The extent to which encapsulation of eggs of Bathyplectes curculionis (Thomson) reduces the effectiveness of this parasitoid as a control agent for the alfalfa weevil, Hypera postica (Gyllenhal), was evaluated in two studies over the period from 1973 to 2000. Collections of alfalfa weevil larvae were made annually from research areas in Grady and Payne Counties (Study I) and in statewide surveys (Study II) across Oklahoma to estimate the prevalence of encapsulation of eggs of both first and second generations of B. curculionis. Subsamples of 100 weevil larvae were dissected for each sampling date to determine the numbers and condition (healthy versus encapsulated) of parasitoids in each host larva. Probabilities for encapsulation of solitary eggs ranged from 0.20 to 0.29 for the first generation and from 0.07 to 0.08 for the second generation of B. curculionis. Although probabilities for encapsulation of at least one egg in superparasitized larvae ranged from 0.48 to 0.55 for the first generation, the probabilities of supernumerary eggs being encapsulated were much lower, ranging from 0.03 to 0.19. Regression analyses indicated highly significant relationships between effective (hosts containing healthy parasitoids) and actual (all hosts containing parasitoids) parasitism for both studies across all values (1–100%) for actual parasitism. Regression of efficiency of encapsulation (percentage of reduction in effective parasitism caused by encapsulation) on years indicates that the value of encapsulation as a defense for H. postica against B. curculionis may have diminished over the 28 yr that these studies were conducted. The reduction in efficiency of encapsulation does not appear to have resulted from greater prevalence of superparasitism, as the occurrence of supernumerary parasitoids in larvae of H. postica also decreased during these studies.