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4/23/2012 - 4/29/2012

Moths Abundant Around Iowa

Army cutworm, or miller moths, have been numerous around homes this week. Photo by Ralph Anderson, Buffalo County Extension, Nebraska


By Laura Jesse, Plant and Insect Diagnostic Clinic and Erin Hodgson and Donald Lewis, Department of Entomology

This has been the week of the fuzzy brown moths or FBMs (as entomologists not-so-technically call the hundreds of moth species that fit this description). There is a wide variety of species that can be called FBMs and it seems we are experiencing several. 

Some crop pests fall into the FBM category, and the sight of so many moths can cause concern. So far no serious crop pests have been among the specimens submitted to the clinic for diagnosis. We already are monitoring for black cutworms and have recommended when to scout for caterpillars in corn. In addition to concerns about crop damage there are questions about using insecticides to keep FBMs away from homes as they are bothersome. We do not recommend treating these nuisance moths for several reasons:

  1. Adult moths can be a nuisance, but do not cause plant damage. If they eat, they only feed on pollen and nectar. 
  2. Spraying foliar insecticides is not a cost-effective option for these night-flying moths.  Field applications are particularly not effective because the adults are not feeding on young corn.
  3. Moth species that do not eat as adults will die in less than a week anyway so we expect this to be a short term annoyance.
  4. Reducing outdoor lighting near homes at night is the best way to keep the masses away from homes. They are attracted to windows as well, but as long as screens are in good repair they will not get indoors.
  5. There appears to be a variety of species active right now so we are uncertain where they will lay eggs and if the caterpillars will become a pest. At this point we just have to wait and see.

Pictures and reports submitted so far have been determined to be army cutworm moths and lucerne moths. Lucerne moth caterpillars feed on a wide variety of plants and grasses including alfalfa. They are not considered pests in Iowa.

Army cutworm moths are also commonly called “miller moths.” (Fig 1) The adult moths are migratory and well known for gathering around homes and accidentally getting inside. Normally the army cutworm moth migration is more noticeable in states to our west (Kansas, Nebraska, Colorado) but it seems that this year Iowans are experiencing more moths than normal and they are early this year. Army cutworm moths are annoying, but harmless. 

We do not expect an increase in damaging caterpillars because moths are abundant. Army cutworms and lucerne moths have such a broad host-range they often end up feeding on weedy plants in ditches and other areas. We will have to wait and see if there is any more caterpillar activity than normal, but there is no need for preemptive treatments.

Figure 1. Army cutworm moths can be variable in size and color. Photo by Whitney Cranshaw,


Black Cutworm Scouting Advisory

By Adam Sisson, Integrated Pest Management; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology

With the unseasonably warm temperatures occurring earlier this year, we asked black cutworm monitoring participants 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. Our 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. The map (Fig. 1) 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 dates are based on mid-April captures.

Figure 1. Estimated black cutworm cutting dates based on 2012 peak flights of moths. Where there are two dates, the top date is an estimate based on moth captures that occurred near the end of March; all other dates are based on mid-April captures. Scouting should begin several days before the predicted cutting date.
* There were intermittent captures in this area throughout the first part of April besides the two peak flights used to estimate cutting dates.


Estimated 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 the central district; May 9 and 17 in the east central district; May 2 and 14 in the southwest district; May 5 and 16 in the south central district; and May 4 and 15 in the southeast district.

Predictions are based on actual and historical degree day data accumulated from the dates of 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 weather.

Since the early peak flights near the end of March, freezing temperatures have been observed in Iowa. 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.

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. Black cutworms are light grey to black; with granular-appearing skin and four pairs of fleshy prolegs on the hind end (Fig. 2). 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.  

Figure 2. Black cutworms are best identified by the dark tubercles found along the middle of the back. On each body segment, the pair of tubercles closest to the head is about one third to one half the size of the pair nearest to the abdomen.

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 in each field. Look for plants with wilting, leaf discoloration and damage, or missing plants (Fig. 3). 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.

Figure 3. Black cutworm larval damage usually starts above the soil surface. Larvae are capable of clipping young corn plants. Photos by Steph Marlay

Thresholds. If larvae found in the field are smaller than three-fourths inch, then a threshold of 2 to 3 percent wilted or cut plants indicates an insecticide application is warranted. If larvae are longer than three-fourths 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. The Excel spreadsheet.xlsx has calculations built-in and be downloaded here to aid in management decisions regarding black cutworm.

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.

Biology. 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 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 and clicking on “Iowa Black Cutworm Monitoring 2012.” 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 This information could help us to refine our prediction efforts in coming years.


Adam Sisson is an Integrated Pest Management program assistant. Sisson can be contacted by email at or by calling 515-294-5899. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at or by phone 515-294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847.

Subsoil Moisture Levels are Still a Concern for 2012 Crops

Paul Kassel, Extension Field Agronomist  

The Iowa State University spring survey of subsoil moisture in northwest Iowa indicates below normal amounts of subsoil moisture in most areas. The exception was near Rossie where the subsoil moisture was 1.3 inches above normal. Subsoil moisture levels are surveyed in pre-determined areas each spring and fall.

Typical soils in northwest Iowa have the potential to hold from 10.0 to 11.0 inches of moisture in the top five feet of soil. The dry conditions last summer and fall left the soil moisture reserve very low. Soil moisture readings from sites near Spirit Lake, Rossie and Pocahontas had 1.4 to 5.0 inches of plant available moisture last fall.

These sites gained an average of 4.0 inches of subsoil moisture over the winter and spring months. This moisture recharge has occurred from winter rainfall and early spring rainfall. The winter rainfall contributed to subsoil moisture because mild winter conditions left soils unfrozen during much of the winter.

Sites near Schaller, Pocahontas and Spirit Lake have below normal levels of subsoil moisture. Soil moisture levels at these sites range from 5.7 to 7.2 inches of plant available moisture. Moisture amounts at these sites are located in the top three feet.  

The subsoil moisture at the Schaller location illustrates this point. There is 4.0 inches of moisture in the top two foot of soil, 1.3 inches of moisture in the third foot and virtually no moisture in the fourth and fifth foot of the soil profile. The location of this moisture in the soil profile will make these soils very dependent on spring and early summer rainfall to produce a normal crop.

However, if spring rainfall does not replenish soil moisture reserves in late April or May, crops will be more dependent on summer rainfall. Corn and soybean crops require about 22 inches of soil moisture to produce a normal crop. Therefore, normal summer rainfall – which is about 18 inches for May to mid September – will be needed to produce a normal sized corn and soybean crop.  Rainfall usually contributes about 80 percent to soil moisture levels.  

Table 1.   Subsoil moisture levels, 2012.


Table 2.  Rainfall amounts since sampling date.


Table 3.  Rainfall amounts since fall subsoil moisture sampling.


Paul Kassel is an Iowa State University Extension and Outreach field agronomist in northwest Iowa. He can be reached at or 712-260-3389.

Corn Nitrogen Rate Calculator Update

by John Sawyer, Department of Agronomy

What is the Corn Nitrogen Rate Calculator?
The Corn Nitrogen Rate Calculator is an online tool that allows determination of nitrogen (N) application rates for corn production and is helpful in determining the effect of fertilizer and corn price on needed rates. The method for calculating suggested N rates is based on a regional (Corn Belt) approach to N rate guidelines. Details on the approach are provided in the regional publication Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn. This approach and the Corn Nitrogen Rate Calculator are now being used by seven states across the Corn Belt: Iowa, Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin.

Nitrogen (N) Response Trials Added
The Iowa N response database in the calculator was recently updated, with response trials added from 2011 research. There are now 214 trials for corn following soybean and 111 trials for corn following corn. Being able to easily update the database with recent data is one of the many advantages to this dynamic database approach for corn N rate guidelines. Having new response trial data allows rapid updating with changing hybrid genetics, rotations and climatic conditions.

With the updated database, calculated N rates have increased slightly from previous years. The table below gives the N rate at the maximum return to N (MRTN) and the profitable N rate range from the updated calculator for several N: corn grain price ratios (price of N fertilizer in $ per lb N divided by the price of corn in $ per bu). You can work with any price of N and corn you wish when running the calculator. Output information includes the N rate at the MRTN, the profitable N rate range, the net return to N application, the percent of maximum yield and the selected N fertilizer product rate and cost.

Nitrogen rates determined from the calculator are directly the total fertilization amounts for each rotation, with no need to further adjust rate for previous crop. That is, for the soybean-corn rotation, there is no need to subtract a “soybean credit” as the rotation effect is already accounted for by the N rate trials that the database is derived from.

Resources for N Rate Decisions
The Corn Nitrogen Rate Calculator Web tool is located at:

The regional publication Regional Nitrogen Rate Guidelines for Corn (PM 2015) can be ordered through any ISU county extension and outreach office, from the ISU Extension Online Store at , or by calling (515) 294-5247. An electronic copy of the publication is available at

The ISU Agronomy Extension Soil Fertility website is located at:

John Sawyer is professor with research and extension responsibilities in soil fertility and nutrient management.

This article was published originally on 4/30/2012 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.