Thursday, April 26, 2012

Iowa: Black Cutworm Scouting Advisory

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

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 here 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 bcutworm@iastate.edu. This information could help us to refine our prediction efforts in coming years.

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