Jonathan Lundgren crouches down in a South Dakota corn field at sundown. He is busy pinning Western corn rootworm larvae to the ground. As he studies his miniature captives in the growing darkness, Lundgren's mind replays a message from the past voiced by other entomologists: "There are no natural predators of the Western corn rootworm."
Using infrared light, he waits to see what fanged predators will crawl out of the lengthening shadows, attracted to the squirming bait. He doesn't have to wait long. Nocturnal carabid beetles, daddy longlegs (harvestmen) and various spiders come calling. The "no predator" myth dies along with the larvae.
WHAT'S OUT THERE?
Lundgren, a USDA-ARS entomologist in the North-Central Region, is unlocking the secrets of insect social interactions within farming operations. He contends there's crop-protection strength in both insect diversity and numbers if farmers will only manage for it. "For every pest, there are 1,700 species that are either beneficial or that we simply don't understand," says Lundgren, who is based in Brookings. He calls beneficials "the friendlies."
Lundgren leads a team that makes up the Corn Pest Risk Project Survey. It kicked off in 2010 to determine the scope of insect numbers and species in a typical South Dakota corn field.
The scientists sampled Bt-free fields with no insecticide seed treatments on 53 farms in 20 counties. They found a diverse community of 147,000 corn-pest predators per acre that included spiders, lacewings, pirate bugs, lady beetles and syrphid flies. That's a lot of predators attacking corn pests such as Western bean cutworm, European corn borer, corn earworm, rootworms and aphids.
THE GMO PAYOFF
Out of 107 insect species found in the corn canopy, 13% are known to be herbivore pests of the corn plant. These pests never exceeded economic thresholds at any of the sites. Lundgren's team thinks this can be attributed to not only the predator count but also the high adoption rates of genetically modified crops.
With so many predators working on corn pests, why are there still periodic pest problems? Lundgren's bioinventory research shows the biodiversity in corn fields rotated with soybeans is reduced by 70 to 75% when compared to the number of species found in the native grasslands that corn replaced. The implications are telling. "Our data from corn fields shows that predation on key pests [i.e. corn rootworm] increases as predator diversity increases," he says.
The Western corn rootworm research also shows that as total predator populations increase, rootworms are consumed more frequently. Since larvae aren't very mobile, they are easy prey and the first to go with adequate predator pressure. Rootworm larvae aren't totally defenseless, however.
"Rootworm larvae have a nasty antipredator defense that scares a lot of predators away," Lundgren says. Sticky coagulated blood from predator bites tends to glue a predator's chewing mouthparts shut, he explains. Sucking insects like harvestmen aren't deterred.
In a gut analysis study, Lundgren explains that out of 2,000 predators collected, 6% of the sampled predators had rootworm DNA in their digestive tracts. "That's 6% of up to a billion predators per acre that had just had a rootworm meal in the last eight hours," he stresses. "That's a lot of predation."
Many of the insect predators that attack crop pests also like a varied diet, consuming large amounts of crop-field weed seed. Known as "granivores," insects like millipedes, crickets, roly-polies, and beetles can consume 1 to 10% of seeds on the soil surface per day. "It doesn't sound like a lot until you extrapolate it out over a 180-day growing season," he reasons.
One discovery that came from Lundgren's bioinventory of prairies, pastures and corn fields is that the bacteria that live in the stomachs of beneficial insect species are reduced significantly within a corn/soybean rotation system. Gut bacteria like these are known to increase the seed consumption of granivores. Though they act as a major filter that stops the majority of seeds from reaching the seed bank, Lundgren says they are only part of the solution.
APPLY THE PRESSURE
Lundgren suggests increasing the number of beneficial insect predators in crop fields by reducing soil disturbance and pesticide use, and by adding vegetation diversity. That might include installing native prairie field strips or seeding cover crops in the rotation.
Lundgren says corn rootworm biology is impacted negatively by these habitat changes. His research shows crops like spring wheat or winter oats preceding soybeans result in decreased soybean aphid populations later in the season. Also, these crops increase beneficial insect predator populations when compared to soybean fields solely in a simple corn/soybean rotation.
Lundgren says cover crops help foster predator communities by creating habitat and alternative plant food resources like nectar, pollen, vegetation, fungus and seeds. "In tilled ground [without covers], a farmer comes in and plants a monoculture crop, and the first insects to arrive are pests.
"There is no biotic resistance [predator pressure] to stop the pests from having an outbreak," Lundgren continues. "Cover crops keep the friendlies in your field and allow these predators to find equilibrium with the crop pests, but also make it easier for [beneficial insects] to be there when the crop pests arrive."
Lundgren's research insinuates if growers remain ignorant of the beneficial services some insects provide, those insects will remain the invisible, unheeded and, therefore, ineffective allies in the war on weed and insect pests.
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