By Adam Sisson, Corn and Soybean Initiative; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology
Several peak flights of black cutworm moths have been reported by cooperators in various parts of Iowa this year. 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 6-9. The map (Figure 1) shows the predicted cutting date for each region.
Trap data shows that moths have 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 larval activity in Iowa may occur for an extended length of time and 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.
Figure 1. Estimated black cutworm cutting dates for each Iowa climate division based on the first recorded peak flights of moths occurring in 2011. Scouting should begin several days before the predicted cutting date.
The estimated cutting dates for the climate divisions in Iowa are as follows: 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 divisions. 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 climate conditions.
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.
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. Black cutworm may be more troublesome in fields where corn is planted late as plants are smaller and more vulnerable to damage. Also, if high numbers of larvae exist in a corn field, they may cause problems despite the use of Bt hybrids.
Fields should be scouted for larvae weekly until the corn reaches V5 by examining 50 corn plants in five areas of each field. Look for plants with wilting, leaf discoloration and damage, and those that are missing. Note areas with suspected damage and return later to assess further damage. Larvae can be found by carefully excavating 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 (Figure 2, top). They can be confused with another insect that may be found in fields during spring, the dingy cutworm (Figure 2, bottom). However, there are some characteristics that can help to set species apart which are outlined further in a 2004 ICM News article Blacks and Dingys: confusing cutworms.
Figure 2. The black cutworm (top) can be confused with the dingy cutworm (bottom).
If larvae found in the field are smaller than ¾ inch, then a threshold of 2 to 3 percent wilted or cut plants indicates an insecticide application is warranted. If larvae are longer than ¾ inch, the threshold increases to 5 percent cut plants. Remember to take into consideration the 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 management decisions regarding black cutworm.
Adult moths migrate on the wind from southern states near the start of spring, then 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
In 2011, traps have been established in at least 64 Iowa counties, 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." Please 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 infestation exists.
If you see any damage from black cutworm larvae while scouting, please let us know by sending a message to firstname.lastname@example.org. This information could help us to refine our prediction efforts in coming years.
Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at email@example.com or by calling 515-294-5899. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at firstname.lastname@example.org or by phone 515-294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at email@example.com or phone 515-294-2847.