Bertha army worms are not expected to be a major problem in most of the Prairies in 1998.
A large percentage of bertha army worm larvae were destroyed by parasites in 1997, said Owen Olfert, a research scientist with the Agriculture Canada Research Centre in Saskatoon.
Olfert and other provincial agriculture officials measured bertha army worm populations last summer and found hot spots in some areas of Saskatchewan.
“Pupae dropped roughly in those areas, so it is the region where to expect activity this coming year,” he said.
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A large precentage of the pupae observed appeared to have been damaged by parasites.
“So probably what we’ll get is not moths emerging but parasites,” he said. “The news is pretty good for ’98.”
Bertha army worm damage peaked in 1995 and has been dropping off since then. Nevertheless, Olfert said an early warning system will probably be set up again this year.
That means “sentinel traps” with pheromones that attract the moths will be set up and checked regularly to see if alarming numbers turn up.
If so, producers have about three weeks before the eggs laid by the moths turn into damaging larvae.
According to a Saskatchewan Agriculture fact sheet, larval monitoring should begin about two weeks after peak trap catches and continue until the number of larvae exceed economic thresholds and the crop is sprayed, or until the crop is swathed.
Each field should be monitored. Adjacent fields might have different larval densities, depending upon how attractive the crop was when the moths were laying eggs. The maturity of the insects can also vary from field to field depending on when the eggs were laid.
Sample at least three locations a minimum of 50 metres apart.
At each location, mark an area of one-metre square and beat the plants in it to dislodge the larvae. The larvae are difficult to see and might be hidden underneath clumps of soil, in cracks or within curled leaves. Carefully search the soil and leaf litter.
Use the average number of larvae at the sites surveyed within each field to determine if it is economically viable to spray.
Twenty larvae per square metre can reduce yields by about one bushel an acre.
