Sources of Life History Variation in the Pacific Spider Mite

Background/Question/Methods   Life history variation is a general feature of arthropod systems, but is rarely treated directly in models of population dynamics. Most models assume that local processes occur identically across individuals, ignoring any genetic variation in life history traits. Additionally, we expect some inherent phenotypic variation at the individual level, suggesting a distribution of developmental outcomes from the identical individuals. In this study, we tested whether field populations of Pacific spider mites (Tetranychus pacificus) display significant intraspecific life-history variation using a common garden experiment of mites in the laboratory. We also investigated what models of variation in individual development and reproduction are sufficient to produce the variation present in the laboratory populations. Spider mites were sampled from eleven vineyards in San Joaquin county in California and used to found lab cultures. Cohorts of mated adult females and eggs were removed from these cultures and each transferred to identically-sized bean plants (Phaseolus lunatus). The dynamics of these focal populations were followed for twelve days (1-2 generations). The dynamics were then simulated using distributions of stage-specific development times and fecundity patterns to assess what combinations of genetic and phenotypic variation in life history traits could explain the observed variation in dynamics.

Results/Conclusions   Dynamics of the lab populations varied widely, resulting in a large range of abundances at the end of twelve days. This variation could not be attributed to site differences, sampled cultivar, or spatial pattern in either adult-founded populations or egg-founded populations. In the simulated dynamics, the variation may be explained by a range of combinations of genetic and phenotypic variation, represented by model parameters for the means, variances, and correlations between developmental times. Models with fixed life history parameters often fail to reproduce the large variation present in natural populations. In field data, the effects of some driving factor may be masked by the large variation between (and within) individuals of a population. Intraspecific variation in life history traits such as fecundity, growth and survival, may prove integral to any realistic model of arthropod dynamics.