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, clutch size declined from a mean of 5.8 to 3.4 as host encounter rate increased from 1 to 8 cocoons per day. In contrast, when life expectancy was greatly reduced by withholding honey as a food source, females did not adjust their clutch size. Mean brood size produced by females foraging in walnut orchards was significantly greater than that under laboratory conditions with excess hosts. In addition, fitness (searching success under field conditions) was found to increase exponentially with body size suggesting that under field conditions M. ridibundus was host limited, producing brood sizes that approximated the Lack brood size, but under laboratory conditions they were egg limited, produced brood sizes notably smaller than the Lack brood size.
Species 1: Hymenoptera Ichneumonidae Mastrus ridibundus
Species 2: Lepidoptera Tortricidae Cydia pomonella (codling moth)
Keywords: fitness, search efficiency
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