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This research investigated parasitism of the buffalograss mealybugs Tridiscus sporoboli (Cockerell) and Trionymus sp. by Rhopus nigroclavatus (Ashmead) through rearing and dissection studies, paired comparison tests, and a field survey. Rates of parasitism reported in this article reflect combined parasitism of the 2 mealybug species. In the rearing study, parasitism of adult female mealybugs by R. nigroclavatus was 48.6%, with an average emergence of 1.77 parasitoids per parasitized mealybug. A maximum of 7 R. nigroclavatus adults emerged from a single mealybug female. Mealybug dissections documented parasitism rates by R. nigroclavatus of 78.5, 67.5, and 4.3%, respectively, for adult females, 3rd and 4th (male pupae) instars, and 1st- and 2nd-instar mealybugs. Results revealed the preference of R. nigroclavatus for adult female mealybugs and later instars, and suggested a potential regulating effect of R. nigroclavatus on buffalograss mealybug populations. Paired comparison tests with and without R. nigroclavatus demonstrated the effectiveness of this parasitoid as a biological control agent for buffalograss mealybugs under greenhouse conditions. Significant differences were detected in the number of nonparasitized mealybugs between treatments containing only mealybugs and treatments containing both mealybugs and parasitoids. Sticky trap captures in the field suggested a functional relationship between the seasonal abundance of R. nigroclavatus and its mealybug hosts.