To the dismay of many gardeners, hornworms are rather fond of garden tomatoes.
“The larva feeds on the leaves and stems of the tomato plant, leaving behind dark green or black droppings,” said Kansas State University horticulture expert Ward Upham.
Hornworms are a common insect invader of home gardens, Upham said. Though they favor tomatoes, they are also known to feed on eggplants, peppers and potatoes. In their adult moth stage they are relatively harmless to the garden. However, in their larval stage, they can be quite damaging to plants.
Upham said the larval hornworm appears as a long, pale green caterpillar. It grows to an impressive 3.5-4 inches in length. Hornworms may differ in appearance of their stripes and horn. Two distinct types most commonly found in Kansas include:
Tobacco Hornworm, characterized by seven white diagonal stripes and a red horn.
Tomato Hornworm, characterized by v-shaped markings and a blue or black horn.
Over a month’s time, the hornworm goes through about 4 or 5 different larval stages where it changes in color and size, according to Upham. It reaches its biggest size in the later stages, and soon after it will drop down in soil to pupate. Even so, they can be difficult to spot amongst foliage.
“Missing foliage is often the first clue that you have an interloper,” Upham said.
“Handpicking is the most effective way to protect a small home garden from these unwelcome guests.”
If hornworms prove too difficult to handpick, Upham suggests a variety of insecticides, including Bt, spinosad, and cyfluthrin. He urged gardeners to closely follow guidelines related to the harvest interval, or the number of days between when the spray is applied to fruit harvest.
“Keep watch for hornworms that may be infected with parasites,” Upham said. “Small braconid wasps will lay their parasitic eggs on the hornworm. The eggs will then morph into a cocoon and appear as small white projections protruding from the hornworm’s body.”
If hornworms are infected with parasites, Upham said they should be left alone. Upon emergence from the cocoons, the newborn wasps will kill the hornworms and seek others to parasitize – making them a natural control method.
Upham and his colleagues in K-State’s Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes. The newsletter is available to view online or can be delivered by email each week.
Interested persons can also send their garden- and yard-related questions to Upham at email@example.com, or contact your local K-State Research and Extension office.