Western bean cutworm at peak flight in northeast Michigan

With high numbers, growers are encouraged to scout and consider control.

Map of trap locations
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 will begin to emerge from the soil in July to mate, and then proceed to deposit eggs in both corn and dry beans. If corn has already tasseled at the time of emergence, most eggs will be laid in dry beans. Once deposited, egg masses will hatch in five to six days, and feeding on the crop will begin.

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 can lead to poor pollination of corn and undesirable ear damage, particularly in sweet corn. In dry beans, reduced yields and seed quality can be expected.

To help producers mitigate the risk of western bean cutworm damage in dry beans, Michigan State University Extension Presque Isle County cooperates with dry bean growers across Presque Isle, Montmorency and Alpena counties to monitor western bean cutworm moth flight. By using pheromone traps, weekly moth counts are recorded to identify peak flight, as well as if/when the economic threshold is met to help determine if insecticide applications are necessary. This year, 10 traps were set near Onaway, Millersburg, Moltke, Hawks, Bolton, Long Rapids, Lachine, Herron, Royston, and Hillman (see photo). 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.

Despite unusual weather and later emergence than the previous growing season, we have recorded our highest moth numbers since monitoring began in 2013. As of Aug. 12, 2019, six of the 10 locations have exceeded the economic threshold established for Michigan dry beans, which is 150 cumulative moths trapped (see table).

In areas where trap counts reach threshold, begin scouting dry bean fields approximately one week after threshold is met for feeding injury on blossoms and pods. Observing western bean cutworm larvae directly is difficult because they are mostly active at night.

2019 Western bean cutworm data (shaded/bold totals above threshold)

Location

Trap set

July 22

July 29

Aug. 5

Aug. 12

Total

Onaway

July 19

0

2

23

49

74

Millersburg

July 19

0

5

75

219

299

Moltke

July 19

0

12

210

696

918

Hawks

July 19

0

7

50

1043

1100

Bolton

July 18

0

7

43

91

141

Long Rapids

July 18

0

7

100

245

352

Royston

July 18

0

8

41

86

135

Hillman

July 18

0

21

45

112

178

Lachine

July 18

0

2

19

51

72

Herron

July 18

0

6

103

98

207

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, egglaying 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), dry weather and coarse soil texture.

When deciding if an insecticide application is appropriate, dry bean growers must consider their market and tolerance for damage. Insecticide applications should be timed to control the bulk of larvae shortly after they hatch, 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.


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