Covering the soil to reduce pest insects

Farm Forum

Cover crops are all the rage these days, and may be a part of the solution for problems confronting the long-term sustainability of agricultural production in the U.S. Simply defined, cover crops are plant “place-holders”, designed to cover bare soil during fallow periods and in between growing seasons. Their benefits are myriad, from reducing soil erosion, to replenishing soil nutrients, to suppressing weeds, to growing soil microbial communities. I would argue that another benefit of cover crops that is too often overlooked (arguably the most IMPORTANT benefit) is in reducing pest insects and growing beneficial insect communities.

Bare soil kills a lot of insects. Actually, bare soil kills most biology in a system. For this reason, farmers and entomologists alike have long touted that a good way to remove pests from a farm or garden is to till the heck out of it and get rid of any non-crop vegetation. And they are right: you can really kill a lot of pests this way. But the problem is that you remove all of the friendly insects from the system as well. Predators (things like lady beetles, ants, spiders, ground beetles, lacewings, pirate bugs, etc.), pollinators, and detritivores are also driven off the farm. And farms NEED these beneficial insects to function properly.

Then the farmer comes back in the spring and plants entire fields of a single plant species, and guess who the first to arrive is? The pest. And there is no biotic resistance to the pest’s proliferation because all of the predators that keep those pests in check have been driven out of the system too. So the farmer sprays insecticides to replace the predation that Mother Nature normally gives for free, and further drives down the natural enemy communities, while temporarily suppressing the pests.

An alternative is to provide habitat for natural enemy communities so that they are there when the pests arrive in the spring. And cover crops are a great way to do this!

Research at the USDA-ARS laboratory in Brookings, S.D., has shown that cover crops preceding corn or soybean can have dramatic effects on natural enemy communities, sometimes increasing predator numbers by 10 times those found in insecticide-sprayed fields; these predators eat things like corn rootworms and soybean aphids.

And these predators DO eat the pests: this research shows that cover crops preceding corn (and their associated predators) reduce the abundance of corn rootworms and the damage that they cause to corn plants. Now we are testing whether cover crops could be economically competitive with “traits-based management” (Bt corn, neonicotinoid seed treatments, etc.) on farms throughout the Plains region.

But like any technology, cover crops need to be used correctly. Simply replacing one monoculture cash crop with a monoculture cover crop has been shown to foster pest populations like pea weevils and armyworms. The science is still rolling in, but it seems that diversifying the cover crop mix to include several plant species may help to overcome these unique and infrequent pest problems.

Using plant diversity to foster natural processes- especially those that foster soil health and biology- seems like a good alternative to constantly fighting for control against natural systems. Cover crops may dramatically reduce farming input costs for things like pesticides; now we need to get the science involved to make it as easy as possible for interested land owners and gardeners to implement these tools with predictable success.

Jonathan Lundgren is a research entomologist with USDA in Brookings, S.D. Although interested in all aspects of insect biology, he specializes on insect conservation and reducing crop pests through the use of beneficial insects.