It's time to scout cornfields for black cutworms. Rather than wait until this insect pest begins cutting off plants, farmers and crop scouts are encouraged to look for early signs of leaf feeding as a potential indicator of cutting that may ensue. An insecticide application as a rescue treatment may be needed. Also, don't assume that all Bt corn hybrids offer the same level of cutworm protection. Plants in the 1 to 4 leaf stage are most susceptible to cutting.
This reminder comes from Adam Sisson, an integrated pest management specialist at Iowa State University. Sisson, along with ISU Extension entomologist Erin Hodgson, and Laura Jesse, who heads up ISU's Plant and Insect Diagnostic Clinic, offer the following information and scouting guidelines.
The black cutworm is a migratory pest. The adult moths fly into Iowa from southern states each spring and lay eggs in fields. The eggs hatch and the larvae (worm stage) feed on leaf tissue and can cut off young corn seedlings. "Scouting for black cutworm larvae helps to determine if applying an insecticide as a rescue treatment will be cost effective," says Sisson. "When to scout for these black cutworm caterpillars is based on the peak flight of black cutworm moths and on the number of accumulating degree days that occur after the peak flight. A peak flight is when eight or more moths are caught over two nights in a wing style trap."
Few peak flights have been recorded, but you still need to scout
As adult moths migrate into Iowa and other Corn Belt states from the south, it can be difficult to determine exactly when the moths arrive. To find out when these moths arrive in Iowa, a volunteer network of farmers, agronomists, Extension employees and others monitor a network of black cutworm moth traps. To prepare for the flights in 2015, ISU Extension early this spring placed moth traps in about two-thirds of Iowa's 99 counties, with many counties having multiple traps.
This spring the volunteers were asked to start checking traps at the beginning of April, "and the first black cutworm moth was recorded in Woodbury County in northwest Iowa on April 2," says Sisson. "However, as of May 15 many trappers were reporting low or no moths; about 40 of the 106 active traps have reported two or less moths and there have been few recorded peak flights. As a point of comparison, total moths captured in cooperator traps in 2015 is less than half of what was reported at approximately the same time in 2011."
Surrounding states may not be capturing high amounts of moths either this spring. Over the past four years, Iowa has generally started degree day accumulation from peak flights in April, he notes.
Some crop scouts are seeing signs of cutworm feeding on leaves
So what does this highly variable black cutworm moth trap capture mean for predicting cutting dates in corn? "So far this spring, we have limited moth activity to generate scouting recommendations for many parts of the state," says Sisson. "However, some experienced crop consultants have notified us of caterpillar feeding in vegetative corn this past week. For example, we just received a report of dingy and sandhill cutworm feeding in a field around southeast Iowa. It may be that peak flights of black cutworm moths occurred earlier and before traps were out this year; late March peak flights were observed in Iowa in 2012."
As you are out in fields assessing stands, be on the lookout now for early season insect injury in corn: black cutworm or otherwise. The map (Figure 1) shows predicted black cutworm cutting dates for the nine Iowa areas, based on actual and historical degree day data and peak moth flights during early May.
Figure 1. Estimated black cutworm cutting dates for each Iowa climate division based on peak flights of moths occurring early May 2015.
"Remember, trap captures of moths have been highly variable this year and we encourage you to scout all cornfields for potential pest insect issues," says Sisson. "Wright County had a peak flight recorded about two weeks earlier than the other peak flights, thus earlier cutting may be possible in this area."
Follow these scouting guidelines for black cutworm
Poorly drained, low lying, or weedy fields, as well as those next to natural vegetation or with reduced tillage, may have higher risk of black cutworm damage. Late-planted corn can be smaller and more vulnerable to larval feeding. Some Bt corn hybrids provide suppression of black cutworm, but larvae can still cut young plants.
Scouts are encouraged to start looking for any activity during the early season when stand assessments are taking place, or at least several days before the estimated cutting dates (see map). This is because local larvae development may be different due to weather variation within an area. Fields should be scouted for larvae weekly until corn reaches V5 stage of growth. Examine 50 corn plants in five areas in each field for wilting, leaf discoloration and damage, or those that are missing or cut (Figure 2).
Flag the areas of the field where you see suspected feeding so you can return later to that same place and assess if there is further injury. Cutworm larvae can be found by carefully excavating the soil around a damaged plant.
Figure 2. Black cutworm larval damage usually begins above the soil surface. Leaf feeding (left) may be observed. As larvae mature, they can do more serious damage to plants (right). Photos by Marlin Rice.
How to correctly identify black cutworm larvae
Black cutworm larvae have grainy, light grey to black skin and four pairs of fleshy "prolegs" on the end of the abdomen (Figure 3). There are pairs of dark tubercles, or bumps, along the side of the body. The pair of tubercles nearest the head is approximately one-third to one-half the size of the pair closest to the abdomen (Figure 4). Black cutworm larvae can be confused with other cutworms and armyworms. Certain characteristics can be used to tell species apart and are summarized in this article on cutworm identification at ipm.iastate.edu.
Figure 3. Black cutworm larvae have grainy and light grey to black skin. Photo by Adam Sisson.
Figure 4. Black cutworms can be distinguished from other larvae by the dark tubercles on the middle of the back. Photo by Adam Sisson.
Economic thresholds to use for deciding when to treat
Common thresholds to use for deciding when it will pay to apply an insecticide as a rescue treatment for seedling corn, and for the V2, V3 and V4 stage corn plants, are when you see two, three, five and seven plants cut out of 100, respectively. A dynamic threshold for black cutworm may be useful with corn price and input fluctuations, says Sisson. An Excel spreadsheet with calculations built in can be downloaded at extension.iastate.edu and can be used to help with black cutworm management decisions.
What about using preventive insecticide applications, instead of waiting and scouting to see if cutworms are present in your field? And then using a rescue insecticide only if a treatment is needed? "Preventive black cutworm insecticide treatments applied as a tank-mix with herbicides are a questionable practice," says ISU Extension entomologist Erin Hodgson. "Black cutworm is a sporadic pest and every field should be scouted to determine the presence of insects before spraying insecticides.
"If you see any fields with BCW larvae while scouting, please let us know by sending a message to [email protected]. This information could help us to refine future predictions," says Hodgson.
Erin Hodgson is an assistant professor of entomology with Extension and research responsibilities; contact at [email protected] or phone 515-294-2847. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at [email protected] or by phone 515-294-0581. Adam Sisson is an extension specialist for the Integrated Pest Management. He can be contacted by email at [email protected] or by calling 515-294-5899.