Young canola seedlings are especially vulnerable to cutworms, flea beetles and seedling diseases, particularly if they are struggling through cool temperatures or frost, damage from wind or hail, and drought or excess moisture.
Early scouting can help ensure canola crops make it through the first few weeks in good shape. Here are a few pests and scenarios to watch for:
Flea beetle seed treatment may not be enough. Depending on the seed treatment rate and how long it took canola to emerge, flea beetle protection could start to taper off within the first 21 days after emergence. When 25% of the leaf area is damaged or missing, farmers should have their sprayers ready to go. The economic threshold is about 50% of leaf area missing or damaged, but flea beetle damage can move from 25% to 50% within hours if the insects are active and canola is growing more slowly than the rate of feeding.
Under warm calm conditions, fields should be scouted 4 to 5 times a week through the high-risk period. And while flea beetles do slow down in cool, wet weather, they may take refuge on stems and the undersides of leaves. Growers are advised to watch for stem feeding, which can be even more damaging.
Big bald patches? Could be cutworms. Growers are advised to dig down and check for cutworms in fields with distinct patches where plants are missing entirely, or where plants are drooping or appear cut off. Cutworm damage usually appears first on light textured soils, hilltops and south facing slopes where soil temperatures are warmer. To scout, growers can use a trowel dig in the top 2″ of soil and sift to uncover cutworms. Cutworms prefer dry soil, but in 2010 were found feeding in moist fields. If digging in moisture, cutworms tend to be close to the soil surface.
The preferred time to spray is in the evening or at night. Click here for more on cutworm identification, scouting and management.
Fast-emerging canola can outgrow seedling diseases. Canola seeded shallow into warm, moist soils will emerge quickly, growing through the seedling disease window while the seed treatment is still strong. Continue to watch for signs of disease while scouting for insects.
Seedling diseases caused by fusarium and pythium species tend to be worse in cool, wet conditions while rhizoctonia tends to be worse when canola emergence is delayed due to cool, dry conditions. The broad spectrum fungicide seed treatments used to treat certified seed typically provide good protection from these seedling diseases.
The tell-tale sign of disease is patchy emergence up to the four-leaf stage of the crop. Seeds or seedlings may decay prior to emergence or the seedlings may emerge and appear normal, then stagnate at the two- to four-leaf stages. These plants will often die.
But some of these symptoms also point to cutworm damage. This is why close scouting early in the season is important. Act quickly because at these early stages, dead plants will decay and disappear quickly, making it impossible to accurately identify the cause. Determining the exact cause of emergence issues will determine if control measures can be used. It is the first step in avoiding similar problems in the future.
For more on early season pests, read the Canola Council of Canada’s factsheet, “Factors affecting canola survival from seedling to 21 days after emergence”.