Western Bean Cutworm Moths Flying, Scout Field and Sweet Corn
Eileen Cullen, Extension Entomologist, UW Madison Entomology Department
Key Points
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Western bean cutworm (WBC) moth flight is underway in Wisconsin.
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Pheromone trap network is recording WBC moth emergence, peak flight and cumulative catch at 131 sites across 25 Wisconsin counties for 2009.
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In field and sweet corn, pheromone traps (a.k.a. “milk jug traps”) and degree-day accumulations (Base 50F) help time scouting activities.
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WI DATCP Wisconsin Pest Bulletin reported high moth trap capture counts for the period of July 12-16 of 17 moths near Montello in Marquette County and 35 moths in the black light trap near Grand Marsh in Adams County. http://pestbulletin.wi.gov/
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Close inspection of field and sweet corn should begin in the week ahead to determine the percentage of plants infested with egg masses and small larvae.
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Start scouting at 1,320 DD (Base 50F). Approximately 25% WBC moth emergence.
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Continue scouting 7-10 days after peak emergence. Peak emergence expected at 1,422 DD (Base 50F). Pheromone trap catch also monitors peak flight.
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WBC moths are most attracted to corn just before tasseling, so scout fields in this growth stage first.
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Eggs are deposited primarily on the upper surface of the flag leaf, while the larvae can be found in the developing tassel and on the plant migrating toward developing ears.
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Treatment is justified when 8% of plants are infested (4% for processing sweet corn), and should be applied at 90-95% tassel emergence.
WBC is a late season corn ear pest. WBC has one generation per year, with six or seven larval stages, and overwinters as a full-grown larva, inside a soil chamber. Spring development begins when temperatures exceed 50°F. Larvae pupate in May and moths begin to emerge from soil chambers in early July. Multiple WBC larvae can infest one ear and cause yield loss and impact quality of grain (damaged, decaying kernels, secondary pathogen infection).
Thank you to Krista Hamilton and WI DATCP Pest Survey, along with UW-Extension county agricultural agents, and cooperating crop consultants for establishing the most extensive WBC pheromone trapping network in WI yet, with 131 traps across 25 counties. Pheromone traps use a WBC pheromone lure to attract male WBC moths. Traps have been operating since July 1 and monitored weekly. This trapping effort is recording moth emergence, peak moth flight, and flight decline which usually occurs in early to mid-August.
Pheromone trap captures and/or Degree Days are essential for timing field scouting. WBC numbers (economic threshold) and treatment decision must be made before WBC larvae enter ears. Therefore economic threshold is based on percentage of plants with egg masses and/or small larvae. Economic thresholds are 8% infestation for field corn, 4% for processing sweet corn.
Begin scouting once WBC degree-day accumulations reach 1,320 DD, Base 50F, (25% moth emergence), or when the first moths are detected in a pheromone trap in your area. In southwestern and southern WI, begin scouting in the week ahead.
Examine 20 consecutive corn plants at five locations in the field to obtain a representative field sample. Check the upper three or four leaves of each sampled plant for presence of WBC egg masses and small larvae. Scout field corn hybrids or processing sweet corn varieties in different stages of development separately. Foliar insecticide treatment is justified when 8% of sampled plants have egg masses and/or small larvae for field corn, and 4% for processing sweet corn.
Western bean cutworm egg mass. Photo: Marlin E. Rice.
Eggs are laid on the upper leaf surface, on the upper part of the plant. Eggs are round are cream colored, then turn purple approximately 24 hours prior to hatch. First instar, small WBC larvae pictured here. Photo: Marlin E. Rice.
In field is planted to a Bt corn hybrid with WBC resistance, remember to scout the non-Bt refuge. In the event of heavy WBC moth flights, Bt corn WBC protected corn can sustain some feeding damage to ears, so it is a good idea to check Bt corn fields occasionally and make sure damage does not exceed 8% eggs and/or larvae threshold.
Half the season’s WBC moth population will emerge at 1,422 degree-days (Base 50F), known as peak flight. Continue scouting fields for 7-10 days after peak flight to detect eggs and larvae.
Twenty five percent moth emergence is expected in southern and central Wisconsin by July 20-27, following the accumulation of 1,320 degree days (base 50°F). Field scouting for egg masses and small larvae should begin at this time, and control decisions made shortly after peak flight is documented. This event usually occurs during the final week of July or first week of August in Wisconsin.
Remember degree-days accumulate at slightly different rates, depending on how cold or hot our temperatures are, and your part of the state. Central and northern locations tend to reach DD accumulations after southern parts of WI. However, there are local variations and microclimate considerations, so check DD accumulations from a weather station or daily weather reports for your area.
Information on insect degree-day accumulations and how to calculate can be found at our Field and Forage Crop Entomology website, authored by Sarah Schramm, Cullen Lab.
http://www.entomology.wisc.edu/cullenlab/insects/degreedays.html
WBC is also being captured in pheromone traps east of WI in Ohio, throughout MI, and all the way to Pennsylvania (although at low numbers in PA to date). Wisconsin’s trap catch is still relatively low as of July 22nd compared to IN. The number of moths in traps does not tell us economic threshold in a field, field scouting is needed to determine that. Heavy flights generally translate to more feeding damage. However, Wisconsin had significant moth flight and corn ear feeding damage in 2008 in southwest, south central, central and northeast WI locations.
The take home message of this article is to be prepared and proactive, and begin scouting field and sweet corn now, at the appropriate time as described above. Later this summer, in August and early September, if large WBC larvae are found in ears, it will be too late to control.
2009 Resources for Western Bean Cutworm Management in Field and Sweet Corn
Western Bean Cutworm: A Pest of Field and Sweet Corn
UW Extension Publication A3856
This 4-page color fact sheet explains WBC appearance, life cycle, corn scouting protocols, economic thresholds, and management options including: biological and natural controls, chemical control, and the importance of insect resistance management when using Bt corn hybrids for WBC management. Download the PDF from:
http://www.entomology.wisc.edu/cullenlab/extension/ext_pubs.html
Western Bean Cutworm Links
Compilation of all WBC resources on Cullen website,
prepared by Sarah Schramm, Associate Research Specialist
http://www.entomology.wisc.edu/cullenlab/insects/info/wbc.html
Webcast …. Western Bean Cutworm Management in Field & Sweet Corn for Wisconsin
January 2009, A 38 minute webinar presentation by Dr. Eileen Cullen
http://www.entomology.wisc.edu/cullenlab/insects/info/wbc.html#Webcasts
How to Calculate Insect Degree Days
Information on insect degree-day accumulations and how to calculate can be found at our Field and Forage Crop Entomology website, authored by Sarah Schramm, Cullen Lab.
http://www.entomology.wisc.edu/cullenlab/insects/degreedays.html