Insecticides could be costing growers yield

BRANDON — Extensively spraying fields when crop-eating bugs are first seen might be costing farmers 10 to 15 percent of their canola yield.

And that’s just from killing honeybees. It doesn’t account for damage done to parasitic flies, wasps and other insects that prey on crop pests.

“If you’re not at that economic threshold, you may be throwing away money,” agronomist Angela Brackenreed of the Canola Council of Canada said during Ag Days in Brandon.

“We had reports of really high numbers of beneficials this year.”

John Gavloski, a bug specialist with Manitoba Agriculture, said during St. Jean Farm Days that farmers have told him their advisers have urged them to include an insecticide as a tank mix when they spray for blackleg.

He said that is bad advice.

“If you are tank-mixing in an insecticide because it is cheap, you may be reducing your overall yield by 10 to 15 percent (because of honeybee deaths), plus you’re killing off beneficial insects like parasites,” he said.

“Don’t do it because it is cheap.”

Gavloski is part of a project studying natural, parasitic control of cutworms. A number of insects will infect, parasitize or eat cutworms, so their presence eventually helps crush outbreaks.

However, insecticides kill them along with the crop pests.

Gavloski said diamondback moths weren’t a big problem for the past two years partly because of the massive numbers of parasites that prey on the pest. Eighty percent of diamondback larvae were parasitized in some fields surveyed in 2011.

Viruses and fungi are also effective curbs on bad bugs. 

Gavloski said farmers should get a good assessment of their problem bugs before they choose to spray so that they don’t overreact. Zebra caterpillar outbreaks, which look much like bertha armyworm outbreaks, often occur just in clumps and are not widely spread in a field.

Brackenreed urged farmers to stick closely to threshold levels so that they are not doing more harm than good with insecticide spraying.



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