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Crop Scene Investigation - 33: Which pest invaded Jeffrey's corn field?

Sunday, April 3, 2011

As expected, the western bean cutworm was part of the problem. But what was this other interloper?

by BERNARD TOBIN

As the 2010 growing season progressed, DeKalb agronomist Bob Thirlwall knew he would be getting calls from growers asking him to help identify western bean cutworm (WBC).

The pest had invaded Michigan corn fields the previous year. Extensive press coverage and presentations at farm meetings had fuelled speculation that the insect, which can reduce yields by up to 30 bushels per acre under heavy pressure, was set to sink its teeth into Ontario's corn crop.

"Farmer awareness was really high and I was expecting to hear from a lot of growers," says Thirlwall.  The pest did indeed take a bite out of some farmer's fields, but when it came to WBC sightings, things were not always as they appeared. That's something Thirlwall learned after he received a call from a grower named Jeffrey in early September.

"He said he had tremendous western bean cutworm pressure and it was eating up his corn crop," recalled Thirlwall. When he arrived at Jeffrey's farm near Bothwell in Lambton County, the first thing Thirlwall noted was the corn field's soil type. "The research so far tends to show that western bean cutworm likes light, sandy soils and that was exactly the profile of Jeffrey's field."

The next job was to scout the field and conduct insect counts. Thirlwall counted the number of insect-infected plants in 10 random areas across the field. In some areas, he found that eight out of the 10 plants he inspected were infected, and in others he found two or three, but overall the degree of pressure was very high.

As Thirlwall inspected the field, he observed that the overall health of the plants was quite good. There was no leaf feeding and no evidence of stalk tunnelling.

There was plenty of damage on the cobs, but the level of injury varied from plant to plant. Some cobs showed grazing on the tips of the kernels, which was evident all the way down the body of the ear, while other cobs showed heavy feeding only on the tips.

The evidence quickly confirmed that Jeffrey indeed had significant WBC pressure. As he observed the pests feeding on cobs, Thirlwall identified the tan-coloured larvae from their shield-like structure just behind the head and two broad, dark brown stripes. But Jeffrey was surprised when Thirlwall told him he also had another pest to contend with.

About one-third of the insects Thirlwall observed in his counts were not WBC. These pests had yellow and greenish bodies, a tan-coloured head and measured about 3.5 centimetres. They also had distinctive black dots along their body.
Do you know what pest had joined WBC in Jeffrey's field?

Send your solution along with complete contact information to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993. All the answers will be pooled and one reader will win a Wireless Weather Station. BF

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