Iowa farmers need to watch for black cutworm on corn this spring. Iowa State University Extension specialists have issued an advisory, saying cutting could possibly begin as early as May 10 or May 13 in areas of northwest Iowa although May 20 is the predicted date. ISU has published a map showing predicted cutting dates in various areas of the state, ranging from May 20 in the northern third of Iowa, May 22 in central Iowa and May 18 in the southern third. The map is included in this article.
This insect pest can begin feeding on corn leaves and cutting off young corn plants as soon as corn starts to emerge. Scouting needs to begin early and fields should continue to be scouted once a week until corn is in the V5 growth stage (8 to 12 inches). The adult black cutworm moths fly into Iowa each spring and lay eggs in fields and the eggs hatch into cutworm larvae which damage young corn plants. If infestations reach the economic threshold in your field, a rescue application of insecticide may be needed.
Scouting is essential to determine if a rescue treatment is needed
Iowa State University Extension integrated pest management specialist Adam Sisson and ISU Extension entomologist Erin Hodgson provide the following guidelines.
The black cutworm (BCW) is a migratory pest that cuts and feeds on early vegetative-stage corn. Black cutworm moths arrive in Iowa with spring storms each year. These moths (the adult stage of black cutworm) lay eggs in and around fields. The eggs hatch and the emerging BCW larvae feed on corn leaves cut off the seedling corn. The sporadic nature of this pest makes scouting essential to determine if management is needed. Scouting for BCW larvae helps to determine if an insecticide application will be cost effective.
When to scout for BCW caterpillars is based on the “peak flight” of moths and accumulating degree days after the peak flight. Degree days are a measure of temperature used to gauge the developmental progress of the insect. A peak flight for BCW is defined as capturing eight or more moths over two nights in a wing style trap baited with a pheromone lure.
Predicted dates when cutting of corn plants may likely begin
To find out when moths arrive in Iowa, cooperators around the state monitor pheromone traps and report moth captures. Cooperators started checking traps in the beginning of April, and the first BCW moths captured were part of a peak flight in Woodbury County on March 28. Moth captures peaked in several parts of the state in mid-April, with several peak flights recorded. The peak flights recorded during this time period were in line with the predicted corn cutting dates in surrounding states.
The map accompanying this article shows predicted BCW cutting dates for the nine Iowa crop reporting districts, based on actual and historical degree day data and peak flights during late March and April. We continued to see a few peak flights after mid-April this spring, indicating several populations of BCW moths moving to Iowa. However, adult moth trap captures do not necessarily mean there will be economically significant BCW infestations in a particular location. Field scouting is essential to determine if an economically damaging infestation exists.
PREDICTED CUTTING DATES: Estimated black cutworm cutting dates for each Iowa crop reporting district are based on peak flights of moths occurring in 2016. The two dates in red are cutting estimates based on a late-March peak flight observed in Woodbury County in northwest Iowa. Scouting should start at the earlier date in these two areas.
How to scout cornfields for black cutworm
Poorly drained, low lying or weedy fields, as well as those areas next to natural vegetation such as a wooded area or areas with reduced tillage, may have higher risk of BCW injury. Cornfields with poorly terminated cover crops may also be attractive to the egg-laying female moths. Also, keep in mind that late-planted corn can be smaller and more vulnerable to larval feeding.
What about Bt corn hybrids? All cornfields need to be scouted for black cutworm, even those fields planted to Bt hybrids. Some Bt hybrids provide suppression of BCW (such as hybrids with these Bt genetic traits: Vip3A, Cry1A.105, Cry2Ab2 and Cry1F proteins), but larvae can still cut young corn plants even if the corn has these traits.
Early scouting is important and keep scouting weekly
Scouts are encouraged to start looking for any activity during early season stand assessment, or at least several days before the estimated cutting dates. Early scouting is important because local larval development may be different due to weather variation within a climate division.
Fields should be scouted for larvae weekly until corn reaches V5 growth stage (8- to 12- inches tall). Examine 50 corn plants in five areas in each field for wilting, leaf discoloration and damage, or those that are missing or cut (see photo A). Flag the areas with suspected feeding and return later to assess further injury. Larvae can be found by carefully excavating the soil around a damaged plant.
Black cutworm larval injury to corn plants usually begins above the soil surface. Leaf feeding (left) may be observed. As larvae mature, they can severely damage or kill the young corn plant (right) by chewing on and clipping the stalks. Photos copyright Marlin Rice
How to correctly identify black cutworm from other cutworms
Keep in mind there are other types of cutworms, which may chew on corn leaves but don’t cut off corn plants. Young corn plants can recover from leaf damage as they grow new leaves. The black cutworm is the species you need to be concerned about—it not only chews on the leaves but it also can clip off the plants. You may need to apply a rescue insecticide treatment to control the BCW larvae if they are doing damage.
BCW larvae have grainy, light grey to black skin and four pairs of fleshy prolegs on the end of the abdomen (see photo B). There are pairs of dark tubercles, or bumps, along the side of the body. The pair of tubercles nearest the head is approximately ? to ½ the size of the pair closest to the abdomen (see photo C). BCW larvae can be confused with other cutworms and armyworms. Certain characteristics can be used to tell the species apart and are summarized in an article you can read by clicking on: article on cutworm identification.
Black cutworm larvae have grainy and light grey to black skin. Photo by Adam Sisson
Black cutworms can be distinguished from other cutworm or armyworm larvae by the dark tubercles on the middle of the back. On each segment, the tubercle closest to the head is about 1/3 the size of the tubercle closest to the rear, as shown above. Photo by Adam Sisson.
Use economic thresholds to decide when to apply an insecticide
If you see cutworms present and threatening to do damage to corn plants in a field, when should you apply an insecticide? When will it pay to treat? Use economic thresholds to make that decision.
Common thresholds for seedling corn and the V2, V3 and V4 stage corn plants, are two, three, five and seven plants cut out of 100 plants, respectively. A dynamic threshold for BCW may be useful with corn price and input fluctuations. An Excel spreadsheet with calculations built in can be downloaded by clicking here. The spreadsheet can be used to help with black cutworm management decisions.
What about using a preventive treatment of insecticide?
So, what we have discussed in this article is making a rescue treatment of insecticide to control cutworm on corn. What about using a preventive insecticide product and applying it before you see any cutworms in the field or see any damage? Some farmers apply an insecticide mixed with their herbicide application. Is that a good idea?
ISU entomologists say using preventive BCW insecticide treatments applied as a tank-mix with herbicides is a questionable practice. That’s because BCW is a sporadic pest and every field should be scouted to determine insect presence before spraying insecticides, advises ISU Extension entomologist Erin Hodgson.
REPORT BLACKCUTWORM SITINGS TO ISU: If you see any cornfields in Iowa with BCW larvae in them while scouting, please let the ISU crop pest management team know by sending a message to: [email protected]. This information could help the ISU specialists to refine future black cutworm cutting predictions.