Female size affects provisioning and sex allocation in a digger wasp

Investment ratios in field populations of the European beewolf, Philanthus triangulum F. (Hymenoptera, Sphecidae), are strongly biased towards males. Possible explanations are conditional sex allocation and/or constraints on provisioning females: daughters need at least three prey items (honey bees), whereas sons need only one to develop and reproduce. If small females are not able to procure these three bees quickly enough they might have to produce sons instead. Larger females had a higher proportion of successful hunting flights, carried heavier loads, and could fly continuously with a load for longer than small females. They brought in more and heavier bees but needed less time for a single hunting trip. An estimate of the sex allocation of the food bees, based on the timing pattern of successful hunting trips, suggested that female size is positively correlated with the proportion of daughters produced. However, this effect explains only a small part of the overall variation in investment ratio.