Evaluation of field border management on predation of weed seeds

Weed seed predation was studied in nine organic crop fields (between 2.5 and 4 ha each) (three fields each of corn, soybeans and hay) each surrounded by four field border treatments (planted native grass and prairie flowers, planted prairie flowers only, fallow vegetation, or mowed vegetation) from September to November 2009 and 2010. We used predator exclusion cages to determine the amount of weed seed removal caused by invertebrates (control cages had <1mm openings and insect accessible cages had 12.7mm openings). Twenty-five seeds of three common weed species (redroot pigweed, Amaranthus retroflexus L., broadleaf signalgrass, Urochloa platyphylla (Munro ex C.Wright) R.D.Webster, and sicklepod, Senna obtusifolia (L.) H. S. Irwin & Barneby) adhered to individual cards were placed inside each cage once a month for seven days. Activity density was measured with pitfall traps. Preliminary results show that invertebrates were an important factor in seed removal, but field border treatments had no effect on seed removal rates. Similarly, the activity density of the predominant weed seed eating ground beetle, Harpalus pensylvanicus De Geer, did not change between field border treatments. However, both weed seed removal and Harpalus pensylvanicus activity density increased closer to the field edge for all border treatments in the corn and soybeans. These preliminary results suggest that increasing vegetative diversity in field borders is not an effective method for conserving weed seed predators, but that reducing the distance between the edge and the center of a field may enhance weed seed predation, especially in annual row crops.