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.