Ecologists Agree: We're No Smarter Than Nature
Jay Withgott, BioMedNet News 6 August 2001,
http://news.bmn.com/news/story?day=010807&story=2

Corporate models of farming took it on the chin at the Ecological Society of America conference today, as participants railed at heavy pesticide use as an unsustainable practice for agriculture.

Traditionally, most ecologists have been reluctant to apply their understanding of natural systems to the much-simplified artificial ecosystems that farm fields represent. But ecologists at the symposium on ecology and agriculture felt no such hesitation. "The more effectively insects are killed, the more effectively they evolve resistance," said University of Illinois entomologist May Berenbaum.

Berenbaum honed in on the central irony of chemical usage: The strategies currently used in pesticide application - application of insecticides to large areas for prolonged periods of time every generation, with the goal of destroying every last bug - are a perfect recipe for natural selection of evolution resistance. More than 500 arthropod species are known to have evolved resistance to insecticides and more than 140 weed species to herbicides thus far. Higher doses of the chemicals are being used with less effect every year, says Matt Liebman of Iowa State University.

Some insects have shown behavioral changes along with changes in physiology. For instance, Illinois farmers held the corn rootworm at bay for 30 years by rotating crops, planting corn in a given field one year and soybeans the next. The corn-munching rootworms would lay their eggs, which would overwinter in the field. Come spring, the rootworm larvae would hatch into a sea of soybeans, and promptly starve to death.

Recently, however, the bugs have evolved two ways of dealing with their predicament, Berenbaum says. Some fly to nearby soybean fields and lay eggs there, and others stay dormant for two winters instead of one, waiting to hatch in the "correct" year.

Using chemicals to fight insects that eat plants is nothing new: Plants themselves have been at it for millions of years. But, Berenbaum says, plants conduct their chemical warfare differently, and more effectively.

Rather than broadcast a chemical wantonly in every direction before an insect is even on the scene, many plants conserve their defenses until they are attacked, and limit their toxins to specific sites. They also boast a complex arsenal of many chemicals, whereas humans often rely on one at a time.

Berenbaum studies wild parsnip plants and the webworms that devour the plant's flowers and fruit. The plants defend themselves with a battery of different furanocoumarins, so webworms choose plants with lower furanocoumarin concentrations. In the co-evolutionary arms race between plant and insect, Berenbaum has found that the plants produce many fruit lacking seeds, which pack only one-third as many furanocoumarins as seeded fruits. The webworms eat the seedless fruit of no value to the plant, while the fruit with seeds survives. Seedless fruits thus serve "as decoys that divert webworms away from viable fruit," said Berenbaum.

The plant's strategy may appear to cut into its immediate yield, but "plants are less concerned with maximizing yield than they are with maximizing reproductive output" in the long term, she said. That's a lesson Berenbaum thinks humans need to take to heart: Farmers and their crops both share the same goal but "plants are in it for the long haul, and don't spend a lot of time and energy in revenge mentality."

The next frontier - crops bioengineered with genes to fight insects - may be no more immune from the forces of evolution. "We have every reason to believe that resistance will be a problem with transgenic crops," said Berenbaum. As Bt-corn and Roundup-ready soybeans occupy more fields, cases of resistance have already popped up.

What then is the solution? Berenbaum suggests using the decoy principle in a different way: Mixing in non-crop plants might lure insect pests away from crops and onto the decoy species, a strategy now practiced to lure lygus bugs off cotton and onto alfalfa. "One of the best hopes for resistance management is to diversify the landscape," Berenbaum said.

Liebman echoed many of Berenbaum's concerns and solution and says that in crop rotation, as compared to monocultures, weeds decreased in 82% of cases and increased in only 3%. Following the other presentations, Harvard University's Richard Levin suggested that pesticide practices may indeed change. "The chemical dependency we have now is a brief successional stage in the history of agriculture, and we're already passing out of it," he said.