Response of Herbivores and Natural Enemies to Organic and Conventional Management of Field Crops

Organic agricultural production in the United States has continued to expand; the
amount of organically managed land doubled from 1990-2002 and again from 2002-2005
(USDA 2009). The rising prevalence of organic farming systems in the agricultural
landscape has necessitated efforts to understand their ecological impacts. Numerous studies
have found greater abundances of predatory or beneficial insects on organic compared to
conventional farms (Drinkwater et al. 1995; Letourneau and Goldstein 2001; Bengtsson et al.
2005). However, many of these same studies found no significant effect of farm management
on insect pest abundances (Bengtsson 2005). Variable within-crop and off-crop plant
diversity associated with farm management can affect insect populations through reducing
the concentration of host plants for herbivores and increasing the diversity and stability of
resources for natural enemies. Increased within-crop plant diversity, including weed
diversity, is commonly attributed to organic farming systems and has been correlated with
greater abundances of natural enemies and lower abundances of herbivores (Degooyer et al.
1999; Skelton and Barrett 2004; Cai et al. 2007; Brewer et al. 2009; Lundgren et al. 2009).

The objective of this study was to examine how conventional and organic management of alfalfa and soybean crops at the Wisconsin Integrated Cropping Systems Trial (WICST) affect herbivores and their natural enemies. The major herbivorous pest of soybean is the soybean aphid; for alfalfa it’s potato leaf hopper.

Authors: R.E. Mallinger and D.B. Hogg