The utility of biological control for weed management in agroecosystems will increase with a greater understanding of the relationships between common weed and granivore species. Giant foxtail is an introduced, summer annual grass weed that is common throughout the United States and problematic in numerous crops. Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) is a common, native, omnivorous carabid beetle with a range that overlaps giant foxtail. In 2004 and 2005, H. pensylvanicus was captured from farm fields in Centre County, PA, and subjected to laboratory feeding trials to test the preference of giant foxtail and other species on predation by H. pensylvanicus. Weed species seed preference experiments that included “Choice” and “No Choice” treatments were conducted using giant foxtail, common lambsquarters, and velvetleaf. When given a choice amongst the three weed species, H. pensylvanicus preferred giant foxtail and common lambsquarters seeds equally compared to velvetleaf seeds. When given the choice, H. pensylvanicus preferred newly dispersed giant foxtail seeds over field-aged seeds. Phenology of giant foxtail seed shed relative to H. pensylvanicus activity density was also quantified in field experiments in 2005 and 2006. Giant foxtail seed rain was determined by collecting shed seeds from August through October using pan traps. Activity density of H. pensylvanicus was monitored for 72-h periods using pitfall traps from June to October. Peak activity density of H. pensylvanicus occurred at the onset of giant foxtail seed shed in both years; however, giant foxtail seed shed peaked approximately 30 to 50 d after H. pensylvanicus activity density. Future research should focus on management practices that enhance and support H. pensylvanicus populations later in the growing season to maximize suppression of giant foxtail and other weeds that shed palatable seeds later in the season.
Nomenclature: Harpalus pensylvanicus DeGeer, Giant foxtail (Setaria faberi Herrm.), common lambsquarters (Chenopodium album L.), and velvetleaf (Abutilon theophrasti Medik.).