Soybean aphid and alfalfa weevil were the top insect pests in Manitoba last summer, says John Gavloski, an extension entomologist with the provincial agriculture department.
“For soybean aphid, it was more in east and in the central and south Interlake of the province. It was fairly widespread in our intensive soybean growing areas,” he said.
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“For the alfalfa weevil, I had reports of heavy damage right across the province. In the past, it has been more localized and restricted to certain areas, but this year every region seemed to have at least some fields that were affected.”
Soybean aphids don’t typically wipe out entire crops, but if present in large enough numbers, they can affect overall yield. Gavloski said dimethyl-8-based insecticides are registered for the aphid.
It’s still unclear whether they will continue to be a problem next year because the sap suckers, which drain the fluids of host plants, aren’t able to overwinter in Manitoba.
“Even in Minnesota, they did a study that found they could overwinter in the southern part but not very good in the northern part. So we’re making the assumption that they are blowing in,” he said.
For that reason, infestation levels for next year will depend on whether enough of them can become airborne and drift in from the United States and find favourable weather and a crop to call home.
“Their population varies wildly from year to year. We had a really bad infestation in 2006, and then in 2007 you could barely find them. And now in 2008, we’re back in an outbreak.”
Growers hit hard last year by aphids shouldn’t let that affect their seeding plans, he added, because the bugs might not show up in 2009.
Alfalfa weevils, which are defoliators that eat the leaves of alfalfa plants, are able to overwinter in Manitoba. It is still not understood what regulates their population: natural enemies or another factor.
“I don’t see that one disappearing within a year like the soybean aphid,” Gavloski said.
“We can reasonably expect relatively high levels of them in at least some areas next year.”
In severe infestations, the weevil can significantly reduce yields for seed and forage producers.
“They were numerous enough this year that some fields were being heavily defoliated and had almost taken on a white sheen to them because the foliage was disappearing,” Gavloski said.
Hay producers can cut their losses by harvesting the crop early, but seed growers don’t have that option. Organophosphate insecticides containing dimethyl-8, such as Matador or Decis, are their main tools.
Weevil larvae don’t have legs and can’t easily migrate to the next field if their food source is cut down.
“They are small insects and they don’t do very well if their food source is drying up,” Gavloski said.
“If you do the cut when the weevil is at a really vulnerable stage, you can get a really good kill.”
He said such a measure can also prevent further damage.
Alfalfa growers with a perennial weevil problem needn’t tear up their established stands and seed it to another crop. A better strategy, he said, would be to wait and monitor the pest’s development each year and cut early if necessary.
Buyers of hay grown in Manitoba shouldn’t worry about importing bugs in their shipments because most hay contains only larvae, which can’t survive without fresh, green plants.
Cutworms, both redback and dingy, may be a problem in the coming year, Gavloski said. Reports from last spring point to an upswing in their population cycle.
Natural enemies play a big role in keeping cutworm numbers down, but the pests need to reach a suitable peak before the predator species can catch up enough to have a significant impact.
“They build up, and you get a few bad years, then they naturally begin declining,” he said.
“They follow your typical bell curve, much like a Bertha armyworm population.”
A plague of grasshoppers may also be looming in some areas, with higher than normal levels reported in western Manitoba in 2008.
“Over the last two years we’ve seen a gradual increase in population. So what that means for farmers is they should at least consider looking for grasshoppers in their field scouting and monitoring.”
If the weather is dry and mild next June and early July, producers may uncover hopper hot spots by poking around in lighter soil, south facing slopes, ditches and field edges.
If numbers of concentrated juveniles look high enough to be a threat, a timely dash of spray or pesticide-laced bran to neutralize problem pockets might be a worthy investment, he said.
“Spraying can work if it’s timed well. Grasshoppers are quite sensitive to most of our insecticides when they are young. But once they get to be adults, good luck.”
Waiting too long can make spraying a waste of money because the adults are far hardier. The population is also often more dispersed, forcing producers to cover a larger area.
