Field border and crop vegetation effects on weed seed predation

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], broadleaf signalgrass, [Urochloa platyphylla], and sicklepod [Senna obtusifolia]) adhered to individual cards were placed inside each cage once a month for two weeks. Activity density of invertebrate weed seed predators was measured with pitfall traps. Results show that field border treatments had no effect on seed removal rates but that crop species heavily influenced both weed seed predation and invertebrate seed predator activity density. Weed seed predation was highest in the dense, perennial hay fields and lowest in the more open corn fields. Activity densities for field crickets (Gryllus sp.) and the ground beetle Harpalus pennsylvanicus were also high in the hay fields and low in the corn fields while the red imported fire ant seemed to prefer the open corn fields. These results show that increasing vegetative diversity in field borders is not an effective method for conserving weed seed predators, but that higher quality habitat inside the crop field can be achieved by increasing ground cover.