Temporal variation in the response of parasitoids to agricultural landscape structure

Abstract

Temporal variations in the relationship between agricultural landscape complexity and parasitoid abundance were assessed in Ingham County, MI. The study site consisted of a 3.2 km×13.9 km area including a complex and a simplified agricultural landscape. Landscape structure was quantified using black and white aerial photographs and digital land-use data. A heterogeneous mixture of crop and non-crop habitats characterized the complex landscape (40.6% of non-crop habitat), while the simple landscape was more uniform and contained less non-crop habitat (28.6%). In the complex landscape fields were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and 81% more field edge in wide hedgerow than in the simple landscape. In June–July 1993, 1996, 1998, 2000, and 2001, a total of 15,786 armyworm ( Pseudaletia unipuncta) larvae were released into maize fields. A total of 3752 larvae were recovered and reared in the laboratory with 1410 individuals parasitized by 15 identified species of parasitoids. Two species, Glyptapanteles militaris and Meteorus communis, represented 96.3% of the parasitoids recovered. Agricultural landscape structure influenced the temporal dynamics of P. unipuncta parasitism. While G. militaris was equally present in the simple (48.8% of total recovery) and complex landscape (51.2%), M. communis was found mostly in the complex landscape (78.9%). Overall, percentage parasitism differed between landscapes and among years with different trends in the simple and complex landscapes. These results indicate that effects of landscape structure on parasitism are not adequately characterized by short-term studies.