Michigan State University Extension is encouraging farmers in northeastern Lower Michigan to scout their dry beans and corn for western bean cutworm over the next two weeks.
“Our moth numbers are the highest I have seen since we started monitoring in 2013,” said MSUE educator James DeDecker, who works out of the Presque Isle County office in Rogers City.
Western bean cutworm (WBC) is a pest of dry beans and corn that first migrated into Michigan from the Western United States in 2006.
Adult WBC moths emerge from the soil in July and early August to mate, laying their eggs on the leaves of dry bean and corn plants (Figure 1).
In years when corn has yet to tassel during WBC flight, eggs masses are first deposited in corn before the moths eventually move on to dry beans. If corn has already tasseled when the moths emerge, as was the case this year, they tend to lay eggs mostly in dry beans.
Once hatched, developing larvae feed on the silks and young ears of corn, or the blossoms, pods and immature seeds of dry beans, ultimately reducing crop yield and quality.
MSU Extension in Presque Isle County has monitored WBC flight through pheromone trapping since 2013. This year we recruited six local producers to participate in the Western Bean Cutworm trap network managed by the Canadian Corn Coalition.
Nine pheromone trapping sites were established in Millersburg, Moltke, Hawks, Posen, Bolton, Lachine, Heron, Hillman and Rust, Michigan adjacent to dry bean and corn fields (Figure 2).
An interactive map of all sites participating in the 2018 WBC trap network can be viewed here: https://www.cornpest.ca/wbc-trap-network/.
Traps have been checked weekly from July 11 through July 30, and we will continue monitoring until Aug. 15.
On July 30, four of our nine monitoring sites rapidly surpassed the cumulative threshold of 150 moths established for Michigan dry beans (Table 1).
Moth counts at these locations are already 2 – 3.5 times the previous record number we observed in 2016, and trapping will continue for another two weeks. We suspect that at least two other sites (Hillman and Rust) will likely reach threshold before monitoring is complete.
WBC flight has historically peaked during the second week of August in northeastern Lower Michigan. Once deposited, egg masses hatch in five to six days.
In areas where trap counts reach threshold, growers are encouraged to scout dry bean fields during the second and third weeks of August for feeding injury on blossoms and pods.
Observing WBC larvae directly is difficult because they are mostly active at night. Scouting for this pest is particularly 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.
WBC 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 WBC moth numbers include susceptible crops in rotation (corn and dry beans), dry weather, and coarse soil texture.
Dry bean growers must consider their market and tolerance for damage when deciding whether or not an insecticide application to control WBC is justified.
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 WBC are restricted-use pesticides requiring an applicator license to purchase and apply.
Insecticides can also kill non-target and/or beneficial insects, which may cause secondary problems, as in the case of flaring spider mite infestations by removing their natural predators.
Additional information on WBC management can be found at http://msuent.com/extension/.
2018 WBC monitoring locations across Northeast Lower MI
WBC moths trapped at monitoring sites since July 11, 2018