A number of studies have found higher biodiversity on organic than conventional farms. However, these studies do not fully account for a broad variety of pest management practices that do not fit neatly into “organic” or “conventional” categories, such as Integrated Pest Management (IPM). This study examines how a range of pest management practices affects the abundance and diversity of arthropod natural enemies in apple orchards.
From May through August 2009, I conducted monthly vacuum sampling of canopy arthropods in six apple orchards in southeastern Michigan, and identified natural enemies to family and morphospecies. I used pesticide application records and grower interviews to order the orchards along a management gradient from certified organic to fully conventional practices. The four intermediate orchards used a range of IPM and conventional practices, including one orchard which the owner calls “all but organic.” Arthropod species and family richness were analyzed using EstimateS, and mean abundance was compared using ANOVA. I predicted that natural enemy biodiversity would be greatest in the organic orchard and lowest in the conventional, with intermediate levels between these two end points.
June samples of natural enemy wasps included 69 morphospecies in 13 families. Species richness was highest in the organic orchard (35 species), and decreased gradually in accordance with the management gradient down to zero wasps in the conventional orchard. Mean wasp abundance fell into three statistically significant categories: the organic orchard with 6.4 wasps per tree, the “all but organic” with 3.4, and the remaining orchards with low mean abundance. Family richness varied with spatial scale: mean family richness per tree was highest in the organic orchard, but total family richness per orchard was highest in two of the IPM orchards. June results will be combined with other months to examine temporal patterns in the six orchards.
These results suggest that examining a broad range of pest control practices reveals a rich picture of the underlying relationship between farm management and biodiversity. In fact, the organic and “all but organic” orchard did not differ significantly in species or family richness, indicating that a variety of pest control methods may preserve arthropod natural enemies. This study indicates how pest management practices can affect the potential for conservation biological control by arthropod natural enemies in apple orchards.