How to help beneficial insects keep pests in check

When scouting fields for pest insects, don't look just for the bugs that can potentially damage crops. Also take stock of the good bugs. If beneficial predatory insects are present, they may keep pest bugs in check, saving farmers the time and expense of applying insecticides.

"If you're scouting for aphids in soybeans, for instance, and you find aphids along with the seven-spotted ladybugs, consider that each adult ladybug can feed on several hundred aphids a day," says Travis Prochaska, area Extension crop protection specialist at North Dakota State University North Central Research Extension Center.

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While an economic threshold of 250 aphids per soybean plant typically serves as the trigger for applying insecticides to control aphids, Prochaska says letting the ladybugs do their work could save money in both the long and short term, when aphid numbers are below the economic threshold.

"Applying insecticides may control the aphids, but it may also kill the ladybugs," he says. "It will take awhile for populations of ladybugs to rebound."

Harming ladybug populations in the field can reduce your chances of successfully controlling aphids, especially in instances when aphid populations could rebound in the short term. That's largely because of the differences in life cycles between insects such as aphids and ladybugs.

"Aphids have a life cycle of five to seven days with temperatures between 70° and 80° Fahrenheit," says Prochaska. "Ladybugs have a life cycle of four to eight weeks."

That means aphids could reemerge or be blown in with the wind relatively soon after an insecticide application. Meanwhile, the ladybug population would struggle to rebound and provide crop protection within the growing season in which the insect suffered a setback from a chemical application.

Other Beneficial Bugs

Ladybugs are not the only insects to provide benefits to farmers. Beneficial insects include pollinators, dung beetles, ladybugs, lacewings, soldier beetles, predatory wasps, and others.

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The economic value of the ecological services that native insects provide amounts to $57 billion in the United States alone, says Prochaska. This value represents the losses that would accrue if insects were not functioning at their current level. The services these insects provide include:

• Pollination
• Pest control
• Soil fertilization
• Wildlife nutrition

Through their work, pollinators alone are responsible for 35% of global plant population, says Prochaska. Globally, that represents an annual market value of $577 billion, he says, with the U.S. share of that amounting to $40 billion annually.

The potential work of pest control in crops is accomplished by a mind-boggling array of helpful insects. North America alone has 150 species of "assassin and ambush bugs" whose common prey includes flies, aphids, and grasshoppers. Forty species of damsel bugs eat aphids, leafhoppers, and spider mites.

Besides the 475 species of ladybugs, beneficial insects include pirate bugs, soldier beetles, and predatory wasps, plus more than 1,000 species of tachinid flies prey on grasshoppers and sawfly larvae.

Beneficial insects need help to keep their populations strong. Today's farming practices often work against them. A focus on maximizing crop production often leads to the loss of those islands of natural, undisturbed plant habitat that beneficial insects require to thrive.

Prochaska suggests two key ways farmers and ranchers can help good bugs maintain their numbers so they can provide the services that in turn help farmers and ranchers:

1. Use chemicals as a last resort.

Reducing the use of pesticide preserves the lives of beneficial insects, of course, but it also helps to reduce potential incidences of pesticide resistance in the populations of pest insects. "Using pesticides as a last resort helps protect the effectiveness of chemical ingredients for those critical circumstances where they're really needed," he says. Herbicides also can have a harmful effect on beneficial insects. "By spraying field edges, you could be eliminating habitat for some good species," says Prochaska. "The monarch butterfly, for instance, in the caterpillar stage feeds on milkweed."

2. Provide habitat for beneficial insects.

Practicing no-till leaves soil undisturbed and creates the surface residue insects need for shelter. Growing season-long cover crops provides flowering plants for pollinators. Including tree shelterbelts and perennial phases in the rotation provides ground habitat for certain species of beneficial insects.

"Some insects, like native bees, are ground dwellers," he says.

Managing grasslands so that native flowering plants have a chance to get started and flower also provides habitat for pollinators. The pollinators help the flowering plants, and the resultant vibrant plant diversity bolsters soil health and grass production.

Establishing field borders of perennials is another way of providing habitat for beneficial insects. (See "Conservation Programs Benefit Insects," right.) "An effective field border for providing shelter for beneficial insects might be a 30-foot strip running along one side or two sides of a field," says Prochaska. "Planting native grasses and wildflowers in the border makes ideal habitat for beneficial insects."

In the main field, planting a variety of crops and other plants across the farm is a good way to ensure the diversity that supports populations of beneficial insects.

"Vary the blooming times of plants in order to provide pollen and nectar throughout the growing season, especially in years of drought," he says.

Learn More

Travis Prochaska, 701/857-7682, travis.prochaska@ndsu.edu