Crop Report – August 25, 2021

It is safe to say that western bean cutworm (WBC) is a pest that we can rely on entering corn and dry beans every year in Ontario. Though a frustrating pest to scout for, using traps to monitor moth flight has helped us better predict when to expect peak moth flight occurs, which is shortly followed by peak egg laying. For several years now, moth flight has started to ramp up in Ontario during the week of July 15th, three weeks prior to peak flight. Based on average trap counts from the 500+ traps in Ontario on the Great Lakes and Maritimes Pest Monitoring Network (GLMPMN) this year, peak moth flight occurred during the week of July 28th to August 4th; the same week it has been doing so since 2018 (Figure 1).

Figure 1. Weekly WBC trap catches for Ontario. Peak flight occurred during the week of July 29th to August 4th.
Figure 1. Weekly WBC trap catches for Ontario. Peak flight occurred during the week of July 29th to August 4th.

Peak flight occurred the same week for both field corn and dry bean traps while sweet corn traps peaked a week earlier. As always, dry bean traps captured more moths during that period than traps next to field and sweet corn fields (Figure 2).

Figure 2. WBC trap catch by host crop. Dry bean and field corn traps peaked during the week of July 29-Aug 4th, while sweet corn traps peaked one week earlier.
Figure 2. WBC trap catch by host crop. Dry bean and field corn traps peaked during the week of July 29-Aug 4th, while sweet corn traps peaked one week earlier.
Figure 3. Average WBC trap counts by county, sorted by peak flight periods.
Figure 3. Average WBC trap counts by county, sorted by peak flight periods.

When we look more closely at trap counts by county however, peak flight timing varied and wasn’t entirely dependent on geography or degree day (DD) accumulation (Figure 3). Some counties peaked the week of July 22-28, while others peaked the week of July 29 – August 4th. Grey county observed peak flight later than all other counties across Ontario (August 5th – 11th). Several DD models have been developed to help predict WBC flight activity. Hanson et al. 2015 is considered the most reliable so far by many states. It uses 3.3C as the base temperature, calculating degree days starting on March 1st. According to the model, 50% of moth flight (peak flight) occurs when 1502 DD have been accumulated. Using weather data and trap counts for Ontario this year, the model accurately predicted peak flight for some locations but not all. Table 1 shows the predicted versus observed peak flights by county across Ontario. Predicted peak flight was too early or too late for some counties. We hope to make refinements to this model to predict WBC development more accurately in Ontario. This will help improve both scouting and application timing to enable the most effective management possible.

Table 1. WBC predicted versus observed peak moth flight by county using the Hanson et al. 2015 model and trap counts from the GLMPMN.


Peak WBC Moth Flight WeekAccuracy of Prediction
GDD3.3C PredictedObserved
EssexJuly 15 – 21July 22 – 28Too Early
Chatham-KentJuly 22 – 28July 22 – 28Accurate
LambtonJuly 22 – 28July 22 – 28Accurate
MiddlesexJuly 22 – 28July 29 – Aug 4Too Early
ElginN/AJuly 22 – 28N/A
NorfolkJuly 22 – 28July 22 – 28Accurate
HaldimandN/AJuly 29 – Aug 4N/A
NiagaraJuly 22 – 28July 22 – 28Too Late
BrantJuly 22 – 28July 22 – 28Accurate
OxfordN/AJuly 29 – Aug 4N/A
PerthN/AJuly 29 – Aug 4N/A
WaterlooJuly 29 – Aug 4July 22 – 28Accurate
WellingtonAug 5 – 11July 29 – Aug 4Too Late
HuronJuly 29 – Aug 4July 29 – Aug 4Accurate
BruceAug 5 – 11July 29 – Aug 4Too Late
GreyN/AAug 5 – 11N/A
DufferinAug 5 – 11July 29 – Aug 4Too Late
SimcoeAug 5 – 11July 29 – Aug 4Too Late
Kawartha LakesAug 5 – 11July 29 – Aug 4Too Late
NorthumberlandAug 5 – 11July 29 – Aug 4Too Late
Prince EdwardAug 5 – 11July 22 – 28Too Late
Lennox and AddingtonN/AJuly 22 – 28Too Late
FrontenacAug 5 – 11July 22 – 28Too Early
RenfrewJuly 22 – 28July 29 – Aug 4Too Early
Leeds and GrenvilleJuly 29 – Aug 4July 29 – Aug 4Accurate
LanarkN/AJuly 29 – Aug 4N/A
OttawaJuly 22 – 28July 29 – Aug 4Too Early
S, D and GN/AJuly 29 – Aug 4N/A
Prescott and RussellN/AJuly 29 – Aug 4N/A
N/A = no weather data available for this county

With peak flight consistently occurring for the last four years during the last week of July, it is safe to plan to start scouting corn no later than the week July 15th each year, starting first in fields that are approaching tasseling stage. Scout weekly, looking for egg masses, up to the peak flight period or until R3 stage of corn has been reached. If during those scouting periods, 5% of the plants have had egg masses on them, an insecticide is required. Soon to be published Ontario research indicates that some WBC insecticides provide up to 2 weeks of protection against WBC. Therefore, an application made the week prior to peak flight (i.e. July 22nd-28th) or during peak flight (July 29th-August 4th) will provide the most protection during the critical period of WBC development.

Over that two-week period, many of the eggs will be laid in these fields and larvae will have hatched. Keep in mind however, no insecticide provide 100% control and larvae will still be found in some ears later in the season. Egg laying does continue after peak flight, especially in later planted fields still in R1 or R2 stage. Some escapes should be expected. Yield loss is not the main concern with this pest. The concern is the wounds they leave in the ear that helps encourage ear mould development.

In a year like this one, with ideal weather conditions for ear mould development, scouting in early to mid-September is important, regardless of whether an insecticide or fungicide application was made. Any fields found to have larvae in the ears or ear mould developing should be harvested early. Plan to segregate the grain from these problem areas from the rest of the field to avoid contamination.

WBC management in dry beans is different, given that this pest is nearly impossible to scout and successfully find eggs or larvae in the field. Dry bean fields with trap catches of 150 or more moths should receive an insecticide application 10 to 21 days after peak moth flight if pods are developing on the plants. This would mean somewhere between August 10 and 21st would be a good time to apply an insecticide to protect from WBC feeding, targeting the earliest planted fields first.

Always make sure to rotate your management tools annually for this pest. Don’t rely solely on one particular insecticide or Bt corn trait (i.e. Vip3A) as this pest has great potential for developing resistance to these tools.

More information western bean cutworm can be found at:

Great Lakes and Maritimes Pest Monitoring Network:

Canadian Corn Pest Coalition:

Ontario Dry Bean Agronomy:

WBC Degree Day Prediction Model Paper:

Field Crop News:

OMAFRA Publication 812, Field Crop Protection Guide:

Weather Data



Weekly August 16- August 22


Highest Temp (°C)Lowest Temp (°C)Rain (mm)Rain (mm) April 1stGDD 0C April 1st GDD 5C April 1stCHU May 1st
20193011 274222715502350
Mount Forest2021291133361217815002300
20192910 129200613472143
20193111 203210414372188
Thunder Bay20213370247193212741983
Fort Frances202133413215204313792141
Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at