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Crop advisor casebook: Pest leaves puzzling pinholes in corn silage

A Crop Advisor's Solution from the January 8, 2019 issue of Grainews

An insect was causing these pinholes in Lenny’s crop. But which one? Not all corn varieties were impacted.

Lenny is a central Alberta producer who owns a farm west of Irma where he grows corn silage for his purebred Simmental cattle herd. We have been working with Lenny and his corn silage plot for three years now, and I examine the plot about once every week during the growing season to monitor its growth and development.

While I was scouting the plot in early August last year — that week I was examining varietal differences — I noticed some corn stalks had small round holes with a sawdust-like material surrounding those holes.

A closer inspection revealed only certain corn varieties had been affected. When I showed Lenny the stalks, he didn’t know what to think, especially since a plant of one variety had pinholes in its stalk while another variety a few feet away didn’t. This was consistent throughout the field.

When I cut the stalks of affected plants open, I could see they were starting to turn brown inside.

Sawdust-like material surrounded the small, round holes in the corn, and when I cut open the stalks of the affected plants, I could see they were starting to turn brown. photo: Supplied

This was the first time I had come across damage of this kind in this area. I knew it was an insect causing the pinholes, but which one? And why was it only infesting certain varieties in the plot?

Crop Advisor’s Solution: Check your silage corn plots for European corn borer

It was then I remembered discovering weaker stalks of this variety while examining some grazing corn the previous winter.

Although I hadn’t yet come across this pest in this region, the evidence all pointed to the European corn borer! I cut open another plant and found a corn borer inside the stalk. We had our proof.

Now it made sense that only certain varieties were being infested: those with the built-in Bt Bacillus thuringiensis trait were protected from the corn borer whereas those varieties without the trait were susceptible to the pest.

In this region, I’d never had this issue before in corn grown for silage. We thought we wouldn’t have corn borer issues in this area, so our weekly scouting turned out to be very beneficial. I was happy we identified the problem before we had a complete wreck on our hands.

This year we seeded all Bt corn, so the plants were resistant to corn borer feeding. And we saw a lot less damage. The Bt corn also has a five per cent refuge in the bag, which helps with resistance management.

Bt corn genetics costs a bit more than other varieties, but at the end of the day Lenny was happy to invest more for corn that wouldn’t be infested by this pest, and his yield won’t be affected in the future.

Growing corn for silage or grazing is becoming more popular in this area. Start your year off by choosing a variety with built-in Bt protection. This is especially true if you are grazing, as corn borer larvae overwinter in stalks, cobs and plant debris.

If you are silaging, tillage helps with stalk management, which will aid in the reduction of overwintering corn borer populations.

And, remember to scout your corn fields regularly — look for the small holes and sawdust-like frass.

Colette Thurston, BSC (Ag), RTAg, works for Richardson Pioneer Ltd. in Vermilion, Alta.

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