Crop and field border effects on weed seed predation in the southeastern U.S. coastal plain

Abstract

Weed seed predation was studied in nine organic crop fields (three each of maize, soybeans and hay; 2.5–4.0ha each) surrounded by four experimental field border types (planted native grass and prairie flowers, planted prairie flowers only, fallow vegetation, or mowed vegetation) during the fall of 2009 and 2010 in eastern North Carolina. We used predator exclusion cages to determine the amount of weed seed removal caused by invertebrates and vertebrates. Three common agricultural weed species, redroot pigweed (Amaranthus retroflexus), broadleaf signalgrass (Urochloa platyphylla), and sicklepod (Senna obtusifolia), were adhered to individual cards and placed inside the exclosure cages once a month for two weeks. Activity-density of invertebrate weed seed predators was measured with pitfall traps. Results show that field border type had no effect on seed removal rates, but that crop type 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 harvested maize 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 maize fields, while the red imported fire ant (Solenopsis invicta) seemed to prefer the open maize fields. These results show that increasing vegetative diversity in field borders is not always an effective method for conserving weed seed predators, but that higher quality habitat inside the crop field can be achieved by increasing ground cover.