Late-season insects and pre-harvest intervals

August 22, 2018 - Issue 21

We had reports this week of bertha armyworm at higher numbers (maybe not at thresholds) in some very localized areas, lygus (it’s getting late), flea beetles (don’t spray them, it won’t help for next spring) and aphids (probably don’t spray them either).

While doing a pre-harvest scouting for disease, take a look at the insect situation.

Bertha armyworm. If only a small area is at threshold, a localized spray can be very effective, but find a product that fits the pre-harvest interval. Go to spraytoswath.ca, enter the days until cutting (swathing or straight combining) and see which products are available for that window. Bertha armyworm scouting tips and thresholds. Check leaves for chewing and identify the insect responsible. Once the leaves die off, or are eaten, the next source of food is the pods – where berthas are easy to spot from the road.

Lygus. Count lygus and compare numbers to thresholds. Note that if swathing is a week away, pods may be too tough for lygus to penetrate. Lygus scouting, thresholds and timing. If choosing to spray, find a product that fits the pre-harvest interval. If lygus are at thresholds and you still have a spray window, insect specialists are looking for canola fields to validate economic thresholds for lygus. If you want to participate in this trial, email CCC agronomy specialist Keith Gabert at gabertk@canolacouncil.org.

Flea beetles. Flea beetles feeding on canola leaves and pods are unlikely to cause an economic loss. Entomologists have not set thresholds for late season flea beetle feeding, but it’s generally believed that numbers have to be very high — perhaps 100 per plant — before economic losses occur. You may also note that flea beetles can be highly variable at this time of year, with high numbers on some plants and next to none on others. Read more.

Aphids. Noticeable aphid clusters can sometimes be found on a few plants this time of year. Even if aphids cover the entire top 10 to 15 cm (4 to 6″) of plants, damage is rarely significant because so few plants are affected (usually) and the bulk of pod formation has been completed. Even if the top few small pods are damaged, they contribute little to the overall yield. The nominal thresholds for turnip aphid or cabbage aphid in canola is when 10-20% of stems have aphid clusters. Read more.

Further reading:

Spray-to-swath calculator to check on product pre-harvest intervals for canola
Scouting and field survey tasks for this week

Canola Watch