Adelges cooleyi (Hemiptera: Adelgidae), a host-alternating gall-making insect pest native to the Rocky Mountains and Pacific Northwest and introduced into the eastern United States during the mid-19th century, was studied to address questions about phylogeography, to determine effects of introduction on genotypic and phenotypic variation, and to compare genetic variation associated with host use in native and introduced ranges. In Chapter One, sequence data from two mitochondrial (mtDNA) genes and amplified fragment length polymorphisms (AFLPs) were used to quantify the structure of genetic variation in the insect's native range. Several well-supported, divergent mtDNA lineages were identified. The structure of genetic variation among sampled locations is consistent with patterns shaped by glaciations. Samples from the southern edge of the insect's distribution are genetically isolated from the rest of the species, and hybridization of divergent mtDNA lineages via secondary contact was inferred from AFLP data.

Changes in genetic and phenotypic variation associated with introduction were quantified in Chapter Two. Introduced populations had decreased genetic variation relative to native populations. Variation in an ecologically important trait, host preference, was also significantly lower in introduced populations than in native populations. An association between mtDNA haplotypes and host preference was identified. Adelges cooleyi in the eastern US have low genetic and phenotypic variation but appear to be sufficiently adapted for persistence. My results call into question the utility of neutral genetic variation to assess the probability of persistence in new environments by introduced species.

Host-plants that A. cooleyi requires to complete its lifecycle are not native to the eastern US and occur together in patches that are often widely separated. In Chapter Three, analyses of mtDNA and AFLP genetic variation were conducted to determine the distribution of genetic variation within and among host plants in the native range and identify discrepancies that may be consistent with an incomplete lifecycle in the introduced range. Distribution of genetic variation within and among host-plants in the introduced range was not significantly different than that in the native range, as indicated by fixation indices, and I found no evidence for asexual populations in the introduced range.