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4/19/2010 - 4/25/2010

Is Preventative Black Cutworm Control Worth It?

By Erin Hodgson, Department of Entomology

Many growers are getting their corn planted ahead of schedule and thinking about making the first weed control application. Some might think tank-mixing a preventative insecticide with a herbicide will save money and protect yield from early season corn pests such as cutworm and grubs.

Black cutworms are only an occasional corn pest in Iowa, but this insect still deserves attention due to its potential to cause economic loss. Currently seed treatments and genetically modified hybrids do not fully protect against cutworms, so corn is still at risk every year to migrating black cutworm moths.

Production practices that favor black cutworm damage include late tillage and late planting. That is good news for all the fields planted early this year. Corn planted after soybean is also more likely to get infested than in continuous corn. Poorly drained, weedy fields or fields located near native vegetation can also be attractive to egg-laying females.

The price of corn seed is rising fast and a prophylactic treatment to protect young corn is a convenient option for high risk fields. But there are several reasons, previously explained by Marlin Rice, why tank-mixing an application right now is not a financially or environmentally sound decision.

  1. Black cutworm infestations have been historically sporadic and patchy, and the last serious outbreak was in 1984. A just-in-case treatment does not follow IPM guidelines and may be a waste of money.
  2. So far, adults have been trapped in low numbers throughout Iowa. We haven't reached peak moth flight yet and predicted cutting dates are still not determined for 2010. Appling an insecticide too early may not fully protect plants after emergence and a second treatment may be necessary to knockdown damaging cutworm larvae.
  3. A typical foliar insecticide product ranges from $5-15/acre. Even a generic product will increase overall production costs, especially for large operations.

Scouting for young black cutworms will help determine infestations and damage potential. Start looking for small, irregular holes on leaves (Figure 1) after corn emerges. Look at 50 plants in five locations per field. Signs of cutworm activity include discolored or wilted leaves, and missing plants. Only older larvae (more than 1/2 inch in length) are capable of cutting plants and can be found in the soil during the day. Continue scouting for cutworms until plants reach 15 inches tall or the V5 growth stage. That doesn't mean to stop scouting in corn because mid-season corn pests, like stalk borers, will begin their summer feeding activity.

A dynamic threshold calculator was developed last year and can be a useful management tool. In general, a foliar insecticide may be warranted if:

• Two to three percent of plants are wilted or cut, and black cutworm larvae average less than 3/4 inch in length.

• Five percent of plants are wilted or cut when black cutworm larvae are greater than 3/4 inch in length.

• One to two percent of plants wilted or cut if poor plant populations (more than 20,000/acre) exist.

black cutworm damage 
Figure 1. Common young black cutworm damage in young corn

 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.

Black Cutworm Monitoring Network Update 2010

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

Monitoring for black cutworm, an occasional corn pest in Iowa, has begun. Cooperators in the black cutworm monitoring network are observing the arrival of adult moths (Figure 1) throughout the state. Since April 1, reports of moths captured in pheromone traps are coming from a number of counties, but most captures have been below peak flight levels.

The black cutworm monitoring network helps growers determine when they should start scouting. Scouting fields is important in determining if cutting larvae are a problem in a specific field. Adult moth trap captures alone are not enough to justify an insecticide treatment. Scouting should be done before treating for black cutworm is considered.

black cutworm adult moth
Figure 1. The black cutworm adult life stage

 

Biology
The black cutworm, a pest of corn in Iowa, causes damage early in the season. The insect does not overwinter here. Instead, adult moths migrate on the wind from southern states near the beginning of spring, mate and lay eggs. Approximately 1300 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 (Figure 2) injure corn plants by feeding on leaf tissue and older larvae can cut seedlings (Figure 3). Once corn reaches the V5 stage, it becomes harder for the pest to cut plants. Three generations of black cutworm occur per year.

Black cutworms 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 such as skin type and tubercle size which are outlined in detail in ICM News article Blacks and Dingys: Confusing Cutworms 

 

blackcut worm larvae
Figure 2. Black cutworm larva

 

black cutworm damage to corn 
Figure 3. Damage to corn plant from the black cutworm

 

Monitoring network
After checking pheromone traps for the presence of the male moths, monitoring network cooperators enter the number of captured moths on the network website. The arrival of moths indicates egg laying will soon take place. Once a large number of moths are recorded in a particular region in Iowa, we use degree days to estimate insect development. Degree days are based on temperature which is a better way to estimate insect development than calendar days – if it is warm insects will grow faster than when it is cold. We use past temperature data combined with this year’s temperature information to determine when hatched larvae will begin cutting corn (cutting date). Cutting dates are projected to occur at about 300 accumulated base-51 F degree days from a peak flight. 

As the season progresses, we will keep you posted about the status of the black cutworm in Iowa. Keep your eyes open for the black cutworm predicted cutting dates article on the Integrated Crop Management News website. If you wish to join the monitoring effort in coming years, please send an email to bcutworm@iastate.edu with your name and address and we will add you to the list for 2011.

 

 

Adam Sisson is with the Department of Plant Pathology. He can be reached at ajsisson@iastate.edu or 515-294-0581. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic. She can be reached by email at ljesse@iastate.edu or by phone (515) 294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.

Crop and Weather Report – April 19, 2010

By Elwynn Taylor, Department of Agronomy

Crop planting is progressing rapidly in much, if not most, of the Corn Belt. Nineteen percent of Iowa’s expected corn acres were planted as of April 18. This is well ahead of the five-year average of 5 percent, which is where planting was in 2009 on this date. Most of the central production area is also progressing well. The USDA weekly “Crop Progress” report is searchable by state. 

precip map

West Central Iowa has areas receiving less than one-fourth of normal precipitation since April 1. Graphic from http://water.weather.gov/precip/    


Seventy percent of Iowa’s farm land has received less than usual precipitation during the past three weeks. Temperatures have averaged above normal for all but two days of the past three weeks in most of Iowa. The conditions to date are considered favorable for planting and crop establishment. Forecasts for cooler and moist weather in the western half of the Corn Belt may signal a shift to the persistent pattern of cooler than usual temperatures and above normal precipitation that were characteristic of 2009. 

The relatively brief El Nino of the past several months appears to have diminished strength.  The atmospheric pressure aspect of the El Nino event retreated to -0.8 standard deviation from (below) normal today (19 April 2010) just five months after reaching the level of significance.  This is in keeping with the past two El Nino episodes that persisted three and five months only, somewhat of a departure from the three-year episode experienced in the early 1990s.

The departure of El Nino does not signal adverse impacts on Midwest crop success. Still there is a memory of several instances of a sudden change from El Nino to La Nina and accompanying drought-like conditions. Historically the chance of below average corn yield for the U.S. is about 45 percent, but during El Nino the chance of a below trend corn yield reduces to 30 percent and during La Nina summers the risk increased to near 70 percent chance of below trend crop yields. Most forecasts give the chance of switching to summertime La Nina odds of less than one chance in five.

 

Iowa soil temperatures
Daily soil temperature reports for Iowa are available online. Observed temperatures are interpolated to provide estimated values for each county. The 2010 soil temperature is exactly on schedule according to the history of observations in Iowa. By the second week of May it is extraordinary to have central Iowa soil temperature drop to 50 F or below. Although some colder than average temperatures must be anticipated between late April and mid-May, each day of normal temperature diminishes the risk of a damaging cooling of soils.

The damage of a soil cool down this time of year is not so much freezing as it is experiencing of temperatures below the threshold for normal crop growth and development. When temperatures are not sufficient for vigorous plant growth the crop becomes increasingly susceptible to injury from agricultural pests (disease, weeds, and insects).

 

soil temps

Weekly average soil temperatures (beginning with March 1 as the first day of the first week of the climate year) are indicated by the central curve in the graphic. A shaded region about the line shows the historical standard deviation around the average for the week. The irregular lines above and below the shaded track delimit the extremes of temperature observed for specific weeks. The lowest temperature for plant growth is called the base temperature.  The base temperature for Midwest corn is very near 50 F. Data from Iowa State University Department of Agronomy.

 

 

                 soil temps on crop emergence

Effect of soil temperature on emergence of corn and soybeans. Emergence is slow at temperatures near 50 F (10 C). Emergence is rapid near 90 F (32.2 C). Graphic assumes that soil moisture is near ideal for plant establishment. Graphic from Elwynn Taylor

 

Elwynn Taylor is Iowa State University Extension Climatologist and can be reached at setaylor@iastate.edu or by calling (515) 294-1923.



This article was published originally on 4/26/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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