Brian Handwerk, National Geographic News 12 Aug 09;
Hungry pests cost farmers millions in crops losses each year, resulting in the widespread use of chemical pesticides, to which many bugs eventually become immune.
But one effective deterrent is clean, green, and sustainable—the wasps, flies, ladybugs, and other predators that happily feast on crop pests.
Scientists and farmers are learning to make better use of this powerful weapon by reshaping the landscape to create abundant habitat for pest predators, encouraging them to make farms their homes.
Rebecca Chaplin-Kramer, a doctoral candidate at the University of California, Berkeley, released the results of her multiyear "biological control" study last week at the Ecological Society of America's annual meeting in Albuquerque, New Mexico.
Promoting predator habitat doesn't have the instant impact of pesticides, but it could be highly sustainable, as established predator populations can thrive indefinitely, Chaplin-Kramer noted. (Learn more about sustainable agriculture.)
Inviting pest predators might also save money. The U.S. Environmental Protection Agency estimates that farmers spend more than U.S. $30 billion a year on pest control—and the cost continues to climb. (Related: "Pesticides Float From Distant Farms to Protected Forests, Study Says.")
Yet pests still wipe out more than a third of potential crops. In addition, more than 500 pest species have developed resistance to chemicals that once kept them in check.
Happily Harboring Killers
Previous studies had shown that diverse landscapes promote bigger populations of pest predators. But it's tough to evaluate the effect that farmers really care about: how much pest control such predators actually provide.
Chaplin-Kramer is hoping to find out.
She's been working in the fields of California's Salinas Valley since 2006, examining how predator habitat can provide pest control on both the local level—that is, within a farm's fields—and in the surrounding landscape.
Chaplin-Kramer specifically studies the cabbage aphid, the plague of many broccoli crops and favored prey of the syrphid fly.
She first surveyed a handful of test farms that mixed their crops with hedgerows, weeds, and even strips of flowers or plant cover designed to attract predatory bugs. Other sites contained only rows of broccoli plants.
But quantifying the role of habitat in pest control can be tricky, since a variety of factors, such as weather variables, can affect how many aphids might be found in a given field.
"Surveys can't really tell me the whole story," she explained. "If a site has low pest levels, are predators keeping them in check, or did they just not show up there?"
So Chaplin-Kramer put out broccoli plants infested with 50 aphids each in closed and open cages. Open cages let natural predators do their best at pest control, while closed cages allowed aphids to live or die depending only on prevailing conditions.
The cages were then stationed in a variety of environments ranging from pure, diverse natural habitats to monoculture farm fields.
The results showed that natural pest control is boosted significantly when predator populations are booming in more diverse natural landscapes surrounding a field. Lacking such surroundings, farmers can still gain key benefits by planting locally complex fields.
"Natural landscapes exhibit about five times the level of pest control of agricultural landscapes in the early season," Chaplin-Kramer said.
"In the late season, within the agricultural landscapes, the locally diverse farms have more than four times the pest control of the locally simple farms," she said.
The studies also showed that predators tend to arrive in farm fields a bit later in the growing season. This suggests that farm habitats may not be able to hold aphids in check early on, and that pesticides or other controls may be needed to prevent pests from gaining an irreversible foothold.
Also, the pest-control prowess of even the most predator-friendly landscapes may be trumped by natural factors, such as temperature or moisture, which regulate pest populations.
Farmers' Friends?
The results paint a picture of a promising but complicated endeavor, as many farmers will attest.
Phil Foster, who runs Pinnacle Organics in San Juan Bautista and Hollister, California, worked with Chaplin-Kramer on the study. He typically plants 30 or 40 different crops from eight families. He also rotates crops and plants predator-friendly hedgerows.
"I've found that a whole farm system is pretty important," he said. "We're in our 20th season of organic production, and for us it's a learning process all the time."
Foster's lettuce crop provides one example of how he helps make pest predators feel right at home.
"Probably 5 to 8 percent of the acreage goes to insectary plants, like dill and coriander and cilantro, that are known to attract beneficial insects like syrphid fly adults and parasitic wasps," he said.
Foster doesn't harvest the herbs but instead plants them solely to attract predators—though he ponders the possibility of pairing dill with his cucumber crop to produce pickles.
Steve Stevens, a Tillar, Arkansas, cotton farmer who has to contend with aphids, doesn't run an organic operation but still counts on lots of pest protection from natural sources.
"We try to hold back any sprays until we absolutely have to," he said, "because we also want to preserve our beneficial [insects] as long as possible in the season."
Stevens explained that for him, predatory insects provide a way to increase "resistance management."
"If we continue to overspray with some of these insecticides, we're going to use them up, because insects can become resistant to insecticides," he said.
One Weapon in an Arsenal
Insecticide limitations are one reason biological control is gaining traction, according to Robert Wiedenmann, head of the University of Arkansas entomology department. (Related: "Are Birds Best Hope for Pest-Ridden Coffee Crops?")
"Also, there's not universal acceptance [of biological control], because there's not universal knowledge about it," he added. "It's not an easily understood or 'off the shelf' kind of technology, and that is limiting a lot of its use."
There are other drawbacks, he said.
"Many natural enemies, especially parasites of insects, are fairly specific. They are not going to provide a benefit against all pests, a universal benefit similar to what a pesticide would provide. A habitat that provides benefit against aphids may not provide any benefits against army worms."
That's why Wiedenmann stresses that biological control, while full of promise, is likely to be only one weapon in a farmer's arsenal.
However, he believes it may be an important long-term solution, potentially resulting in farm environments with widespread natural pest control that kicks in automatically each season.
"The future of integrated pest management, if it's really integrated, [means we] will not rely almost exclusively on pesticides," he said, "or on waiting until we have a problem and then trying to solve it."
