Ninety-one percent of the 1500 million hectares of the worldwide cropland are mostly under annual crop monocultures of wheat, rice, maize, cotton and soybeans (Smith & McSorley, 2000). These systems represent an extreme form of simplification of nature's biodiversity, since monocultures, in addition to being genetically uniform and species-poor systems, advance at the expense of natural vegetation, a key landscape component that provides important ecological services to agriculture such as natural mechanisms of crop protection (Altieri, 1999). Since the onset of agricultural modernization, farmers and researchers have been faced with a main ecological dilemma arising from the homogenization of agricultural systems: an increased vulnerability of crops to insect pests and diseases, which can be devastating when infesting uniform crop, large-scale monocultures (Adams et al., 1971; Altieri & Letourneau, 1982, 1984). The expansion of monocultures has decreased abundance and activity of natural enemies due to the removal of critical food resources and overwintering sites (Corbett & Rosenheim, 1996). With accelerating rates of habitat removal, the contribution to pest suppression by biocontrol agents using these habitats is declining and consequently agroecosystems are becoming increasingly vulnerable to pest outbreaks (e.g. Gurr et al., 2004).

A key task for agroecologists is to understand the link between biodiversity reduction and pest incidence in modern agroecosystems in order to reverse such vulnerability by increasing functional diversity in agricultural landscapes. One of the most obvious advantages of diversification is a reduced risk of total crop failure due to pest infestations (Nicholls & Altieri, 2004).