This research compared effects of the weed control practice, soil cultivation, and the conventional practice, glyphosate application on weed seedbank, in a vineyard system. The experiment was conducted in a commercial wine-grape vineyard in the Napa Valley of northern California from 2003 to 2005. The annual treatments were “winter–spring glyphosate,” “spring cultivation,” “fall–spring cultivation,” and “fall cultivation–spring glyphosate,” and were applied “in-row,” under the vine. Composition of the weed seedbank collected in 2002 before treatment establishment did not differ among treatments. After 3 yr of weed treatments, detrended correspondence analysis indicated that the composition of spring cultivation and winter–spring glyphosate tended to differ from each other, but the remaining two treatments showed little differentiation. As determined by linear discriminant analysis, the specific weed species were associated with seedbanks of certain treatments. These were Carolina geranium, annual bluegrass, brome grasses, California burclover, and scarlet pimpernel, which do not pose problems with regard to physical aspects of grape production. Although ‘Zorro’ rattail fescue was ubiquitous among treatments, its distribution between depths in the cultivated treatments indicated that tillage provided some homogenization of seedbank along the vertical soil profile. The seedlings from the seedbank study were not congruent with those measured aboveground in the field, suggesting that both treatment and microclimatic effects in the field may have influenced germination, and thus, aboveground composition.

Natural enemies that respond to prey in a density-dependent manner may be able to quickly suppress pest populations before they reach economically damaging levels. Although it is primarily the combination of a natural enemy's functional response and a population numerical response that will influence the maximum number of pests attacked, other factors may influence a density-dependent response. We conducted large-scale field experiments, both artificially inoculating grapevines with larvae and using naturally occurring populations, to quantify and characterize the response of a parasitoid, Dolichogenidea tasmanica (Cameron) (Hymenoptera: Braconidae) to different densities of its host, the pest of grapevines, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae). We showed that the response of D. tasmanica to the density of E. postvittana was inversely density-dependent, and that the degree of parasitism was consistently and significantly higher in the grape variety Cabernet Sauvignon compared with Chardonnay. While the significant effect of variety on the degree of parasitism may provide an option for increasing the parasitism of E. postvittana by D. tasmanica, it also highlights how differences in host plant can influence trophic interactions.