EAST LANSING, Mich. — 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.|
|Hawks||Dark Red Kidney||17||739||240||996|
|Spratt||Light Red Kidney||254||506||404||1164|
|Lachine||Light Red Kidney||54||157||236||447|
|Metz||Dark Red Kidney||19||280||329||628|
|Millersburg||Dark Red Kidney||10||250||111||371|
|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.
— Christian Tollini, Michigan State University Extension
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