Can field‐scale habitat diversification enhance the biocontrol potential of spiders?

The study investigated the effect of strip management in alfalfa on the abundance and diversity of spiders. In strip management, narrow strips were left unmown in the crop, which were then shifted at next mowing. In an experimental field, out of the six 50 × 50‐m blocks, three received strip management and three were managed traditionally (ie the whole block was mown). Our main hypothesis was that unmown strips will contain a more diverse and abundant spider assemblage, which will increase spider numbers in the mown parts of the crop. Over the 3 years of the study, unmown strips contained a spider assemblage of more than 50% higher abundance than in control alfalfa. Species diversity was also greater in the strips, and the presence of specific indicator species could be shown. We found that five dominant species comprised over 75% of spiders in the strips, and they were the same species as those that are dominant in alfalfa, where they represent 85% of all spiders. In contrast, a neighbouring meadow, which was a control in the study, also had high diversity, but different dominant species. The main result of the experiment was negative in that elevated abundance of spiders in unmown strips did not raise spider numbers in the mown strips of alfalfa, where abundance was only marginally higher than in the control alfalfa. We can speculate that better ecological conditions in unmown strips attract and conserve spiders from neighbouring areas and from the crop at mowing. We suggest that if diversification is more interspersed within the crop, we have a higher chance of increasing the number of natural enemies and bringing them closer to pests, so that they can interact more intensively. © 2003 Society of Chemical Industry