Black Cutworm Scouting Advisory For Iowa

Predictions of cutting dates are based on recorded peak flights which took place near the end of March and approximately two weeks later in Iowa.

Published on: May 1, 2012

Unseasonably warm temperatures occurred earlier this year. So Iowa State University Extension entomologists asked the participants in a program that helps ISU monitor black cutworm moth flights coming into Iowa each spring--to place moth traps during the end of March. The first moth was recorded in Muscatine County on March 20. Peak flights have been reported by cooperators in many parts of Iowa this year.

ISU's predictions of cutting dates (the date when black cutworm larvae are likely to be damaging corn) are based on recorded peak flights which took place near the end of March and approximately two weeks later in Iowa, says ISU Extension entomologist Erin Hodgson. The map accompanying this article shows the predicted cutting dates for Iowa climate divisions. Where there are two dates, the top date is an estimate based on moth captures that occurred near the end of March; all other predicted cutting dates are based on mid-April captures of black cutworm moths.

Predictions of cutting dates (the date when black cutworm larvae are likely to be damaging corn) are based on recorded peak flights which took place near the end of March and approximately two weeks later in Iowa.
Predictions of cutting dates (the date when black cutworm larvae are likely to be damaging corn) are based on recorded peak flights which took place near the end of March and approximately two weeks later in Iowa.

Scouting for cutworm should begin several days before predicted cutting date

* What's the asterisk doing there? There were intermittent captures in this area throughout first part of April besides the two peak flights used to estimate cutting dates.

Map shows estimated cutworm cutting dates for Iowa climate divisions

Estimated cutting date is May 19 in the northwest district; May 21 in north central district; May 15 and 21 in the northeast district; May 17 in the west central district; May 10 and 18 in central; May 9 and 17 in east central; May 2 and 14 in southwest; May 5 and 16 in south central; and May 4 and 15 in the southeast.

Predictions are based on actual and historical degree day data accumulated from the dates of peak flights of black cutworm moths. Crop scouts looking at fields for signs of black cutworm damage are encouraged to start looking a few days before the estimated cutting dates. That's because development in some areas may be sped up (or slowed down) by localized weather.

Since the early peak flights occurred near the end of March, freezing temperatures have been observed in some Iowa areas. However, there is evidence to suggest that black cutworm eggs are able to survive for at least one night of sub-freezing temperatures. So it may be that these peak flights recorded in late March will produce cutting larvae; however, scouting a field is the only way to tell if an economic infestation is occurring in an emerged crop.

Cutworm moth trap data also shows that moths have been observed flying into the state at other times during April than the posted peak flights. Because of this, black cutworm larval activity may occur before and/or after the estimated cutting dates. 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—what you should look for. Black cutworms are light grey to black; with granular-appearing skin and four pairs of fleshy prolegs on the hind end. They can be confused with another insect that may be found in fields during spring, the dingy cutworm. However, there are some characteristics that can help to set species apart.

Certain fields of young corn may be at a higher risk for black cutworm damage than other fields. 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. Black cutworm may be more troublesome in fields where corn is planted late--as the plants are smaller and more vulnerable to damage by the cutworm. Also, if high numbers of larvae exist in a corn field, they may cause problems despite the use of Bt hybrids.

High numbers of rootworm larva can cause problems, even if you use Bt hybrids

Fields should be scouted for larvae weekly until the corn reaches V5 by examining 50 corn plants in five areas in each field. Look for plants with wilting, leaf discoloration and damage, or missing plants . Note areas with suspected damage (with a flag) and return later to assess further damage. Larvae can be found by carefully excavating the soil around a damaged plant.

* Thresholds. If the cutworm larvae found in the field are smaller than three-fourths inch, then a threshold of 2% to 3% wilted or cut plants indicates an insecticide application is warranted. If larvae are longer than three-fourths of an inch, the threshold increases to 5% cut plants. Remember to take into consideration the corn plant population in a particular field and adjust the threshold numbers accordingly when making your decision whether or not to spray the young corn with an insecticide.

However, with a strong corn price and input cost fluctuations, a dynamic threshold may be more useful. This Excel spreadsheet.xlsx has calculations built-in and be downloaded here to aid in management decisions regarding black cutworm.

What about spraying so-called preventative insecticides on the plants? "Preventative black cutworm insecticide treatments applied as a tank-mix with herbicides are of questionable worth," says ISU's Erin Hodgson. "Black cutworm is a sporadic pest and therefore, every field should be scouted to determine the presence of the insect prior to spraying insecticides."

* Biology. Adult black cutworm moths migrate into Iowa on the wind from southern states near the start of spring, then 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 seedlings.

Trap catches in Iowa now--traps are monitoring moth flights in 53 counties

In 2012, traps have been established in 53 Iowa counties, with several counties having multiple traps. The moths trapped in Iowa so far can be viewed by going to www.ncipmpipe.org and clicking on "Iowa Black Cutworm Monitoring 2012."

Hodgson asks that you "Please consider that adult moth trap captures don't necessarily mean there will be economically significant black cutworm infestations in a particular location. Field scouting is essential to determine if an economically damaging infestation exists. If you see any damage to corn plants from black cutworm larvae while you are 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."