By Harry Brook
The crop is starting to emerge. What insect pests should you be on the lookout for?
Canola starts out as a tiny, fragile plant, once germinated. Insignificant flea beetles can really devastate a canola crop at the cotyledon stage. It isn’t helped when we have cool periods in spring where the canola growth stalls. This is the time to be vigilant about flea beetles.
Flea beetles overwinter as adults under litter, at the edges of a field. Both the two-stripe and the crucifer flea beetle really like canola. They also feed on cruciferous weeds like shepherd’s purse and stinkweed. They emerge in the early spring when temperatures rise to 14˚ C. Hot, dry, weather favours beetles moving into a field. At temperatures above 18˚ C, they readily fly. When it is colder than 18˚C, they move into canola fields by hopping and will mostly be a problem around field edges.
They are most destructive and should be controlled when they have consumed 25% or more of the cotyledons and first true leaf. They also kill seedlings when they feed on the growing point. Under good growing conditions the plants can outgrow the damage if the plants are rapidly growing. The insecticide part of a seed treatment is only effective for a couple of weeks after seeding. Also, it is less effective on two-stripe flea beetles as compared to crucifer flea beetles, and flea beetle populations are shifting to mostly two-stripe.
Wireworms can also be a problem in germinating crops. Especially as pastures and hay are being broken up and used for annual crops. Wireworm are larvae of the click beetle and there are several problem species that feed on a wide range of germinating crops. They feed on the roots as the seedling starts to grow, killing it before or just after emergence. They prefer grass species but will feed on any root. The larvae take 3 to 5 years to mature to adults and larvae feed primarily in early to mid-spring. Once the soil surface heats up, they go deeper into the soil and become dormant.
Our most common seed treatments, like neonics, do not kill the larvae, they just prevent them from feeding. Often, they show up as bare patches in a field of emerging wheat or canola. If you suspect wireworm, look in the top 3 to 4 inches of soil along the edges of the blank area. The worms are light brown bodies with dark brown head capsules. Seed treatment is the only control.
Cutworms are also found in crop fields. There are about half a dozen species that will be found feeding on emerging crops. Different cutworms like different crops. The damaging larvae are from eggs laid by adult cutworm moths in the previous fall. Most cutworms overwinter as eggs, hatching in the spring and begin feeding on available plants. Scouting your fields frequently during the first two or three weeks after seeding is necessary to spot any bare areas developing, particularly if they first occur on south facing slopes. Those areas need to be investigated to determine the cause. Most cutworms feed above ground, but usually at night. Pale Western cutworm stays below ground all the time, making it difficult to control.
A rough rule of thumb to decide if they should be controlled is when there has been a 25 to 30% stand reduction. It may be possible to patch spray for control. It is essential to check on the larvae to see if they are still feeding or approaching the pupal stage, when they stop feeding. The most damaging stage is when larvae are young, one-quarter to half an inch long, as they still have lots of feeding to go before pupation. If they are approaching the pupal stage, there is very little to be achieved by spraying and you may inadvertently be killing off beneficial insects.
Once again, I cannot overemphasize the importance of scouting your fields. Early detection gives you time to make the right control option. Sometimes, the right thing to do is nothing.