Watch For Black Cutworms On Corn

A black cutworm scouting advisory has been issued for cornfields in Iowa. Based on several peak flights of egg-laying black cutworm moths this spring, ISU Extension entomologists have made predictions as to when this insect pest will likely begin damaging corn plants.

Published on: May 9, 2011

Several peak flights of black cutworm moths have been reported recently in various parts of Iowa. The moths fly into Iowa from the south each spring and lay eggs on weed seedlings in fields. Those eggs hatch to produce the larvae that can chew on emerging corn seedlings as well as on young corn plants, and can cut the plants off and reduce stands.

Scouting your cornfields for signs of black cutworms and applying a rescue insecticide treatment may be necessary, says Erin Hodgson, an Iowa State University Extension entomologist. You need to keep an eye on fields to see if the pest is present. If cutworm numbers are high enough in your field and the threshold for economic damage to corn is reached, you may need to spray an insecticide as a rescue treatment.

Hodgson, along with Adam Sisson of ISU's Corn and Soybean Initiative, and Laura Jesse of ISU's Plant and Insect Diagnostic Clinic at Ames, offer the following guidelines and related information regarding black cutworm scouting and treatment thresholds for 2011.

Black cutworm scouting advisory for Iowa cornfields in 2011

"Our cutting date predictions (the date when black cutworm larvae are likely to be causing damage in corn) are based on the first peak flights which took place from April 6 to 9," notes Hodgson. The map accompanying this article shows the predicted cutting date for each region.

Black cutworm moth traps are placed in various areas of Iowa to monitor the moth flights. The trap data shows that moths continued to come into various parts of the state at peak flight levels during the middle and later parts of April. Because of this, black cutworm larvae (worm stage of this insect pest) activity in Iowa may occur for an extended length of time. Thus, corn growers are urged to scout fields on a regular basis as scouting is the only way to tell if a field is infested by black cutworm larvae.

Scouting should begin several days before the predicted cutting date.

The estimated cutting dates this year for the various crop reporting districts in Iowa are: May 15 in the southwest; May 17 in the south central and southeast; May 19 in the west central and east central; May 20 in the central; and May 22 in all three northern districts. These predictions are based on actual and historical degree day data accumulated from the dates of the first peak flights.

Scouts are encouraged to start looking a few days before the estimated cutting dates as development in some areas may be sped up (or slowed down) by localized weather conditions.

What if you've applied an insecticide mixed with a herbicide when you made your weed control application? Should you still scout for cutworms? "The answer is yes," says Hodgson. "Actually, preventative black cutworm insecticide treatments applied as a tank-mix with herbicides are of questionable worth. Black cutworm is a sporadic pest and therefore every field should be scouted to determine the presence of the insect prior to spraying insecticides. The scouting and rescue treatment approach is the best strategy."

Guidelines to use when scouting cornfields for black cutworm

Certain fields may be at a higher risk for black cutworm damage than others. These fields include those that are poorly drained and low lying; those next to areas of natural vegetation; and those that are weedy or have reduced tillage. Another consideration is black cutworm may be more troublesome in fields where corn is planted late, as the corn plants are smaller and more vulnerable to damage. Also, if high numbers of black cutworm larvae exist in a corn field, they may cause problems despite the use of Bt corn hybrids.

Fields should be scouted for larvae weekly until the corn reaches the V5 growth stage. Scout by examining 50 corn plants in five areas of each field. Look for plants that show wilting, leaf discoloration and signs of leaf damage and cutting damage. Look to see if plants are missing. Note any areas that have suspected damage and return later to the field to assess further damage. Larvae can be found by carefully digging the soil around a damaged plant.

Black cutworms are light grey to black; with granular-appearing skin and four pairs of fleshy prolegs on the hind end (see figure 2, top). They can be confused with another insect that may be found in fields during spring, the dingy cutworm (see figure 2, bottom). However, there are some characteristics that can help to set cutworm species apart, and those characteristics are described further in a 2004 ICM News article Blacks and Dingys: confusing cutworms.

What's the economic threshold for cutworm treatment decisions?

If larvae found in the field are smaller than ¾ inch long, then a threshold of 2% to 3% wilted or cut plants indicates an insecticide application is warranted. If larvae are longer than ¾ inch, the threshold increases to 5% cut plants. Remember to take into consideration the corn plant population in a particular field and adjust threshold numbers accordingly. However, with corn price and input fluctuations, a dynamic threshold may be more useful. An Excel spreadsheet with the calculations built in can be downloaded and used to aid your management decisions regarding black cutworm.

You need to understand the biology of this pest. Adult moths migrate into Iowa, riding the wind from southern states near the start of spring. They mate and lay eggs. Around 1,300 eggs are laid by a single mated adult female. Eggs are laid in crop stubble, low spots in the field and in weedy areas. Younger larvae injure corn plants by feeding on leaf tissue and older larvae can cut the young corn seedlings completely off.

Moth trap catches in Iowa provide valuable cutworm information

In 2011, traps have been established in at least 64 Iowa counties across the state of Iowa, with several counties having multiple traps. Iowa's trap catches can be viewed by going to www.ncipmpipe.org and clicking on "Black Cutworm Trapping 2011." You need to consider that adult moth trap captures do not necessarily mean there will be economically significant black cutworm infestations in a particular location. Field scouting is essential to determine if an economically damaging cutworm larvae infestation exists.

"If you see any damage from black cutworm larvae while scouting, please let us know by sending a message to bcutworm@iastate.edu. This information could help us to refine our prediction efforts in coming years," says Hodgson.