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Where wheat is grown on huge areas, ladybirds, spiders, hoverfly larvae and other enemies of aphids don't have enough food in spring, as the pest begins to populate the wheat fields not before May when they start to reproduce. Therefore, the enemies move on to places farther away where there is more abundant supply of food. When pest infestation occurs, the aphids thus encounter ideal conditions since their enemies are low in numbers.
The situation looks different if a variety of different crops grows around a wheat field: Since the natural enemies are around anyway, they are quick to devour the aphids. This effect is all the more pronounced the more diverse the landscape is in a 500 m radius around the field. This is what Sarah Redlich writes in the Journal of Applied Ecology; she is an ecologist and a PhD student of Professor Ingolf Steffan-Dewenter at the University of Würzburg in Bavaria, Germany.
Study - Scientist - Landscapes - Würzburg - Area
For her study, the scientist picked 18 landscapes in the greater Würzburg area that exhibited a maximum crop diversity. The landscapes were six kilometres in diameter and each had a winter wheat field at its centre. "We chose fields in low-diversity landscapes and fields with high landscape-level crop diversity," Sarah Redlich explains. For this purpose, the abundance and area of up to 12 crop plant groups in the landscape were calculated, both in a small radius (up to 500 metres) and in a larger radius (3000 metres) around the fields.
She set up two cages each containing 100 aphids on each winter wheat field. The wheat in one of the cages was completely inaccessible. "This cage was designed to keep out all predators. I wanted to know how quickly the aphids reproduce in this case," Redlich says.
Cage - Birds - Access
The other cage was coarse meshed, denying only birds access but no...
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