Alley cropping agroforestry mediates carabid beetle distribution at a micro-habitat scale

Abstract

Temperate agroforestry systems are being strongly promoted for ecological reasons in many regions of the world. Pest management in these highly-structured environments is however barely studied, despite widespread claims of increased biological control enhanced by the presence of trees and grassy strips at their base. The aim of this study is to assess how different carabid species use space in an alley cropping agroforestry system. We carried out spatially and temporally-tight pitfall trap experiments in an old grown agroforestry plot and characterized how the activity density of two abundant carabid species of the field, Nebria brevicollis (F.) and Anchomenus dorsalis (Pont.), varied with the distance to the strip. The grassy strips were either un-mowed or used to stack branches, and were planted with or without trees. Both species clearly responded to the differences between strips and culture, but in a different way. N. brevicollis had a strong preference for the strip, but only when the strip had trees on it. Conversely, almost no captures of A. dorsalis occurred in the strip, whereas plenty were captured in crops. Our results are similar to those obtained on field margins and beetle banks, implying that the rich knowledge drawn from these studies can be applied to alley cropping agroforestry systems. These systems however differ by the regular arrangement of strips, which may support an assemblage of species with varying habitat requirements. These results imply that grassy strips can be managed for increasing their role in conservation biological control.