| A large-scale biological control program against invasive saltcedars (Caryophyllales: Tamaricaceae) has been implemented in the western United States. The program targets invasive Tamarix parviflora at northern California release sites using the leaf beetle Diorhabda elongata (Coleoptera: Chrysomelidae), and has resulted in delayed establishment success. Herein host plant associations of D. elongata were investigated with regards to behavior, fitness consequences, and native range population genetic variation, to better understand program efficacy.;Effects of larval host plant on adult fecundity and fertility were evaluated under no choice laboratory conditions. There was no correlation between host plant and larval survival, adult egg laying, or egg viability, suggesting that selection factors extrinsic to host plant quality may have shaped the maintenance of this preference trait over time.;Population structure in the agent's native Greek range was assessed using a portion of the mitochondrial gene, cytochrome oxidase I, to explore the role of local adapt on to host plant on population differentiation. Seven northern Greece and 11 Crete D. elongata populations were sampled, as well as U.S. laboratory colonies and northern California field-established D. elongata. There was significant genetic structure in the data set which was best explained by restricted gene flow with isolation by distance, and some allopatric fragmentation at several geographic scales. Structure included a regional divide in haplotype distribution between northern Greek and Crete D. elongata. Host plant had no significant impact on haplotype distribution, but a number of morphologically ambiguous host species samples, prevalent on the island of Crete and characterized molecularly as trn G-S region haplotype C, may have contributed to a lack of resolution. There was a loss of genetic variation in the laboratory population over time, and a unique haplotype was sampled only in the field population.;Our findings document differences between the host preferences of a field-established biological control agent population and its source laboratory colony, namely the relaxation of a host acceptance threshold. These changes may have allowed D. elongata to perform better on the target weed, T. parviflora , at northern California release sites. If trait shifts are common, then release strategies should take the selection process into account, as repeated releases could swamp founding populations with undesirable variation. Our work also suggests that genetic variation in the beetle's native range is not uniformly distributed, but historical processes and unidentified local adaptations are more important variables than host related ecological associations.;Post-release host-specificity changes were detected in both greenhouse and open field host choice tests. In greenhouse experiments, source colony D. elongata showed strong ovipositional preference for T. ramosissima over T. parviflora, consistent with findings of host range testing conducted at the onset of the biological control program. However in the same experiments, D. elongata reared from the field population showed no significant preference between the two host plants. In an open field host choice experiment conducted for the source colony just prior to widespread field establishment (2006), T. ramosissima and mixed host treatments had greater oviposition and retention of adult beetles over time when compared to T. parviflora only treatments. In 2008, source colony and field-established beetles were compared using the same open field experimental design and field-established beetles remained longer than source colony beetles, irrespective of host treatment. There were also significantly more source colony D. elongata on the T. ramosissima treatments than others, but there was no treatment effect on field-established beetles. |
