Bertha armyworms work their way up

Bertha armyworm spraying has been reported in a few fields. Scout lower in the canopy this week, looking for feeding on lower leaves. Spotting them down low will give you a couple weeks’ planning time before leaf drop begins and berthas start moving up toward the pods — where they do their most costly damage. By holding off until bertha larvae reach pods, you also give various natural controls — such as disease, fungal infections and weather — a chance to reduce the population and perhaps eliminate the threat.

Bertha armyworms can be green, brown or black. Each has a light brown head and a yellowish-orange stripe down each side. Black larvae also have three narrow, broken white lines on their backs.

It takes about a week for eggs to hatch, and then about 6 weeks for larvae to go through their growth stages and then pupate. Full size is about 1.5” long. When berthas are 3/4” long or longer and have moved up the plant, take counts and be prepared to spray if levels are high.

If canola is $12 per bushel and spray costs $8 per acre, the economic threshold is 11 larvae per square metre. At that number, a spray just pays for itself. Sprays applied at counts higher than 11 are more likely to provide some return on investment. The table below provides thresholds for various canola price and input cost scenarios.

Once the decision is made to spray, check the pre-harvest interval on the chosen insecticide. If the PHI is 21 days, for example, the product has to be applied 21 days or more before the crop is swathed or straight combined.

Scouting tips

—Go out in early morning or late evening when larvae are mostly active.
—Mark out an area a quarter-metre square (50 cm by 50 cm) and beat the plants growing within that area to dislodge the larvae. Count the larvae that have fallen to the ground and multiply by 4 to get the number per metre square. Larvae will hide under leaf litter and in cracks, so check closely.

—Sample at least 5 locations (10-15 is recommended) a minimum of 50 metres apart. Do not sample headlands and areas within the crop that are not representative of the field. Use the average number of larvae at the sites surveyed to determine if the economic threshold has been exceeded.

—Scout each field. Adjacent fields may have very different larval densities, depending on how attractive the crop was when the moths were laying their eggs. Adjacent fields may also have different-sized larvae, depending on when the eggs were laid.

—For best results, apply an insecticide as soon as economic thresholds are reached. A single well-timed application of any registered insecticide is usually effective. Check provincial crop protection guides for registered insecticides.

—Apply insecticides early in the morning or late evening when the larvae are actively feeding. Do not apply during warm afternoons.

MAFRI’s new bertha armyworm factsheet.

Exit holes. While scouting for bertha and lygus, check pods for exit holes. Growers in southern Alberta and southwestern Saskatchewan where cabbage seedpod weevil counts are highest are encouraged to check pods for exit holes from weevil larvae. If less than 25% of pods have exit holes, the spray could be considered a success. If less than 25% and the crop was not sprayed, it was probably the correct decision. If more than 25% and the crop was not sprayed, it probably should have been.