Black Cutworms

Description of black cutworms

The black cutworm is up to 1 to 1 1/2 inches long, pale gray to black in color and a general greasy appearance.  It has no distinct stripes or markings except for small black tubercles that surround the base of each of 4 tiny hairs on each body segment.  The skin surface is roughened with a granular texture.

Black cutworm (top) and dingy cutworm (bottom).
Black cutworm (top) and dingy cutworm
(bottom).

Life cycle of black cutworms

A few black cutworms spend the winter in Iowa as a moth but most of our cutworms develop from moths that migrate into the state on storm fronts that begin in the southern U.S.  These moths may be active from late March, but moth flight peaks in early May.  The moths are attracted to emerging green vegetation, especially in low, wet areas, where they lay their eggs.  These eggs hatch into tiny cutworms that feed on grasses and weeds at night and hide in the soil during the daytime.

Tillage prior to tomato transplanting eliminates the cutworms food source so that damage to young transplants may occur suddenly and severely.  The black cutworm may have 2 or 3 generations per summer.  Damage from the second and third generation is generally not serious because plants are well established and too woody for the larvae to cut.

On the black cutworm (top), the inside pair of tubercles is about 1/3 to 1/2 the size of the outside pair. On the dingy cutworm (bottom) these tubercles are about the same diameter..
On the black cutworm (top), the inside pair of
tubercles is about 1/3 to 1/2 the size of the
outside pair. On the dingy cutworm (bottom)
these tubercles are about the same diameter..

Damage caused by black cutworms

Cutworms feed on the stems at the soil surface.  They do not consume an entire plant but rather take a few bites from one plant before moving to the next plant or another row.

Damage is often concentrated in low, damp areas and in late planted areas where grass had been a weed problem prior to tillage.  Cutworms can be extremely damaging where transplants are planted through black plastic.  Heat that accumulates under the plastic may attract the caterpillars, and cutworms under the plastic are protected and difficult to control.

Management of black cutworms

Special management operations for black cutworms are generally not necessary in Iowa.  Preplant, soil incorporation or transplant water treatment with insecticide is possible but seldom warranted.  Gardens with a history of cutworm damage should be monitored, and all plants should be watched during the transplant establishment period for the occasional cutworm outbreak.

Low levels of damage can generally be tolerated and damaged plants replaced.  Insecticide rescue treatment using either sprays or granules is not usually practical.  Physical barriers such as tin cans or paper/Styrofoam cups with bottoms removed are effective.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.  

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