Geographic variation in Aedes triseriatus (Diptera: Culicidae): temperature-dependent effects of a predator on survival of larvae

Abstract

Survival of larval Aedes triscriatus (Say) with and without the predator Toxorhynchites rutilus (Coquillett) was evaluated as a reaction norm for 4 populations that cooccur with T. rutilus (sympatric) and 4 that never, or very rarely encounter T. rutilus (allopatric). Survival after 7 and 14 d at 2 experimental temperatures was analyzed. There was no consistent difference in survival between sympatric versus allopatric populations at both 7 and 14 d. Multivariate analysis indicated a significant populations-environment interaction involving population, temperature, and predation. This interaction was only significant in univariate analyses at 14 d, accounting for 29.9% of the random variation in survival, populations surviving best or worst with the predator at 20°C did not also survive best or worst, respectively, with the predator at 24°C. Survival was also unrelated to long-term average temperatures from the collection sites. Net reduction in survival as a result of predation tended to occur earlier at 24°C than at 20°C. At 20°C, net reduction in survival because of T. rutilus predation was statistically undetectable after 7 d, but was significant after 14 d. Populations of A. triseriatus varied considerably in their ability to survive with this predator, and in their survival at the study temperatures, but population cooccurrence with the predator and long-term average temperatures from the collection sites appeared to be unrelated to this variation. Variation among populations is only apparent after a fairly long period of development, and the pattern of variation is highly dependent on the experimental temperature. Geographic differentiation in survival with this predator seems likely to be a by-product of geographic variation in growth, development, and feeding rates of A. triseriatus, and unlikely to be primarily a product of natural selection by T. rutilus predation in nature.