Studies were conducted to determine the utility of overseedingendophyte enhanced varieties of perennial ryegrass, Loliumperenne L., into existing stands of Kentucky bluegrass, Poapratensis L., to manage bluegrass billbug, Sphenophorusparvulus Gyllenhal, larval populations and their damage. Twocommercial varieties of endophyte enhanced perennial ryegrass (RepellII and Triple Play) were overseeded at two different rates in thefield. S. parvulus larval populations, their damage, andturfgrass composition were monitored over the course of 3 yr.Significantly fewer larvae were found in plots overseeded with RepellII, whereas Triple Play had no significant effect on larval populationdensity. Endophyte infection was confirmed in ≈58% of all Repell IIshoots, whereas only ≈31% of Triple Play shoots were infected.Regression analysis, using a segmented model, demonstrated that larvalpopulations declined as the proportion of endophyte-infected perennialryegrass increased. However, no additional reductions in larvalpopulation density were achieved in stands containing >40%endophyte-infected perennial ryegrass. Visual damage varied withcultivar, seeding rate, and year, but generally decreased as theproportion of endophyte-infected perennial ryegrass increased. Althoughseeding rate had an initial effect on turfgrass composition, thesedifferences were not detected after the first year of the study.Results indicate overseeding existing stands of Kentucky bluegrass withendophytic varieties of perennial ryegrass can reduce S.parvulus larval populations and their damage. However, plantgenetic characteristics (i.e., different cultivars) may be overridingdeterminants of many aspects of plant performance includingestablishment, tolerance to insect feeding, and expression ofendophyte-mediated resistance.