Under field conditions, insect parasitoids probably experience lower rates of host encounter and life expectancy than under optimal conditions in laboratory studies. We examined the clutch size response of Mastrus ridibundus, a gregarious idiobiont parasitoid of codling moth, Cydia pomonella, cocoons, to variation in both host encounter rate and life expectancy as possible explanatory variables in a comparison of brood size in the field and laboratory. Under laboratory conditions, mean clutch size (number of eggs laid) declined from 5.8 to 3.4 as host encounter rate increased from one to eight cocoons per day. In contrast, when we reduced life expectancy by withholding honey as a food source, females did not adjust clutch size. Mean brood size (number of progeny surviving to pupation) of females foraging in walnut orchards (3.9) was significantly greater than that under laboratory conditions with excess hosts (3.1). Brood size also increased with host size in the field, but not under laboratory conditions. Brood size fitness curves were derived using both host-finding ability in the field and lifetime fecundity under laboratory conditions as indices of female fitness. Host-finding ability increased exponentially with body size, generating an estimated Lack brood size of 4.3, but lifetime fecundity increased linearly with body size, giving a Lack brood size estimate of 5.5. Under field conditions, female M. ridibundus produced brood sizes that closely approximated the Lack brood size estimated from host-finding ability, but that were significantly smaller than that estimated from lifetime fecundity. These observations suggest that, in contrast to lifetime fecundity measures from the laboratory, host-finding ability in the field provides a more accurate estimate of lifetime reproductive success for parasitoids with a low expectation of future reproduction. Reproduction; Reproductive productivity; Life cycle and development; Development; Evolution; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Habitat; Man made habitat; Land zones; Nearctic region; USA; North America Cydia pomonella (Tortricidae); Hymenopteran parasites; Mastrus ridibundus; Clutch size decisions & fitness implications in parasitoid, orchards; Cultivated land habitat; Walnut orchards; California; Central Valley; Clutch size decisions & fitness implications in hymenopteran parasitoid, field & laboratory study Mastrus ridibundus (Ichneumonidae); Fecundity; Lifetime fecundity; Egg number; Clutch size; Longevity; Life expectancy; Natural selection; Fitness curves; Lepidopteran hosts; Cydia pomonella; Clutch size decisions & fitness implications in parasitoids; Cultivated land habitat; Walnut orchards; California; Central Valley; Clutch size decisions & fitness implications, field & laboratory study, orchards none