Growth, development, and life-history strategies in an unpredictable environment: case study of a rare hoverfly Blera fallax (Diptera, Syrphidae)

Abstract

1. Development in organisms can vary in response to fluctuating environments. In holometabolous insects, variation in adult phenotypic traits is strongly influenced by growth conditions experienced by larvae. The main aim of this study was to assess how much environmental insight can be gained from analysis of the phenotypic changes in an insect's life history parameters in response to realistic food limitations.  2. This investigation was motivated by a need for more information about the developmental requirements of the endangered pine hoverflyBlera fallax(Linnaeus) (Diptera, Syrphidae) in Scotland.Blera fallaxdepends on a scarce and often ephemeral habitat, rot holes of Scots pinePinus sylvestrisL. stumps. We studied how rearing conditions affected growth in captive larvae, and compared these responses with a wild population.  3. The growth curve observed in the field was similar to that in resource-limited, lab conditions, suggesting that resources are limiting in nature. The effects of resource availability on development time and body size depended on sex. Adult females were larger but had more variable size at maturity compared with males. In contrast, males typically were not smaller in resource-limited conditions, but rather continued to develop for another year. Between 2% and 20% of larvae extended development over 2 years regardless of growth conditions, perhaps indicating a semivoltine strategy to circumvent extinction during years with a low breeding success.  4. These results identify life history traits that may be important for other saproxylic Diptera in rot holes, and organisms that experience food restrictions during growth.