Skip Navigation

5/12/2008 - 5/18/2008

Black Cutworm Thresholds: What has Changed with the Price of Corn and New Control Methods?

By Jon Tollefson and Marlin Rice, Department of Entomology


Black cutworm traps across Iowa have been capturing migrating moths for several weeks. Pheromone traps are valuable tools in integrated pest management, but they have limitations. The traps only tell you that the insects are in your area; they do not report in which fields the insects have laid their eggs.


Where moths lay their eggs will be influenced by environmental and agronomic factors such as weed cover, tillage, the date the crop is planted and previous crop residue, etc. The bottom line is that once adult black cutworms are forecast to be in your area, you must scout your fields to determine if there are larvae present and if they are “cutting” your corn plants. If there is sufficient cutting then controls should be applied.


To scout for black cutworm injury, walk along rows of corn at several locations in a field looking for feeding on corn leaves and missing plants. Small larvae will feed on the edges of leaves before they get large enough to cut corn plants.


Cutting may not happen for several days after the first leaf feeding is observed. If leaf feeding is observed, begin looking for missing (cut off) plants. Sort through soil near the surface in the area of missing plants while looking for cutworm larvae. If larvae can be found, determine whether they are black cutworms or dingy cutworms Dingy cutworms rarely, if ever, will cut corn plants. Record the number of missing plants and determine the percentage of the plants cut.


There are some new variables in the control of black cutworms. These include genetically engineered corn, higher seed prices and increased market prices of corn. First, with genetically engineered corn, remember that YieldGard® is not effective against the black cutworm; only Herculex® hybrids give some protection against black cutworm.


Second, with the high cost of seed and expected higher returns from corn the economic threshold could be lowered. Larvae ¾-inch long  are in the 4th stage and will cut several more plants before they finish feeding. If the worms are longer than one inch, they are nearly finished feeding and treatments don’t need to be applied until 5 percent of the plants have been cut. A lower threshold would be 1 percent stand loss, which would be within normal stand variability and very hard to detect. The previous economic thresholds was if cutworms were less than ¾-inch, apply an insecticide when 2 to 3 percent of the plants are cut.


The higher threshold of 5 percent for larger worms could be lowered. If we were to assume that a grower were planting 32,000 plants per acre with the expectation of producing 200 bushels of corn, then 160 plants would produce 1 bushel of corn. If corn is selling for $6.00 per bushel then each plant produces $0.0375 worth of corn (32,000 plants/acre divided by 200 bu/acre = 0.00625 bu of corn per plant; multiplied by $6.00/bu = $0.0375/plant). 


If control costs are assumed to be $15.00/acre then 400 plants lost would equal the cost of control ($15.00/$0.0375 = 400 plants). The 400 plants out of 32,000/acre = 1.25 percent of an acre. The result is that losing 1 to 2 percent of the plants at $5.00-6.00 corn would cover the costs of control. The higher threshold of 5 percent for larger worms could be lowered to 1 to 2 percent plant loss. This new economic threshold is based on calculations using the previous threshold. The actual economic returns will depend on how much cutting the larvae continue to do and the percentage of the plants that fail to re-grow after being cut.


Jon Tollefson is a professor of entomology with responsibilities in field crop pest management. Marlin E. Rice is a professor of entomology with extension and research responsibilities.

Black Cutworm Scouting Advisory—2008

By Marlin E. Rice, Rich Pope, and Jon Tollefson, Department of Entomology

A significant flight of black cutworm adults (moths) arrived in Iowa the weekend of April 18, based on pheromone trap capture data across the state.

This insect is an occasional pest of seedling corn that sometimes causes significant damage in a few fields. This year trap cooperators monitored 66 traps in 47 counties. Based upon their reported data, we anticipate that first cutting of seedling corn should occur May 17-18 across southern Iowa, May 18-20 across central Iowa, and May 22-23 across northern Iowa, as indicated on the map below.

2008 Black Cutworm Forecast Map

These dates represent the earliest possible cutting dates, based on actual accumulated degree days through May 12 and average temperatures for future days. However, it is possible that the cutting period may stretch over two to three weeks because moths lay eggs over an extended period, and the emergence of later planted corn would still be susceptible to cutting.

In fact, one noted exception to projected cutting dates is that while there was a statewide flight the weekend of April 18, there was an additional distinct flight of moths in southern and central Iowa about ten days later. Therefore, cutting from larvae generated from the second flight may occur a week or so later in the southern half of Iowa. .

As a reminder, pheromone traps do not predict the amount of cutting in a field nor the counties where cutting will occur. Each year, one of our concerns is that radio advertisements may predict a cutworm "outbreak" in your county just because moths were trapped there in April. Neither the traps nor our predictions based on the trap catches can predict the amount of cutworm injury in a field. Therefore, scout and be diligent.

Scouting of seedling corn near the first cutting date is the only reliable method to determine whether a problem exists. Then, insecticides can be applied if needed. However, corn hybrids with Herculex® I or Herculex® Xtra should provide significant protection from black cutworm, but fields should still be monitored because some minor cutworm damage can still occur.

Scout fields several days before the first cutting date projection. By doing so, you may be able to find "hot spots" based upon leaf feeding, thereby getting a head start on management decisions. Stop scouting when the field is sprayed or when plants have five fully developed leaves (stage V5). Cutworms have difficulty in cutting plants in the V5 stage because of the larger stalk diameter, although occasionally they chew into the side of the stalk and kill a larger plant.

Look for cutworm injury on corn leaves. Dingy cutworms also feed on young corn leaves but rarely cut corn. If leaf feeding is detected, try to find the cutworms to determine whether they are black or dingy. Very large cutworms found during the earliest black cutworm cutting dates are often dingy cutworms because dingy cutworms overwinter in Iowa as partially grown larvae. Also, fields with winter annual weeds are more likely to have cutworms than clean fields, and soybean stubble is more attractive to the moths than corn stubble.

If you find leaf feeding and only black cutworms, then mark off 100 plants in a row with stakes or flags, and scout these same plants for cutting over a period of several days at several locations across the field. Then you can monitor the cutworm activity and determine whether they are cutting plants and the percent cut plants.

In light of sizable changes in the harvest value of corn grain, economic thresholds for black cutworm have been reexamined, and a full discussion of these thresholds is presented in a companion article: Black Cutworm Thresholds: What has Changed with the Price of Corn and New Control Methods?

Several insecticides are labeled for black cutworms in corn. Several years ago, research showed that after application, rotary hoeing in dry soils increases the effectiveness of Lorsban®, but that the pyrethroids (such as Ambush®, Pounce®, or Warrior®) should not be incorporated into the soil as this decreases performance.

Insecticides and rates labeled for black cutworms in corn


6.4-12.8 oz/acre


1.92-3.2 oz/acre

Asana XL

5.8-9.6 oz/acre

Baythroid 2

0.8-1.6 oz/acre

Discipline 2EC

2.1-6.4 oz/acre

Capture 2EC

2.1-6.4 oz/acre


2.6-6.1 oz/acre

Mustang Max

1.28-2.8 oz/acre

Lorsban 4E

1-2 pt/acre

Nufos 4E

1-2 pt/acre


4 pt/acre

Pounce 3.2EC

4-8 oz/acre

Sevin XLR Plus

2 qt/acre


Marlin E. Rice is a professor of entomology with extension and research responsibilities. Rich Pope is an extension program specialist working in the Corn and Soybean Initiative. Jon Tollefson is a professor of entomology with responsibilities in field crop pest management.

Minimize Soybean Yield Loss from Late Planting

by Palle Pedersen, Department of Agronomy


Soybean responds significantly to early planting. Despite cold soil temperatures and slow plant growth during the seedling phase, there is a yield benefit from early planting, which seems to be influenced by field yield potential.


The yield benefit is a result of increased seasonal canopy photosynthesis, greater number of main-stem nodes, potential for earlier flowering, increased crop growth rate during pod set and greater seed filling rate. Based on 24 experiments conducted across Iowa since 2003 with the support from the checkoff and the Iowa Soybean Association, there is a 79 percent probability of achieving  the highest yield by planting the last week of April (southern two thirds of Iowa) or the first week of May (northern one third of Iowa) compared with approximately May 20.


This year, however, early planting hasn’t been easy and today less than 5 percent of the soybean acres have been planted. The weather conditions since April 1 haven’t been that favorable for planting either corn or soybean in the state of Iowa and many still have more than half of their corn acres to plant. That means that we all start losing part of our soybean yield potential day by day.


We are all getting close to the “desperate mode.” Despite that, we should still try to stay calm since we can still get good soybean yields if we have perfect weather conditions in July and August. However, no one can predict that now.


Soil conditions at and following planting are the primary drivers for planting and stand establishment. Numerous factors influence the decision on when to plant soybeans. “Mudding-in” soybean just to plant early – causing soil compaction and poor seed placement – outweighs any benefit of early planting. Soybean cannot better tolerate wet seedbed conditions than corn. I would say it is the opposite. In addition, seed quality this year is not perfect which can lead to higher plant mortality than normal because of the cool wet seedbed.


Warmer and drier soils will give us a faster emergence and since soybean seed doesn’t stay viable as long as corn seed under cooler and wetter conditions soybean seeds are weaker and more susceptible right now to for example soilborne pathogens. One of the soilborne pathogens that prefer cool wet conditions is Pythium and the only way to protect your crops from this disease is using a fungicide seed treatment.


Every bushel will count this year and we want to give the plant as good as a start as possible. To minimize yield loss from delayed planting it is recommended to plant your high productive fields first since it is often here where delayed planting is most costly (Figure 1.)


Soybean Planting Date


Figure 1. A model of soybean planting date response in Iowa based on soybean yield potential.High yielding environment is above state yield average and low yielding environment is below state yield average.


These fields are often the best drained fields as well so for many this year it will be obvious to plant here first. Other things like timely weed management and planting into weed free field and daily scouting for bean leaf beetles is also critical. More information about soybean management can be found at 


Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.

A Bit Cool, a Bit Wet, but Planting Progresses

By Rich Pope, Department of Entomology


The second week of May continued the 2008 pattern of cool and wet spring weather across Iowa. in spite of a band of locally heavy rain the evening of May 7 that brought over 6 inches of rain to Alden and neighboring areas, corn planting progressed slowly but steadily.


As of May 12, the USDA estimate for Iowa was that 46 percent of intended corn acreage was planted. Although that is only about one half of the five year average progress, it means more than 25 percent was planted in the week, with favorable (read that as dry—for now) weather forecast for the next 10 days.  


A few soybean fields were planted, with posted statewide estimates of 4 percent of soy acres planted.


5-8-2008 Iowa Degree Days

Rich Pope is an extension program specialist working in the Corn and Soybean Initiative.

Consider Effects on Seedbed Before Tilling Wet Soil

By Mark Hanna, Department of Agricultural and BioSystems Engineering, and Mahdi Al-Kaisi, Department of Agronomy


The current weather conditions of frequent rain caused saturated conditions in most Iowa soils. Tilling soil for drying the soil surface or weed control at this time may have significant negative impacts on creating proper seedbed conditions and increase soil compaction.


When early weeds have emerged but surface soil is still too wet for no-till planting, growers often consider doing a shallow field cultivation in an attempt to kill winter annual weeds. Non no-tillers may consider cultivation simply to dry the soil. Before spending time and fuel for cultivation, analyze the potential effects on weeds and soil.


When tilled wet, many soils slab into large blocks, keeping the root system of growing weeds intact and allowing them to continue growth. Larger clods on the soil surface may require a secondary tillage pass before the soil is acceptable for planter operation. 


Pre-plant tillage passes on wet soil add random wheel tracks and produce a greater likelihood that some seedling roots will need to penetrate compacted soil. Less desirable soil conditions for early seedling development caused by tillage may negate the perceived advantage of earlier planting. Further discussion on compaction when working soils wet can be found in our earlier article “How soon should I start field operations? 


Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science

Beware of a Dangerous Invasive Weed -- Updated

By Mike Owen, Department of Agronomy

 May 21, 2008


Upon further investigation about the location of “local” infestations of the highly invasive giant hogweed, it was determined that the Wisconsin location is in Iron County which is located at the northern boundary between Wisconsin and the Upper Peninsula of Michigan (see colored area on the map). Gogebic County, Michigan also has or had infestations of giant hogweed. The U.S. Forest Service is working diligently to eradicate these populations.


Giant Hogweed Infestation Map


The good news is that identified locations of giant hogweed are not that close to Iowa.The bad news is that giant hogweed is aggressive and adapted to Iowa habitats. Thus it is still important to be aware of the possibility of giant hogweed invading Iowa and contact the ISU Extension or the Iowa Department of Natural Resources if you suspect that you have this nasty plant.


By Mike Owen, Department of Agronomy

May 12, 2008


A recent news release from the Weed Science Society of America warns of a particularly dangerous invasive weed; giant hogweed.


The weed is related to species that are commonly found in Iowa, however giant hogweed is highly invasive and is capable of causing considerable damage to anyone who comes in contact with the plant. While giant hogweed has yet to be discovered in Iowa, populations have been found in Wisconsin.


Thus it is possible that giant hogweed populations may exist in northeast Iowa. There are a number of links that can be used to find more detailed information about this weed. Also, pictures of  four common weedy relatives of giant hogweed found in Iowa are included for comparison purposes. 




Giant Hogweed



Wild Parsnip


Wild Parsnip


Water Hemlock


Water Hemlock


Queen's Lace


Queen Anne's Lace


Cow Parsnip


Cow Parsnip


Mike Owen is a professor of agronomy with research and extension responsibilities in weed management and herbicide use.

Delayed PRE Herbicide Applications in Corn

By Mike Owen, Department of Agronomy


Given the way the season has developed, the best intentions to apply an Early Preplant (EPP) herbicide application prior to corn planting has gone out the window and it appears that applying a preemergence (PRE) application immediately after planting is also becoming a slim chance. Thus, many who intend to use a soil-applied residual herbicide treatment in corn may be forced to make the application of an early postemergence (EPOST) to the weeds and possibly the corn.


Below is a table containing herbicide options available in corn modified from an article that appeared in the University of Nebraska CropWatch newsletter. The list likely does not include all options given the generic status of a number of these herbicide active ingredients.


The concern about the delayed PRE/EPOST herbicide treatment, regardless of whether or not residual products are included, is making the application soon enough to protect crop yield. It is typical that herbicides will kill weeds that are larger but often after potential yield has been lost.


Furthermore, some weeds (i.e. winter annuals) become extremely difficult to control as they grow larger and the potential for crop injury also increases as the crop gets larger. Thus, the best option is to make the residual herbicide application immediately after planting.  If this is no longer an option, apply the treatment as soon as possible after planting.


Please recognize that some herbicides are registered for application EPOST to the crop but do not have POST activity on weeds. Also, many of these products can be applied to corn that is considerably larger than appropriate to protect the potential yield and weed control may be variable due to the uneven distribution of the spray on the weeds (the corn canopy interferes with the spray coverage). 


Be sure to follow the label directions and understand the limitations of applications as the weeds and corn get larger. Also, it is highly unlikely that any of the EPOST treatments will meet expectations as a “season long, one pass treatment.” 


Scout the fields prior to application and use the correct product to control the weeds that are present. Recognize that some of the labels are not clear about the size of weeds that may be controlled after they emerge. 


Herbicide Options Available for Corn



1Severe injury may occur if Callisto is applied postemergence to corn that has been treated with Counter or Lorsban.  Do not tank mix with any organophosphate or carbamate insecticide.  Do not cultivate within seven days of application.

2Do not tank mix this product with any other herbicide when applied postemergence.

3Do not mix this product with complex fertilizer mixtures such as 10-34-0 or flowables.  Use only water or liquid nitrogen carrier.

4Apply this product to Roundup ReadyTM corn only.


Mike Owen is a professor of agronomy with research and extension responsibilities in weed management and herbicide use.

This article was published originally on 5/19/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.