Factors associated with feeding injury to grass plants by two species of adult billbugs, Sphenophorus gentilis and S. parvulus, were evaluated. Early season tests utilized adult bluegrass billbugs while later studies involved wildrye billbugs. Types of feeding injury were determined and preferred feeding locations on host plants were identified for each billbug species. Greenhouse and field studies compared different species of grasses, individual plants within a species, and plants from different locations, for billbug susceptibility. The effect of grass plant age and stem size were also tested using bluegrass billbugs. Using analysis of variance and multiple comparison tests, significant differences in amounts of feeding injury were determined among entries. Wildrye plants from different geographic locations showed large differences in susceptibility to wildrye billbug feeding when compared in a greenhouse study. Grass stem size had an effect on amounts of feeding injury incurred to 2 out of 3 wheatgrasses by bluegrass billbugs. A field study suggested that host plant age was not a factor in susceptibility of slender wheatgrass to bluegrass bfllbugs. Implications for screening grasses for resistance to billbugs using adult insects are discussed. Comparisons between greenhouse and field studies are also examined.