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Costing weed control

By Janet Wallace

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The cost of controlling weeds is often measured in terms of the direct expenses related to killing the weeds. This includes the cost of herbicides, fuel for cultivation, or the expense of labour for hoeing or hand weeding. Scientists at Cornell University suggest that there may be greater costs including loss of crop yield and quality due to pest damage. They recommend that farmers look at the big picture when considering how to manage weeds.

It’s common knowledge that weeds can reduce crop yields by competing with the crop for light, nutrients or water. Controlling weeds, however, can backfire. Sometimes weeds help crops. A slightly weedy field can lead to increases in crop quality and yield.

In the Cornell study, scientists found that leaving milkweed plants in a corn field led to less damage by the European corn borer. Milkweed attracts aphids that only affect the weed, not corn. The aphids feed on the milkweed and release a sticky substance called “honeydew.” The sugars in honeydew provide food for wasps, including the parasitoid, Trichogramma. The aphids themselves are prey to ladybugs (ladybeetles). The result is that milkweed plants support a healthy community of insects (ladybugs and parasitoid wasps) that prey on the eggs and larvae of the corn borer.

Of course, letting milkweed take over a field will lead to crop losses but a low density of milkweed can reduce corn borer damage. An additional benefit is that the milkweed supports populations of Monarch butterflies.

The corn-milkweed research is just one of several studies showing a positive effect of weeds. Like a cover crop, weeds can also improve soil quality. Weeds can reduce pest damage by providing food to pest predators as in this example. They can also help hide or camouflage crops, or lure pests away from crops.

The scientists concluded that “Failure to account for the positive roles of weeds could lead to suboptimal weed management decisions, specifically, reducing weed populations when their benefits outweigh their drawbacks.”

Source: Integrating Insect, Resistance, and Floral Resource Management in Weed Control Decision-Making. Antonio DiTommaso, Kristine M. Averill, Michael P. Hoffmann, Jeffrey R. Fuchsberg, John E. Losey. Weed Science, 2016, Volume 64, issue 4, pp: 743-756.