Leaving some weeds between crops can help to combat pests on agricultural land, according to a new study carried out by the University of Bonn. This step has particularly positive effects in combination with other measures: the cultivation of different types of crops and planting strips of wildflowers. The results have now been published in the Journal of Pest Science.
Intercropping, i.e. planting different types of crops on the same field has a number of benefits: The crops have different requirements and the crops face less competition than when grown in monocultures. This means that they make better use of the water and nutrients and deliver a better yield overall. Some types of crops—such as beans—are also able to fix nitrogen from the air, thereby delivering this nutrient as a natural fertilizer. The other crop also benefits as a result.
"Intercropping also makes it difficult for weeds to grow," says Prof. Thomas Döring from the Institute of Crop Science and Resource Conservation (INRES) at the University of Bonn. "The crops are also much less infested with pests. Insects usually specialize on one type of plant and they thus find fewer of the right type of plants with intercropping."
While these benefits have been proven many times, Döring and his colleague Dr. Séverin Hatt have now investigated whether these benefits can be improved even further in combination with other measures.
Strips of wildflowers attract aphid predators
The researchers cultivated two different crop mixes—beans and wheat and poppy and barley—in a field experiment lasting two years. In addition, they planted strips of wildflowers along the edges of the fields. "These strips attract beneficial insects that feed on pests," explains Döring, who is also a member of the PhenoRob Cluster of Excellence and the transdisciplinary research area "Sustainable Futures." "These insects include hoverflies and ladybirds, whose larvae are very effective predators of aphids."
In fact, the researchers found that aphid colonization of the mixed crops fell significantly next to the wildflower strips. They also discovered another effect: Mixing beans and wheat or poppy and barley naturally suppressed the growth of weeds without actually eradicating them completely. If the farmer took no additional measures, wild plants would continue to randomly grow across the field.
Residual weeds make it easier for beneficial insects to spread
"We have now been able to demonstrate that these residual weeds make it easier for beneficial insects to spread deeper into the field," says Döring. "And they did not reduce the yield in the process. In contrast, the study showed that they even helped to control pests."
The results were collected from fields that were cultivated under organic farming conditions. The extent to which these findings can be transferred to conventional farming still needs to be investigated.