Western bean cutworm at peak flight in northeast Michigan

Christian Tollini, Michigan State University Extension - August 10, 2020

High flight numbers recorded across region, growers are encouraged to scout and consider control.

Figure 1. Trap locations across northeast Michigan.

Western bean cutworm is a pest of dry beans, field corn and sweet corn that has been present in Michigan since 2006. In northeast Lower Michigan, the most economically significant crop damage associated with this pest occurs in dry beans. Adult moths emerged from the soil in July to mate and then proceed to deposit eggs in both corn and dry beans. Once deposited, egg masses hatch in five to six days and begin to feed on the crop.

Western bean cutworm larvae feed on the silks and young ears of corn, as well as blossoms, pods and immature seed in dry beans. This feeding leads to undesirable ear damage in corn and reduced yields and seed quality in dry beans. Extra handling to sort damaged beans may be necessary as well.

To help producers mitigate the risk of western bean cutworm damage in dry beans, Michigan State University Extension Presque Isle County cooperates with growers across Presque Isle, Montmorency and Alpena counties to monitor moth flight. By using pheromone traps that were set on July 17, 2020, weekly moth counts have been recorded to identify peak flight, as well as if or when flight numbers are high enough that growers should begin scouting for pod damage.

This year, 17 traps were set near Onaway, Millersburg, Moltke, Hawks, Hagensville, Metz, Posen, Polaski, Lachine, Herron, Ossineke, Spratt, Rust, Royston and Hillman (Figure 1). Weekly moth counts are submitted to the Great Lakes and Maritime Pest Monitoring Network, where an interactive map displaying region-wide moth flight can be accessed.

Emergence this year has been slightly earlier than the previous growing season, which is not surprising as growing degree day accumulations and crop progression are much further advanced than at this point last year. Already, moth counts have exceeded those recorded last year in most areas. Every trap location has exceeded 120 cumulative moths (Figure 2), which is considered high flight and a trigger for field scouting. Peak moth flight has occurred between the weeks ending Friday, July 31 and Friday, August 7 across northeast Michigan locations, and should begin to steadily decline in coming days.

Table 1. Western bean cutworm trapped moth counts for 2020. Counts in each column are total moths counted in the week ending on the date in the heading of each column.
Trap	Class	24-Jul	31-Jul	7-Aug	Total
Hawks	Dark Red Kidney	17	739	240	996
Hagensville	Navy	24	256	305	585
Posen	Great Northern	6	96	191	293
Polaski	Navy	40	154	373	567
Lachine	Black	148	530	525	1203
Herron	Navy	82	176	155	413
Ossineke	Navy	33	261	542	836
Spratt	Light Red Kidney	254	506	404	1164
Lachine	Light Red Kidney	54	157	236	447
Rust	Great Northern	134	179	233	546
Hillman	Navy	15	61	169	245
Royston	Navy	21	26	91	138
Metz	Navy	25	158	274	457
Metz	Dark Red Kidney	19	280	329	628
Millersburg	Dark Red Kidney	10	250	111	371
Onaway	Corn	11	149	47	207
Moltke	Dark Red Kidney	65	650	375	1090
Red shading* indicates high flight trigger to begin scouting the following week.

In areas where trap counts reached high flight, growers are encouraged to begin scouting dry bean fields approximately one week after cumulative counts exceed 120 for feeding injury on blossoms and pods. Observing western bean cutworm larvae directly is difficult because they are mostly active at night.

Scouting for this pest is important because high moth numbers in pheromone traps may not directly translate to lots of eggs and larvae in the field, particularly in a dry year when female moths can struggle to survive and produce eggs. Western bean cutworm flight, egg-laying and damage is also quite variable across the landscape and can be patchy within fields, which makes careful scouting critical. Factors that seem to contribute to higher western bean cutworm moth numbers include susceptible crops in rotation (corn and dry beans) and coarse soil texture.

When deciding if an insecticide application is appropriate, dry bean growers must consider their market and tolerance for damage. Pod damage of 10% will result in a 2% pick for damaged seeds, so thorough scouting is required to determine if insecticide applications are necessary. Insecticide applications should be timed to control the bulk of larvae shortly after they hatch, which is usually about a week after peak flight. As always, applicators need to follow label instructions, including pre-harvest intervals. Many insecticides effective against western bean cutworm are restricted-use pesticides requiring an applicator license to purchase and apply.

Insecticides can also kill non-target and beneficial insects, which may cause secondary problems, as in the case of flaring spider mite infestations by removing their natural predators.

For questions, or updates on current western bean cutworm trap numbers in northeast Michigan, contact MSUE field crops educator Christian Tollini at 231-268-9404 or tollinic@msu.edu.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).