Effects of agricultural diversification on the abundance, distribution, and pest control potential of spiders: a review

A review of the literature showed that spider abundance was increased by di versification in 63% of studies. A comparison of diversification modes show ed that spider abundance in the crop was increased in 33% of studies by 'ag gregated diversification' (e.g. intercropping and non-crop strips) and in 8 0% of studies by `interspersed diversification' (e.g., undersowing, partial weediness, mulching and reduced tillage). It is suggested that spiders ten d to remain in diversified patches and that extending the diversification t hroughout the whole crop (as in interspersed diversification) offers the be st prospects for improving pest control. There is little evidence that spid ers walk in significant numbers into fields from uncultivated field edges, but diversification at the landscape level serves to foster large multi-spe cies regional populations of spiders which are valuable as a source of aeri al immigrants into newly planted crops. There are very few manipulative fie ld studies where the impact of spiders on pests has been measured in divers ified crops compared with undiversified controls. It is encouraging, howeve r, that in those few studies an increased spider density resulted in improv ed pest control. Future work needs are identified.