CROP SCENE INVESTIGATION (2)

What’s eating Joe”s IP soybeans?

by DAVID TOWNSEND

When a strange insect starts chomping on your soybeans mid-season, identification, and control if necessary, is critical to keep it from taking a bite out of your yield.

Late July, Joe, an Identity Preserved (IP) soybean grower from Walkerton contacted me concerned about holes in the leaves of his IP soybeans. “It looks like ladybugs are eating them,” explained Joe.

Before going to the field, I asked about the weather, his rotation and injury symptoms. Joe indicated that his area had received a rain storm earlier in the season, but not hail. Joe also grew a standard alfalfa-corn-soybean-barley rotation for both on-farm feed and commodity marketing.

This particular IP soybean field was pegged for export, making Joe extra sensitive to production challenges that could reduce yield and quality. When I asked about the location of the field, Joe indicated that it was a corner lot off the road surrounded by an alfalfa field and woodlot.

Next, I asked Joe to describe where the damage was located and how quickly it was progressing. The injury seemed to be progressing across the field very slowly, if at all. He also indicated that the damage was located only in the back corner of the field, which was bordered by alfalfa and the woodlot.

It was obvious that something was feasting on Joe’s crop, and I wanted to identify it quickly because insects like grasshoppers and armyworm can reproduce and spread rapidly, and cause significant damage.

When I arrived at the field I noticed the field and plant colour seemed normal – the plants hadn’t yet suffered significant stress. Because of optimal in-season moisture, the soybeans were also growing quickly and seemed to be outgrowing any significant leaf damage.

I conducted a complete scout first to assess the entire field. Using a sweep net, I identified grasshoppers, Japanese beetles and a very small number of aphids and ladybugs throughout the field. All were well below economic thresholds.

As I moved towards the problem area I noticed more holes in the leaves than I had expected. These holes were shot holes straight through the leaves, as opposed to holes that skeletonised or scraped away leaf tissue. Also, the holes didn’t break through the leaf veins, but were located between the veins.

Using my sweep net in this section I noticed a large number of a very colourful beetle. They were small with various markings and colours. Some were yellow, green, red and white. Some had spots and some didn’t. But, all had the same black triangle behind their head. They were treating Joe’s IP soybeans like a buffet.

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David Townsend, CCA, is the Technical Services Manager for NK Brand, Syngenta Seeds. He is based in Arva, Ontario.