In certain seasons, cutworm can be a serious insect pest in field corn and certain vegetable crops. A single black cutworm can consume up to 4-5 plants in the course of its larval cycle. Multiply this across a section of a given field with heavy populations, and you might find entire sections of a field ‘wiped out’ by a serious cutworm infestation.
There are several types of cutworm. The general term cutworm applies mainly to larvae of various species in the Noctuidae, a large family of moths; however many Noctuid species are not cutworms, and some ‘cutworms’, so named because of their habit in the larval stage, are not Noctuids.
Cutworms are notorious agricultural and garden pests. They are voracious leaf, bud, and stem feeders and can destroy entire plants. They get their name from their habit of "cutting" off a seedling at or just below ground level by chewing through the stem, or by virtue of wrapping their body around the stem and in the process of working back and forth, cut the stem off at or just below the ground surface. Some species of cutworm can feed on the roots of plants, as well.
In Agronomic crops, especially corn, the most prevalent and damaging forms of cutworm are Black Cutworm and Variegated Cutworm; and between these, the Black Cutworm is most notorious in field corn. Cutworm adults (moths) are migratory, moving north from warmer regions to arrive in corn and other fields where they will lay their eggs from March through May and into June, depending on how late your spring season arrives.
They will lay eggs in annual grass weeds and winter annual weeds which have actively live vegetative targets for the female moths. This is why some of the best targets are minimum and no-till fields; or soybean fields with winter annual weeds are present prior to growing corn in the rotation. The damage occurs from the hatching larvae after the moths lay their eggs.
There are other field and environmental conditions which increase the risk for cutworm damage. Seasonal shifts in predominant winds, especially in a cold and wet spring; come together to create favorable conditions for cutworm to become an economic pest in a given area. Many farmers and advisors will literally characterize a given season as a ‘cutworm year’.
The following risk factors affect the probability of a black cutworm problem developing one way or another:
- Fall plowing reduces the likelihood of cutworm problems.
- Plowing late in the spring, especially if a sod or with a heavy winter annual or perennial weed presence increases cutworm risk due to the egg laying sites they afford.
- Corn following beans increases the risk when overwintering weed populations are present at egg laying time.
- Late planted corn increases their likelihood for economic damage.
- Minimum tillage or no-till where overwintering weed kill is delayed past egg laying time.
- Low lying, wetter soils, where weeds were prevalent prior to tillage or knockdown; and corn is stressed are also good targets for cutworm.
- Wet years with cold springs and delayed planting are at risk to create a ‘cutworm years’.
At-planting, preventative use of insecticides for cutworm on its own are not usually a good economic choice for a number of reasons. If using a soil insecticide to curb corn rootworm, there may be a minimal amount of control for cutworm; but in heavy infestation ‘years’, the crop still might need a cutworm treatment when scouted properly.
Another factor contributing to challenges with cutworm infestations is timing of treatment. Once the cutworm larvae have advanced into later instars of development three conditions exist. First, insecticides applied are not very effective on large worms, above instar 6-7. Second, once well into these instars, the cutworm actually slow down prior to pupation. Finally, cutworm in these later instars will ‘pupate out’ into their resting phase within 7-10 days. Remember, however, that even though they may slow down, they are also more hungry so if the background population is well above economic threshold, replanting may be your only option. Cutworms must be detected soon after hatch for in-season control to be effective.
Scouting and IPM are the Best Approach to Cutworm
Understanding the life cycle of cutworm, practicing good weed control, and scouting at (or before) egg hatch are the best approaches for cutworm management. Many states now utilize pheromone and sticky traps to predict activity and populations of adult moths. Referring to and tracking growing degree days, which can predict general dates of egg hatch, can also be useful. You’ll know when to look for early feeding evidence in time to implement chemical controls.
When scouting for cutworm, remember they are actually nocturnal in nature, so they may not be immediately evident during the heat of the day. They will also go deeper into the soil if weather conditions are dry. Look for leaf feeding and ‘shothole’ damage. This type of damage occurs when the worm works through the leaf before it unfurls. Upon opening up, a sequence of holes equal in size are evident across the leaf, perpendicular to the veins. When you find what you think is evidence, spread a sample of soil onto a white surface and look closely for the larvae.
Economic thresholds are driven by three factors, considered simultaneously:
- Percentage of cut or damaged plants.
- Stage of growth of the corn.
- Average stage of development of the cutworm larvae (instar)
Consult your state’s recommendations via your County Extension Office or Certified Crop Advisor for thresholds and controls. In addition, sign up for a reliable source of timely scouting data and alerts, especially from regions to your south.