Do Not Let Western Bean Cutworm Catch You Off-Guard

Eileen Cullen, Extension Entomologist

Wisconsin looks to be in for our highest Western Bean Cutworm pressure yet. Don’t let WBC larvae in field and processing sweet corn ears catch you off guard in August and September. Scouting now is critical to determine field infestation and prevent populations from reaching economic injury level. Economic (treatment) thresholds of 8% infestation for field corn, and 4% infestation for processing sweet corn, are set to prevent densities from reaching the economic injury level.

Please refer to last week’s Wisconsin Crop Manger article titled Western Bean Cutworm Moths Flying, Scout Field and Sweet Corn for full information on managing this late season corn ear pest. In addition to the article, several links are provided to the UW Entomology educational package on managing this pest (UW Extension fact sheet, pheromone trapping, economic thresholds for field and sweet corn, webinars, photo galleries, plus a primer on using insect degree days to predict emergence and egg laying for WBC). All of these resources are web based and available for free download.

Andy Barta, Agronomist with Rio Creek Feed Mill, reported 7/28, that Western Bean Cutworm (WBC) moth pheromone trap captures are high in Northern Door County (all north of Sturgeon Bay). Andy is hearing from three different farmers about high trap captures (one farmer stated he is catching about 40 moths per day this past week). Another large grower in that area scouted and found about 10% of plants in fields monitored with white to purplish colored egg masses. That is above the 8% treatment threshold for field corn and the grower is planning to spray next week as small larvae hatch. Areas in and around Door County had economic populations of WBC larvae in corn ears last year in 2008. Based on moth flight trap numbers and egg masses scouting thus far, I expect heavy pressure in the area again in 2009.

Bill Halfman, UW Extension Monroe County, reported this morning heavy WBC moth captures in his blacklight trap. Bill counted 165 moths/week on 7/23, and 73 moths/week 7/29. Neither blacklight nor pheromone trap captures tell you threshold numbers in a field (scouting will give that information, see article link above). However, these heavy moth flight detections certainly indicate we are potentially in for a heavy WBC larval population in the coming 1-2 weeks, depending on your location in the state and degree-day accumulations. 

Processing company agronomists have reported to me this week from the Central Sands area higher moth flights than this time last year, and they too are finding WBC egg masses in later planted fields. Processing sweet corn WBC management should take into account sweet corn planting date, and WBC moth emergence degree days. The earliest planted sweet corn (April) will be harvested in the coming 7-10 days, while the later planted acreage is closer to pre-tassel and row tassel stages. WBC moths prefer to lay eggs on late whorl stage corn through early tassel. Many sweet corn processors and growers run a 2^nd generation European Corn Borer (and Corn Earworm) trapping and insecticide management spray program. Keep in mind the degree-days (DD) for WBC and 2^nd generation ECB moth emergence and egg-laying are slightly off-set from each other. WBC moths and eggs are first. Approximately 25% of WBC moths have emerged (and egg-laying has begun) after 1,320 DD (base 50F). Second generation ECB moths emerge at 1400 DD (base 50F) and first eggs at 1450 (base 50F).

Thank you to Andy Barta, Bill Halfman, and processors for your reports. All field corn and processing sweet corn growers in Wisconsin should be alert and scouting for WBC now. Foliar insecticides effectively suppress WBC small larvae, but only if applied before larvae enter the ear. If you are growing WBC protected Bt corn hybrids (currently the Herculex I or Herculex XTRA traits), remember to check non-Bt refuges.