Trap counts in the black for bertha army worm, diamondback moth surge

Manitoba’s trap counts remained in the low risk classes until mid-July, when diamondback moth populations started to reach threshold in some fields

So far only six of 99 bertha traps indicate risk.

*[UPDATED: July 24, 2018] Anyone worried about bertha armyworm will welcome the provincial trap counts so far, but some farmers may be spraying for diamondback moth.

Diamondback moth is the only one of the two to report threshold populations, according to both trap data and Manitoba Agriculture entomologist John Gavloski.

Eighty-eight out of the province’s 99 bertha armyworm traps sat at “low risk” as of July 18, with less than 300 adults moths caught in each trap. Five of the remaining six traps, all in western or central Manitoba, sit only a step higher on the province’s risk scale.

The numbers are slightly higher than the week before, when 93 or 99 traps were classified as “low risk.”

“Basically, what it means is scout your field,” Gavloski said in a July 11 Crop Talk webinar. “When numbers get into that range, there’s the possibility of some localized issues. Usually, if things stay in that range, we’re not looking at widespread issues.”

Cumulative trap counts must top 900 adults before the province considers it at “moderate risk.” All lower counts are labeled as “low” or “uncertain risk.”

Only Tilsten, Man., tipped past those levels, with a trap there counting 960 adults as of July 18.

Diamondback moth

Canola growers have marked threshold populations of diamondback moth, despite initially optimistic trap counts.

As of July 11, 64 of 91 traps set this year had returned less than 10 moths.

“That doesn’t mean don’t scout,” Gavloski said at the time.

Populations have since passed economic thresholds in some parts of the east and south Interlake.

A “moderate” population blew into the Interlake, also grazing the northern part of central Manitoba this year, Gavloski noted.

“Just like bertha armyworm, their major damage is really to the pods,” he added. “Now that we’re getting some rains, if you’ve got good soil moisture, canola can compensate very well for damage to flowers, to buds, leaf feeding. The one thing it can’t compensate well for is pod feeding.”

Gavloski encouraged producers to take that damage risk into account when deciding whether to spray.

The province is reminding producers to base management on the number of larvae shaken from plants, not counts from a sweep net, while producers may also want to keep their eyes on the weather forecast.

“Larvae can complete their development quickly in hot temperatures, so larval populations can change in a field quickly,” the last insect and disease report said.

Check the forecasts, but still scout

Trap counts may still largely be low, but the province is also warning producers that traps are used for regional forecasting, not management field to field.

“The data is really meant to get you looking in your fields to help you prioritize your monitoring,” Gavloski said. “You cannot make control decisions based on trap counts. The traps only collect male (bertha armyworm) moths. The females are the ones that lay the eggs and they do not always lay their eggs in the same fields traps are in. In fact, there’s been many cases where fields that had a trap and had high trap counts had extremely low larval levels.”

The entomologist made a similar point for diamondback moth.

Keep pollinators in mind

Gavloski has also warned producers to stay away from tank mixing insecticide unless a pest has reached economic thresholds.

Producers might cause an unintentional yield loss if that same insect management kills off pollinators, he said, since crops like canola have shown greater yield if there are pollinators present.

“It’s enough of a yield bump that if you really aren’t at an economic threshold, tank mixing in that insecticide likely will be a losing proposition,” he said. “If you’re at the threshold for bertha armyworm or diamondback moth or any of the other insects that time of year, then it may pay off.”

The entomologist suggests growers might expect a 10 to 15 per cent boost in yield from pollinators.

*UPDATE: Article updated to reflect a changes reported in diamondback moth numbers.

About the author


Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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