Late last summer, a canola field heavily infested with bertha armyworm in western Manitoba created a lot of fuss on Twitter, proving, yet again, just how localized population spikes can be in some areas. While sudden population spikes can cause panic, experts recommend that growers only take measures after they’ve got a good handle on what’s in the field. That means scouting and looking for beneficial insects to see if fields have reached economic thresholds.
Justine Cornelsen, agronomy specialist with the Canola Council of Canada, and John Gavloski, entomologist with Manitoba Agriculture, explain.
Bertha armyworm is one of the most significant canola pests in Western Canada. It likes to feed on nectar, so it is particularly problematic during the crop’s flowering stage. Adult moths emerge from overwintering pupae in mid-June; emergence continues until late July. There is only one generation of bertha armyworm per year.
Within five days of emergence adult moths will begin to mate, laying their eggs on the undersides of canola leaves. Typically, the eggs will hatch within a week. Larvae will drop off the plants, making them difficult to locate during scouting. It takes about six weeks for them to completely develop. During that time they will pass through six growth stages. Making matters more difficult, bertha armyworm larvae look distinctly different in several of those stages.
Diligent scouting recommended
Justine Cornelsen, the Canola Council of Canada’s agronomy specialist for western Manitoba, says producers are typically caught off guard with bertha armyworm because of their timing. “Bertha armyworm typically occurs in the quiet time when producers have finished up spraying a fungicide for sclerotinia and are waiting for seed colour change to begin swathing,” she said.
Cornelsen warns growers not to overreact if populations in the neighbour’s fields are high. Just because your neighbor has them, doesn’t mean you do.
Hot, dry conditions favour the development of both bertha armyworm and diamondback moth, which is why populations were so high across the prairies in 2017. “Manitoba sustained several weeks of little to no precipitation,” she explained.
Continuous and proper scouting is crucial when managing this insect pest. The critical scouting period, said Cornelsen, is after peak flowering. She suggests scouting for economic thresholds and monitoring crops very closely as damage can happen quickly if the conditions are right.
“Economic thresholds have been developed to help producers keep money in their pockets and to further protect the environment,” said Cornelsen. “If a field has not reached the economic threshold, please do not spray. The field may look bad, but the cost of the application is not going to be recovered unless the threshold is hit.”
Environmental conditions and natural predators could take care of pest, so keep a close eye on the situation, said Cornelsen. “A strong rain event can cause the larvae to be pushed down in the canopy, killing them — or fungal pathogens form to help minimize populations,” she said. “Forecast is something to keep in mind when looking at economic thresholds.”
John Gavloski, entomologist with Manitoba Agriculture, agrees. “If you’re not at economic threshold you could be killing parasitoids,” he said.
There are two parasitoids that help control bertha armyworm levels in the field. The first is a parasitic wasp called Banchus flavescens that goes after the young larvae. The second is a parasitic fly called Athrycia cinerea. It’s a hairy fly that lays eggs on the caterpillars. Once born, the larvae crawl inside the bertha armyworm, killing it.
“Make sure you’re above economic threshold,” he said. “Don’t make assumptions based on what your neighbours are finding. In some fields, there might have been traps for the adults, but those traps do not do a good job of indicating what the larval numbers will be like in the field the trap is in.”
“Trap data is only to encourage you to scout,” he said. “It cannot be used to make management decisions.”
Gavloski offers these tips for thorough scouting:
- Measure out a quarter of a square meter.
- Once measured, give the plant a really good shake.
- Bertha armyworm doesn’t like light, so they spend the day on the ground and come out at night to feed. Count them very thoroughly, making sure to look in cracks and under leaves and stubble.
- “Once the count is done, multiply by four,” said Gavloski, who recommends repeating the measurement in five separate locations (at minimum) at least 50 meters apart. “That’s the number you want to use to make your management decision.”
- “Doing five thoroughly is better than doing 10 — 15 quickly,” he said.
Finally, Gavloski warns growers not to panic too early. Bertha armyworm prefers eating leaves and the leaves don’t impact the crop that much, economically speaking. Patience is key; if there’s still flowering going on in the field, wait. Canola yields 10 to 15 per cent better when there are pollinators in the field, and spraying will kill them too. If practical, wait until flowering is done, Gavloski concluded.