By Miguel Angel Altieri; Clara Ines Nicholls
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Extra resources for Biodiversity and pest management in agroecosystems [...] XA-GB
Crop spatial and temporal patterns determine the extent and persistence of crop plants and, thus, the availability of key resources associated with the crop (Kareiva, 1983; Van Emden, 1990). The spatial and temporal availability of those crop-associated resources (and those in surrounding or adjacent unmanaged habitats or in managed refuges) also may be critical determinants of whether, when, or how they respond to herbivores or other resources provided by crops (or other plants). , taking natural enemies away from crops).
Sheehan (1986) suggested that specialist parasitoids might be less abundant in polycultures than monocultures because (1) chemical cues used in host finding will be disrupted and the parasitoids will be less able to find hosts to parasitize and feed upon in polycultures, and (2) the indistinct boundary at the edges of polycultures will be hard to recognize and they will be more likely to leave polycultural habitats than monocultures. In addition, Andow and Prokrym (1990) showed that structural complexity, or the connectedness of the surface on which the parasitoid searches, can strongly influence parasitoid hostfinding rates.
Prey or host populations will recolonize these monocultures and rapidly increase (Andow, 1991a). Finally, both generalist and specialist natural enemies should be more abundant in polycultures than monocultures because more pollen and nectar resources are available at more times during the season in the complex systems (Altieri and Letourneau, 1982). Resource Concentration Insect populations can be influenced directly by the concentration or spatial dispersion of their food plants. There can be a direct effect of associated plant species on the ability of the insect herbivore to find and utilize its host plants.