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)CLB Counts
Brant2 egg masses
Bruce4 adults
Chatham-Kent2 larvae
Haldimand5 adults, 1 larvae
Halton1 adult
Huron18 adults, 6 egg masses
Elgin4 adults
Perth7 adults, 2 egg masses
Simcoe2 adults
Middlesex3 adults, 1 larvae, 6 egg masses
Lambton4 larvae, 3 egg masses
Waterloo2 adults, 2 egg masses
Table 1: Average cereal leaf beetle counts for the week of May 13th to May 17th.
Location (County) TAW Moth Counts
Brant2 moths
Bruce2 moths
Chatham-Kent37 moths
Haldimand1 moth
East1 moth
Elgin1 moth
Simcoe1 moth
Middlesex1 moth
Lambton1 moth
Waterloo2 moths
Wellington1 moth
Table 2: Average true armyworm counts for the week of May 13th to May 17th.

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 at head emergence stages which is approximately 10-14 days ahead of normal. Some fields near Harrow are now at anthesis. While fields in southern Ontario are at or quickly approaching anthesis, 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 47 (Flag leaf sheath opening)
BruceGS 39 (Flag leaf collar just visible)
Chatham-KentGS 57 (3/4 of head emerged)
PerthGS 39 (Flag leaf collar just visible)
PeterboroughGS 37 (Flag leaf just visible)
HaldimandGS 41 (Flag leaf sheath extending)
HuronGS 39 (Flag leaf collar just visible)
LambtonGS 39 (Flag leaf collar just visible)
MiddlesexGS 39 (Flag leaf collar just visible)
WaterlooGS 39 (Flag leaf collar just visible)
EastGS 37 (Flag leaf just visible)
EssexGS 57 (3/4 of head emerged)
ElginGS 39 (Flag leaf collar just visible)
Table 3: Average winter wheat growth stages for the week of May 13th to May 17th.

Winter Wheat Growing Degree Days (GDDs) January 1 – May 16, 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 262-314 GDDs in northern Ontario to 510-780 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 YR Norm (11-20)
MOUNT FOREST667503510412
MOOSE CREEK676497549

Table 4: Growing Degree Day Accumulations across Ontario from January 1, 2024 to May 16, 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.