Refuge habitats modify impact of insecticide disturbance on carabid beetle communities.
Abstract
Carabid beetles are polyphagous predators that can act as biological control agents of insect pests and weeds. While current agricultural practices often create a harsh environment, habitat management such as the establishment of within-field refuges has been proposed to enhance carabid beetle abundance and impact. We examined the joint effects of refuge habitats and insecticide application on carabid activity density (parameter of population density and relative activity) and species composition in a maize field. Our 2-year study (1998-99) conducted at the Entomology Research Farm, Michigan State University, East Lansing, Michigan, USA comprised four treatments: (i) -refuge/-insecticide; (ii) +refuge/-insecticide; (iii) -refuge/+insecticide; (iv) +refuge/+insecticide. Refuge strips consisted of grasses (Dactylis glomerata), legumes (Trifolium repens) and perennial flowering plants. '-Refuge' strips were planted with maize and not treated with insecticide. Before planting and insecticide application, carabid activity density in the crop areas was similar across all treatments. Insecticide application immediately reduced carabid activity density and altered community composition in the crop area. Refuge strips had significantly higher activity density of beetles than -refuge strips before planting and during the summer. During summer, as new carabids emerged and insecticide toxicity declined, the presence of refuge strips influenced carabids in the adjacent crop area. Carabid activity density within crop areas previously treated with insecticide was significantly higher when adjacent to refuge strips. Also, carabid communities within insecticide-treated crop areas were affected by the presence or absence of a refuge strip. The presence of refuge strips did not consistently augment carabid numbers in crop areas where insecticide was not applied. One explanation may be that insecticides decreased the quality of crop habitat to carabids by depletion of prey and direct mortality. However, subsequent rebounds in prey density and the absence of competing predators may make these areas relatively more attractive than unperturbed crop habitats to carabid colonization from refuges. This study demonstrates that refuges may buffer the negative consequences of insecticide application on carabids in adjacent fields. Diversifying agro-ecosystems with refuge habitats may be a viable strategy for maintaining carabid populations in disturbed agricultural landscapes to keep pests below outbreak levels.