Western Bean Cutworm Moth Flight Has Begun
Eileen Cullen, Extension Entomologist
The annual flight of western bean cutworm (WBC) moths began this past week. The first moths of the season were recorded June 23-29 in Adams, Calumet, Iowa, Marquette and Sauk counties according to the WI DATCP Pest Bulletin survey 2011 WBC pheromone trap network. July 5, we captured our first WBC moth (1 moth total) at the Arlington, WI (Dane Co.) pheromone trap. With very low trap counts of 1-2 moths per trap during this first week of detection, the WBC moth flight is just beginning.
Western bean cutworm moth
There is one generation per year of this mid to late-season field and sweet corn ear feeding insect. WBC overwinters in Wisconsin as a late instar larvae in the soil. Thus, we can use insect degree-days to estimate peak moth emergence and egg laying in the field and alert growers, consultants and canners to accurately time field scouting and threshold based treatment decisions before larvae enter the ear.
Western bean cutworm egg mass on corn (2009)
Using the WBC phenology model, you should start scouting field and sweet corn at 1,320 degree days (base 50°F) when 25% of the moth population will be in flight. This scouting cue puts you ahead of peak moth flight (50% emergence; 1,422 degree days) and will allow you to notice when the first egg masses are present in the field.
WI DATCP reports that 1,320 degree days should occur around July 14 near Beloit, July 22 near Madison, August 2 near Eau Claire, and August 12 near Green Bay. Check WBC degree-day accumulation in your area to make sure you do not miss the moth flight and scouting window for eggs and small larvae over the next couple to three weeks (depending on location in the state and WBC degree days for your area).
Keep in mind, degree-day accumulations accrue sooner in southern and central WI than Northeastern and Northern corn growing areas of the state, so it’s important to track WBC degree days and check WBC pheromone traps in your area to fine tune your scouting and treatment decisions.
Although, degree day forecasting gives you a sound indication of when to start scouting fields (1,320 DD; 25% moth emergence), WBC pheromone traps placed on or near your farm fields (corn) are a better indicator of when to start scouting. If you have a WBC pheromone trap on or near your farm fields, scouting for egg masses and small larvae should begin as soon as moths begin appearing in traps.
Adult female WBC moths are most attracted to corn just before tasseling and lay eggs on the upper leaf surface, primarily on upper leaves on the corn plant, but also near the ear zone. Corn crop growth stage (planting date) and planting type (field corn, seed corn, sweet corn) will dictate somewhat where on the plant you can find egg masses, and eventually small larvae as they move from the egg mass after hatching.
As WBC moths prefer to lay eggs on corn just before tasseling, these fields should be scouted first when 1,320 degree days are reached and/or you begin to catch WBC moths in a pheromone trap. Continue to scout post tassel corn as well since moths will lay eggs in these fields too, particularly at the end of the moth flight when corn is older. Later in July at peak moth flight (1,422 degree days; 50% moth emergence) and beyond in the flight trajectory, larvae can be found on corn tassels, in leaf axils, and on silks. They are much easier to scout for when in the egg and small larvae stage, before they have moved to silks, and treatment will be more effective when timed accordingly.
For field corn, the economic threshold level of 5% field infestation with eggs and small larvae is now in use throughout the Upper Midwest (adjusted downward from 8%). Processing sweet corn economic threshold is lower at 4%.
For more information on WBC biology, economic threshold and management, please refer to the Entomological Society of America’s Journal of Integrated Pest Management, J. Integ. Pest Mngmt. 1(1): 2010; DOI: 10.1603/IPM10003, Ecology and Management of the Western Bean Cutworm (Lepidoptera: Noctuidae) in Corn and Dry Beans.
Western bean cutworm eggs are white when first oviposited