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.

County

Peak WBC Moth Flight Week Accuracy of Prediction
GDD3.3C Predicted Observed
Essex July 15 – 21 July 22 – 28 Too Early
Chatham-Kent July 22 – 28 July 22 – 28 Accurate
Lambton July 22 – 28 July 22 – 28 Accurate
Middlesex July 22 – 28 July 29 – Aug 4 Too Early
Elgin N/A July 22 – 28 N/A
Norfolk July 22 – 28 July 22 – 28 Accurate
Haldimand N/A July 29 – Aug 4 N/A
Niagara July 22 – 28 July 22 – 28 Too Late
Brant July 22 – 28 July 22 – 28 Accurate
Oxford N/A July 29 – Aug 4 N/A
Perth N/A July 29 – Aug 4 N/A
Waterloo July 29 – Aug 4 July 22 – 28 Accurate
Wellington Aug 5 – 11 July 29 – Aug 4 Too Late
Huron July 29 – Aug 4 July 29 – Aug 4 Accurate
Bruce Aug 5 – 11 July 29 – Aug 4 Too Late
Grey N/A Aug 5 – 11 N/A
Dufferin Aug 5 – 11 July 29 – Aug 4 Too Late
Simcoe Aug 5 – 11 July 29 – Aug 4 Too Late
Kawartha Lakes Aug 5 – 11 July 29 – Aug 4 Too Late
Northumberland Aug 5 – 11 July 29 – Aug 4 Too Late
Prince Edward Aug 5 – 11 July 22 – 28 Too Late
Lennox and Addington N/A July 22 – 28 Too Late
Frontenac Aug 5 – 11 July 22 – 28 Too Early
Renfrew July 22 – 28 July 29 – Aug 4 Too Early
Leeds and Grenville July 29 – Aug 4 July 29 – Aug 4 Accurate
Lanark N/A July 29 – Aug 4 N/A
Ottawa July 22 – 28 July 29 – Aug 4 Too Early
S, D and G N/A July 29 – Aug 4 N/A
Prescott and Russell N/A July 29 – Aug 4 N/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: https://arcg.is/0Lry5a.

Canadian Corn Pest Coalition: https://cornpest.ca/corn-pests/western-bean-cutworm/

Ontario Dry Bean Agronomy: https://drybeanagronomy.ca/insects/

WBC Degree Day Prediction Model Paper: https://academic.oup.com/jee/article-abstract/108/4/1728/2379857?redirectedFrom=fulltext

Field Crop News: https://fieldcropnews.com/

OMAFRA Publication 812, Field Crop Protection Guide: http://www.omafra.gov.on.ca/english/crops/pub812/p812toc.html

Weather Data

Location

Year

Weekly August 16- August 22

Accumulated

Highest Temp (°C) Lowest Temp (°C) Rain (mm) Rain (mm) April 1st GDD 0C April 1st  GDD 5C April 1st CHU May 1st
Harrow 2021 31 14 9 438 2529 1827 2685
2020 30 12 13 309 2431 1742 2669
2019 29 14 35 405 2379 1687 2517
Ridgetown 2021 31 13 17 414 2391 1697 2532
2020 31 9 17 315 2314 1640 2517
2019 29 11 21 535 2252 1563 2373
London 2021 31 13 13 342 2371 1684 2499
2020 29 9 8 338 2259 1587 2433
2019 29 12 61 513 2153 1489 2291
Brantford 2021 32 12 10 300 2345 1655 2451
2020 32 8 0 211 2285 1619 2420
2019 30 11   274 2227 1550 2350
Welland 2021 30 13 3 306 2360 1667 2500
2020 29 10 3 272 2317 1644 2529
2019 29 15 38 400 2288 1595 2449
Elora 2021 30 11 1 266 2174 1493 2280
2020 29 7 2 282 2097 1450 2265
2019 28 12 53 411 1758 1155 1833
Mount Forest 2021 29 11 33 361 2178 1500 2300
2020 29 8 9 396 2074 1440 2281
2019 29 10   129 2006 1347 2143
Peterborough 2021 31 7 1 301 2184 1492 2268
2020 29 5 18 220 2085 1440 2223
2019 28 6 11 345 1988 1325 2042
Kingston 2021 29 10 10 248 2246 1555 2382
2020 26 10 30 285 2267 1589 2488
2019 28 12 12 359 2203 1514 2344
Kemptville 2021 33 9 2 266 2346 1649 2418
2020 29 7 40 275 2240 1572 2403
2019 31 11   203 2104 1437 2188
Earlton 2021 32 13 0 452 2042 1370 2086
2020 27 10 11 323 1886 1301 2089
2019 30 9 0 349 1652 1074 1760
Sudbury 2021 31 12 0 366 2095 1424 2183
2020 24 7 18 399 1958 1348 2173
2019 28 8 30 381 1699 1116 1819
Thunder Bay 2021 33 7 0 247 1932 1274 1983
2020 26 6 39 219 1759 1180 1925
2019 29 7 26 291 1601 1015 1685
Fort Frances 2021 33 4 13 215 2043 1379 2141
2020 30 5 4 292 1917 1301 2085
2019 29 7 79 372 1727 1112 1813
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 www.fieldcropnews.com.