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Bertha Armyworm is a real risk for 2018 -
Scott Meers, Insect Management Specialist
With some bertha armyworm pheromone monitoring sites now showing above the first warning threshold (and one now above the second warning threshold of 900 moths) it is time to review what this means.
A trap adjacent to a particular field does not necessarily reflect the risk in that field. In fact in a study conducted in Minburn county during the last large outbreak the field adjacent to a trap with a catch in excess of 1500 moths was never sprayed. Many fields in the area were well above larval thresholds and were indeed sprayed. In addition that year there were many traps that exceeded the first (300 moths) and second (900 moths) and even some at the highest warning level (1200+ moths) warning thresholds. There was a widespread outbreak in that area.
So what does it mean when an individual trap is low when all around it are higher? It could mean the pheromone was not handled properly prior to setting out. Also, we have seen traps set up adjacent to pastures with lower counts while nearby ones adjacent to last years canola with higher counts. In our recent study looking at trap density we saw an effect of trap placement relative to last years canola but because we hadn’t set out to measure this we couldn’t provide statistical evidence of the phenomenon.
What does it mean when one trap goes really high when others around it remain low. It could mean that the trap was put in just the right place to catch a high local population. It could also be an unexplained anomaly as we often see. Over the past several years there have been times where a trap has had high counts but no outbreak has occurred. It is one of the hazards of a pheromone system, we don’t have all the answers as to why and how certain traps react the way they do.
In our experience a lone trap over 300 moths usually does not result in an outbreak. Rather some fields approach thresholds and the odd one may be over. More often localized spots within fields approach threshold.
When an area starts to get a mixture of traps near 300 moths, some over 300 and the odd one even over 900 moths then we are into potential outbreak territory. We are on the verge of this in Camrose/Beaver counties this year. In addition there is a smattering of traps that have gone over 300 throughout southern and central Alberta. This is still fairly low overall level risk but one that can not be ignored. It means we need to be scouting canola fields in the vicinity of the elevated trap catches.
Here is a link to our map https://www.agric.gov.ab.ca/app68/listings/bertha/bertha_map.jsp