Ken Wise (NYS IPM) and Mike Hunter (CCE North Country Regional Ag Team)
Western bean cutworm (Striacosta albicosta [Smith]) (WBC) was first discovered in New York State in 2009. It has been expanding its range from its origin in the high plains area of the US over the last 20 years. WBC is an insect pest of corn and dry beans, and can cause significant yield and quality losses to field corn grain. In other parts of the Corn Belt, it has become a pest causing significant economic losses in field corn. WBC is a Lepidopteran Noctuidae moth species that lays eggs on the upper surface of leaf just before tasseling (Fig. 1).
Once eggs are laid on leaves, they appear white and will turn tan, and then a purplish color before hatching (Fig. 2). The 1st instar larvae will eat their egg shells before finding other food and an area of protection from predators or parasitoids. The small larvae will move to the whorl and/or leaf axil, and they will eat pollen, tassels and silks (Fig. 3). By the 4th instar the larvae will bore into the corn ear and feed on kernels of corn (Fig. 4). One difference between WBC and other species of worm pests of corn ears (European corn borer, corn ear worm) is that you can find multiple worms in one ear. Other species are cannibalistic, and allow only one larvae to enter the ear, while WBC does not mind if there are several per ear.
One to several larvae per ear can really affect the yield. Once the larvae reach the 6th instar they drop from the plant to the soil surface, where they burrow into the soil and create a chamber where they will overwinter in a pre-pupa stage (Fig. 5). They will finish the pupation in late spring and early summer, and emerge from the soil from mid-July through mid-August with peak flights during the last week in July to the first week in August.
Since the discovery of western bean cutworm in New York in 2009, we have monitored its progression across the state. In 2010, we developed a WBC pheromone trap monitoring network. This network of Cornell Cooperative Extension Educators, crop consultants and agricultural professionals placed out bucket pheromone traps to capture moths each year from late June through August. A female WBC pheromone lure is placed in the trap which attracts and catches only the male WBC moths. Each week the number of moths are counted and reported by the location of the trap. These traps are deployed to monitor moth presence and determine the peak flight. Traps help us identify fields at risk and when scouting should take place, but we cannot use trap counts to determine when a field should be sprayed with an insecticide.
Since 2010, the population of the WBC in New York has increased exponentially. We started with 19 volunteers and 44 traps in 29 counties, and in 2018, we had 50 volunteers and 118 traps in 45 counties.
The total number of WBC moths captured per trap in New York by year are depicted in Table 1. In 2010 there were less than 15 moths caught per trap with a high of 99. In 2018, we had 118 traps that caught 39,290 moths with an average of 333 moths per trap. Some traps in Northern NY had 1000 to almost 3000 moths in a single trap. Northern NY is the hot spot for WBC, and the number of moths caught in this region of the state far exceeds the rest.
When looking at the average number of moths caught per trap, 67% of the traps caught more than 100 moths and only 15% caught less than 20 moths (Fig. 7). Jefferson County had a single seasonal trap accumulation of 2964 moths. The range of trap counts were 0 to 2964. While the average came down just a bit from 361/trap in 2017 to 333 /trap in 2018, we had many more traps in areas of the state that do not have the same pest population densities of Northern NY. This brought the average number of moths/trap down for the first time since 2016. In 2016, we had drought conditions that might have caused a reduction in population of WBC.
A very important aspect of managing WBC is knowing when peak flight occurs. The annual peak flights are outlined in Figure 8. From 2010 to present, the peak flight has ranged from the last week in July to the first week in August. By knowing the peak flight, you know when most of the moths will be laying eggs in pre-tassel corn because the female moths prefer to lay eggs on this stage of corn growth. And this peak flight time is when we should be vigilant about scouting for WBC egg masses and small larva.
The data is starting to show that the population is beginning to build up in areas of the state that have previously had lower populations of WBC. The data in Figure 9 indicates that the average number of moths caught per trap is increasing across the state outside of Northern NY. In time, WBC populations will likely rise across the state to the point that management will be needed for this insect pest. Widespread, high WBC populations in many areas of Northern NY have resulted in some corn fields being treated with insecticides to manage this pest.
While WBC damage to corn ears can be significant and may have detrimental effects on corn grain yield and quality, the economic impact on corn silage is less understood. For corn silage growers, determining whether or not this pest significantly impacts the yield or quality of the forage is critical to their decision making for managing this pest.
Scouting corn at the pre-tassel stage of growth is an important aspect of managing this pest. The economic threshold is 5% of plants having egg masses and small larvae. The 5% is an accumulated threshold, meaning that if in week one 3% of the plants have egg masses and the folowing week there are 2% more, this equals a cumulative 5%.
Current strategies available for control of WBC in corn are the use of foliar insecticides or selecting transgenic corn hybrids with the Vip3A trait. Foliar insecticide treatments are effective but can be difficult to correctly time applications. If a field is found to be over threshold for WBC, an insecticide should be applied only if fresh silks are present. If no tassel is present there is no reason to spray an insecticide because it would be too early and the larva will not survive. Once the larva make their way into the ear tip it is too late to spray as the insecticide will not come into contact with the larva. Currently, only corn hybrids with the Vip3A trait will provide control of the WBC. There have been reports from Michigan, Indiana, Ohio and Ontario, Canada suggesting varying levels of control of WBC with the Bt corn trait containing the Cry1F protein, (DiFonzo, C., Krupke, C., Michel, A., Shields, E., Tilmon, K. and Tooker, J; 2016). Based on 2016 to 2018 on farm research trials in Northern and Western NY, it was determined that incomplete control from the Cry1F trait was confirmed, (Hunter, M., and O’Neil, K.; 2018, 2017, 2016).
Thank you to Jaime Cumming (NYS IPM) and Marion Zuefle (NYS IPM) for editing and providing additional data for the article.
2018 New York WBC Pheromone Trap Monitoring Network:
Thanks to cooperating growers for allowing us to use their fields for sample sites. Special thanks to the following individuals for their enthusiasm, dedication, excellent data collection and maintenance of the WBC trap network: Adam Abers, Brian Boerman, Chuck Bornt, Elizabeth Buck, Sara Bull, Paul Cerosaletti, Mike Davis, Janice Degni, Dale Dewing, Natasha Field, Cassidy Fletcher, Jennifer Fimbel, Aaron Gabriel, Kevin Ganoe, Jeffrey Gardner, Don Gasiewicz, John Gibbons, Ethan Grundberg, Mike Kiechle, Ariel Kirk, Jeff Kubeka, George Krul, Christy Hoepting, Mike Hunter, Amy Ivy, Joe Lawrence, Jodi Lynn Letham, Jen Masters, Laura McDermott, Carol MacNeil, Sam Meigs, Stephanie Melancher, Sandy Menasha, Jeff Miller, Anne Mills, Eric Nixon, Kitty O’Neil, Jessica Prospers, Bruce Reed, Teresa Rusinek, Erik Kocho-Schellenberg, Jack Steele, Abby Seaman, Keith Slocum, Paul Stackowski, Mike Stanyard, Dan Steward, Crystal Stewart, Allie Strun, Linda Underwood, Katherine Vail, Ken Wise, Anastasia Yakaboski, Glenn Yousey, Marion Zuefle, WNYCMA. The WBC Bt corn trials were made possible with support from both the New York Corn Growers Association and the Northern New York Agricultural Development Program.
DiFonzo, C., Krupke, C.;,Michel, A., Shields, E., Tilmon, K. and Tooker, J; 2016. An open letter to the Seed Industry regarding the efficacy of Cry1F Bt against western bean cutworm. October 2016. Cornell University
Hunter, M., and O’Neil, K.; 2018 Evaluation of the Efficacy of Bt Corn for the Control of Western Bean Cutworm in NNY. Northern NY Agricultural Development Program. Cornell University (report forthcoming)
Hunter, M., and O’Neil, K.; 2017 Evaluation of the Efficacy of Bt Corn for the Control of Western Bean Cutworm in NNY. Northern NY Agricultural Development Program. Cornell University
Hunter, M., and O’Neil, K.; 2016 Evaluation of the Efficacy of Bt Corn for the Control of Western Bean Cutworm in NNY. Northern NY Agricultural Development Program. Cornell University