With no synthetic pesticide use and more habitat kept intact, organic farms tend to have more biodiversity than conventional farms--including beneficial wasps that kill crop pests. But do these extra insects give organic farmers an edge in natural pest control over conventional growers? A study of 20 farms in the southwest of the United Kingdom says no. The lesson, says Wim van der Putten of the Centre for Terrestrial Ecology in Heteren, the Netherlands, is that organic is not always better.
The new study looked in part at whether natural pest management, which is thought to depend on the abundance and diversity of parasitic wasps, works better on organic farms. Led by ecologist Jane Memmott of the University of Bristol, U.K., the group selected five certified organic farms in southwest England and a similar conventional farm within 5 kilometers of each.
Over 2 years, the team identified which pests were eating which plants. They also searched for parasitic wasps that prey on these pests. (The wasps lay their eggs inside caterpillars, which die when the young wasps emerge.) Piecing those relationships together, the group mapped out the food webs, which contained between 143 and 274 species.
As expected, the organic farms were richer in species. They had up to 40 or so additional kinds of plant; up to roughly 30 more herbivore species; and most had 10 or more species of parasitic wasps beyond those found on the conventional farms. Also matching the team's predictions, the crop pests on organic farms were attacked by more kinds of parasitic wasps.
The surprise was that the rate of wasp attack was the same on both kinds of farms. And that means that these organic farmers weren't getting any extra natural pest control from these extra species.
Next, the researchers conducted an experiment to see whether biodiversity helps crops resist the attack of new pests. They added a harmless insect, an alien leaf-miner that is present elsewhere in the U.K. and related to harmful pests such as the apple blotch leaf-miner. Once again their expectations were confounded; the mock pests were killed by wasps at roughly the same rate on both kinds of farms. Again, no benefit for organic farms. The finding were published online 8 January in Ecology Letters.
One reason for the equivalence, the authors suggest, may lie in the fact that conventional farmers in that region of England are relying less on synthetic pesticides than they used to. Another factor is that both the conventional and the organic fields were relatively small and often bounded by hedgerows or woods. So even though the conventional farmers spray synthetic pesticides, the beneficial wasps would thrive in the surrounding habitat.
This similarity in farming systems "may confound the long-held belief that organic farming is 'better' for the environment than conventional farming," the authors write in their paper.
The big message, says van der Putten and others, is that ecosystem services may depend more on the overall landscape surrounding farm fields than they do on whether food is grown organically. The new findings may not apply to North America, where many conventional farms are enormous and much more homogenous than these English fields.