Fewer aphids in organic fields

Farmers who spray insecticides against aphids as a preventative measure only achieve a

short-term effect with this method. In the long term, their fields will end up with even more

aphids than untreated fields. This has been reported by researchers at the Biocenter of the

University of Würzburg in the scientific journal PLoS One.

What’s the status of the biodiversity in differently managed triticale fields? This is what the

biologists at the Department of Animal Ecology & Tropical Biology wanted to find out. Triticale

is a cross between wheat and rye. The cultivation of this crop is on the rise across the globe,

because it delivers good yields even in poor soil conditions.

When comparing conventionally managed crop fields, which were either sprayed with insecticides

or were left untreated, Jochen Krauss, Iris Gallenberger and Ingolf Steffan-Dewenter made a discovery,

which should catch the attention of every farmer: “According to our results, the preventative application

of insecticides against aphids does not produce any advantages even though it consumes a lot of time

and money,” Jochen Krauss sums up.

The scientists studied five triticale fields that were sprayed with insecticides against aphids, comparing

them to ten other fields without any such treatment. “To be sure, the application of the insecticide led to

a short-term decrease of the pest density,” says Krauss. “After four week’s time, however, significantly

more aphids could be found in these fields than in insecticide-free fields. This also astonished the farmers

who made their fields available for our study.

More aphids as a result of a decrease in natural enemies

The scientists offer two possible explanations for this phenomenon. One possibility is: The insecticides

indiscriminately kill off beneficial animals that feed on the aphids, such as ladybugs or the larvae of

lacewings and hoverflies. Because their enemies are missing, the aphids find it easier to return and

proliferate than in insecticide-free fields.

Another possibility is an indirect effect: The insecticide just kills the aphids, after which their enemies

leave the field for a lack of prey. Final result: In this scenario, the aphid population can also recover better

after their return because the natural enemies are missing.

Greater biodiversity in organic crop fields

In conventional fields that have not been sprayed with insecticides, the pest control through natural enemies

seems to work better – thanks to the higher biodiversity in these fields. The biodiversity is far greater still

in fields under organic management, as reported in PLoS ONE by the Würzburg scientists.

The researchers found five times as many plant species and 20 times more types of pollinating insects in

the 15 organic crop fields included in the study than they did in conventional fields. Furthermore, they

detected three times as many natural enemies of aphids and five times fewer aphids in the organic fields

than in the conventional fields.

http://www.farming.co.uk/articles/view/4481