Crop Scene Investigation - 30 Solved: What's clipping Wilson's wheat?

Sunday, February 6, 2011

by BERNARD TOBIN

The pest that was clipping the heads off Wilson's wheat was a grass sawfly.

It is extremely rare for this pest to be found in wheat. Ontario Ministry of Agriculture, Food and Rural Affairs entomologist Tracey Baute consulted her colleagues across North America to help determine why the pest migrated to the crop in Ontario in 2010.

These pests have been observed clipping heads in wheat in Oregon, Delaware and in Michigan. In these cases, the grass sawflies fed on wheat plants during extremely warm spring weather conditions, just as occurred in Ontario in 2010. Baute says the pests were initially difficult to identify because they were hard to find.

Armyworm was pegged as the obvious cause of the wheat damage and Baute's team scouted the fields in the early morning and evening, when armyworm typically feeds. But grass sawflies feed during the day and were not detected until the fields were scouted during the daytime. The pest's green colour, similar to the wheat plant, also helped it go undetected.

Baute says grass sawfly can actually do more damage than armyworm if they are present in high enough numbers. "Thresholds are hard to peg since this is a rare occurrence, but Delaware has a threshold of two larvae per five feet of row. If significant head clipping is noticed, control is necessary. Growers should treat them as they would armyworm, but spraying can take place during the day since that's when they are feeding."

After the 2010 experience, Baute says she'll be watching for the pest during warm springs. "We certainly learned a lot. All fields infested looked fine until you parted the wheat plants and looked at the canopy floor. Only then did you see the number of heads lying on the ground. These larvae didn't seem to bother eating the leaves but instead climbed the plant and clipped the heads right off." Congratulations to: Andy Noorenberghe, Mount Brydges, for his correct answer. BF