, a Carolina Sphinx Moth floated through your garden, unassumingly laying her eggs on your newly planted tomato and pepper plants. Within a few days, from her little green eggs emerged a tiny but very hungry green caterpillar.

Hornworm caterpillars. Photo: Rachel Rhodes, University of Maryland Extension

Since that day, the ravenous little green hornworm caterpillar has spent his days munching away, perfectly hidden by the copious green foliage of your tomato plants, growing bigger and bigger. You begin to notice stems of complete defoliation. Maybe you think it’s a bunny or deer having a nighttime nibble as the little green caterpillar stays camouflaged, until the moment you notice the red-tipped horn and the very large green body of a caterpillar measuring almost 4” in length hanging on your prize winning tomato plant.

During the last month the hornworm caterpillar has gone through 4-5 instars (growth stages) while feasting in your garden. If the hornworm reaches the final growth stage he will begin to wander looking for the perfect site to pupate. Once the perfect site has been found the caterpillar will form a pupal cell below the leaf litter or soil.

However, in our area there are many natural predators that love to make a meal of the delicious protein- rich hornworm caterpillar or eggs. Birds, small animals, and other insects find the hornworm caterpillar particularly delicious. Paper wasps use the caterpillars as a future food source in nest cells containing the wasp’s eggs.

hornworm with parasitic wasps
Hornworm caterpillar parasitized by a braconid wasp. Photo: Rachel Rhodes, University of Maryland Extension

In sci-fi movie fashion, parasitic wasps (Braconid wasps), also use hornworms as a food source for their young, but in a much more diabolical manner. The small parasitic wasp inconspicuously stings the caterpillar depositing her eggs inside the hornworms body. As the larval wasps develop they devour the caterpillar, feeding on its blood as they grow. In the final pupal stage, the immature wasp spins small white cocoons that resemble grains of rice that protrude from the body of the living hornworm. Eventually, the parasitized hornworm will fall victim to the wasp and will stop eating and die. Using nature as your method of control is perhaps the best way to rid your garden of this very hungry
caterpillar, so just sit back and watch the show.

By Rachel J. Rhodes, Master Gardener Coordinator, Queen Anne’s County, University of Maryland Extension. Follow the Queen Anne’s County MGs on Facebook. Visit the UME Master Gardener webpage to find Master Gardener events and services in counties throughout Maryland.

6 thoughts on “From moth to monster: Hornworms return

  1. GEORGE LAMBERT July 8, 2019 / 11:13 am

    I have heard that once parasitized, the hornworm’s death is imminent so should be left alone to encourage the braconid wasp population. However, the hornworm is so ravenous, I hate to let them munch for even 24 hrs. Is there any way to keep the hornworm alive off the tomato until it succumbs to the wasp? Also, exactly how many hours until death after the white eggs are visible?

    • Maryland Grows July 10, 2019 / 1:25 pm

      When you see the white cocoons, the hornworms are already dying. It is actually good to leave them on (or at least near) the plant so the adult wasps will find and parasitize other hornworms that might be present on your tomatoes. The cycle of biocontrol will continue!

      When the adult wasps lay their eggs in a hornworm, it can take 12-16 days for the larvae to emerge. Then they spin their cocoons. Once the cocoons are present, it takes about 3-8 days for the adult wasps to emerge. This timeline will vary by temperature. Here is more information about the lifecycle.

      Also, check out this video from Purdue Entomology.

  2. lilyo3 July 8, 2019 / 11:49 am

    I’m interested in the answer to George’s question (above) also.

    • Maryland Grows July 10, 2019 / 1:26 pm

      Thanks for your interest! See the reply to George’s question (above).

      • lilyo3 July 10, 2019 / 8:24 pm

        Very helpful – Thank you!

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