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.