Weekly Cereal Pest, Winter Wheat Growth Staging and GDD Update

Cereal Leaf Beetle and True Armyworm Update

Cereal leaf beetle (CLB), and true armyworm are common cereal pests in Ontario. To help support growers in their management of these pests and to better understand their prevalence, 28 fields across Ontario are being scouted weekly for cereal leaf beetle (CLB) and true armyworm (TAW). Crop growth stages are also being monitored.

CLB and TAW have been found at some locations but continue to be well below threshold (Table 1 and 2). All trap data can be found at The Great Lakes Pest Monitoring Network.

Location (County)Average CLB Counts
Brant1 adult
Chatham-Kent2 egg masses
Haldimand1 adult, 2 egg masses
Halton7 adults
Waterloo1 adult
Table 1: Average cereal leaf beetle counts for the week of April 29th to May 3rd.
Location (County)Average TAW Moth Counts
Brant1 moth
Simcoe1 moth
Waterloo1 moth
Wellington1 moth
Stormont, Dundas and Glengarry0
Table 2: Average true armyworm counts for the week of April 29th to May 3rd.

The best time to scout for TAW is shortly after dusk when larvae are actively feeding.  In cereals and mixed forages, examine 10 areas of the field, assessing the number of larvae per 30 cm x 30 cm (1 ft2). Pay particular attention to the border area directly adjacent to other grassy host crops. During the day, if it is cloudy and overcast, you might be lucky enough to see larvae on the head of the plant but on sunny days, they will be down on the ground among the crop debris or under soil clods. Brown frass may also be present on the plants and on the soil surface. Birds diving into your field is a good indication that there are good eats there so take a look.

When you do find larvae, look for any white eggs that may be attached to the backs of the armyworm larvae. This is a sign that the larvae have been parasitized by one of its parasites which have done the job for you. Avoid treating with insecticides when large numbers of parasitized larvae are present as they have already been controlled by parasitoids or when larvae are close to 2.5 cm in length, as insecticides will no longer be effective, and the larvae will soon stop feeding.

Chemical control for true armyworm is warranted if there are 4 to 5 un-parasitized larvae per square foot (30 cm x 30 cm). If a significant amount of wheat head clipping is occurring, spray may be warranted if larvae are still actively feeding, are smaller than 2.5 cm and pre-harvest intervals have not been reached.

For CLB, controls are needed if an average of three larvae per tiller are found before boot stage, or one CLB adult or larvae per stem after boot but prior to heading. Chemical control products for winter wheat against TAW and CLB can be found on the Crop Protection Hub. Currently, TAW levels are low across Ontario, and no insecticide treatments are recommended. CLB has been seen in select counties, with some higher prevalence in more southern regions compared to eastern and northern Ontario.

Winter Wheat Growth Stages

Winter wheat is progressing quickly through growth stages with fields in southern Ontario now reaching flag leaf which is approximately 10-14 days ahead of normal. Some fields near Harrow are now at the early boot stages. Once the flag leaf emerges herbicide applications should be avoided. While fields in southern Ontario are at or quickly approaching flag leaf, many areas further east and north are still in the early growth stages and are about 7 days ahead of normal (Table 3).

Location (County)Average Growth Stage (Zadoks)
BrantGS 32 (2nd node)
BruceGS 31 (1st node)
Chatham-KentGS 33 (3rd node)
PeterboroughGS 30 (Stem Elongation)
HaldimandGS 32 (2nd node)
HaltonGS 31 (1st node)
SimcoeGS 31 (1st node)
LambtonGS 32 (2nd node)
MiddlesexGS 32 (2nd node)
WaterlooGS 31 (1st node)
WellingtonGS 31 (1st node)
EssexGS 37 (flag leaf just visible)
Stormont, Dundas and GlengarryGS 31 (1st node)
Table 3: Average winter wheat growth stages for the week of April 29th to May 3rd.

Winter Wheat Growing Degree Days (GDDs) January 1 – May 5, 2024

Growing Degree Days (GDDs) can be a helpful tool to track cereal crop growth and development. GDDs are calculated by adding the average daily temperatures during the cereal crop’s growing season. If the daily average temperature is equal to or less than 0˚C, the degree-day value is zero and no GDDs were accumulated for that day. It takes approximately 100 GDDs for each new leaf to appear on a winter wheat plant, which is a period known as the phyllochron. The environmental conditions at planting plays a role in setting the length of the phyllochron, with early planted wheat generally having a longer GDD requirement than later planted wheat. Once winter wheat emerges, you can begin calculating the number of growing degree days accumulated. Since January 1, 2024, Growing Degree Day (GDDs) accumulations across Ontario have ranged from 175-300 GDDs in northern Ontario to 550-650 GDDs in southwestern Ontario (Figure 1 and Table 4). The Visual Guide to Winter Wheat Staging is a helpful resource for identifying crop growth stages.

Figure 1: Accumulated GDDs since January 1, 2024 across Ontario compared to 2023, 2022, and the 10-year average.
Location20242023202210 Year Norm
St. Catherines582466393
Mount Forest406290248200
Moose Creek384289284
Table 4: Growing Degree Day Accumulations across Ontario from January 1, 2024 to May 5, 2024 compared to 2023, 2022 and the 10-year average.


Watch for True Armyworm and Cereal Leaf Beetle – Field Crop News

Managing Winter Wheat with Growing Degree Days – Field Crop News

Historical Data – Climate – Environment and Climate Change Canada (weather.gc.ca)

Growing-Degree Days and Development of the Wheat Plant

A Visual Guide to Wheat Staging

Haun, J. R. (1973), Visual Quantification of Wheat Development. Agronomy Journal, 65: 116-119.