Some Pests Prefer Organic
News Nov 18, 2009
- Phil Berardelli. November 13, 2009
Contrary to claims made by some proponents of organic farming, natural fertilizers are often no better than chemical fertilizers at defending crops against insects--and sometimes they're worse. That's what British researchers found over the course of a 2-year trial. The results suggest that farmers should tailor fertilizing to individual plant varieties.
Organic farming has gained popularity in recent decades because of its use of natural ingredients. Proponents say that cow manure, for example, is far less harmful to the environment than petrochemical-based products. Some advocates have also claimed that organic fertilizers help plants resist insect pests better than synthetic varieties do. That's because plants absorb the nitrogen and other nutrients from organic fertilizers more slowly, and the pest larvae that rely on those nutrients have a tougher time gobbling them up.
Previous research on the topic proved inconclusive, so researchers at Imperial College London and two other institutions in the United Kingdom studied how three pests--two types of aphid and one species of moth--responded to the application of natural and synthetic fertilizers on cabbage plants. The team used chicken manure and other green fertilizers derived from beans and alfalfa, plus commercially produced ammonium nitrate, and applied all in both high and low concentrations. The experiment spanned two growing seasons at multiple field sites.
The team got surprisingly mixed results. The moth, Plutella xylostella, favored the conventionally fertilized plants, laying its eggs about four times more frequently on them than on organically fertilized cabbage. One aphid, Myzus persicae, also preferred the commercial fertilizer, laying eggs on ammonium nitrate–fed plants twice as often as on plants fed with organic fertilizer. But the other aphid, Brevicoryne brassicae, preferred the organically grown variety by about a three-to-one egg-laying margin, the researchers report this week in the Proceedings of the Royal Society B.
The lesson here, says entomologist and co-author Simon Leather, is that the complex chemical interactions between fertilizer and plant can be unpredictable, repelling some pests but attracting others. "One size does not fit all," he says.
Biologist Gordon Port of Newcastle University in the United Kingdom agrees, calling the work a "robust" study. What remains to be seen, he says, is how the natural enemies of the pests (such as ladybugs for the aphids and wasps and spiders for the moths) react to the two different types of fertilizers