In classical weed biological control, small collections of arthropods are made from one or a few sites in the native range of the target plant and are introduced to suppress the plant where it has become invasive, often across a wide geographic range. Ecological mismatches in the new range are likely, and success using the biocontrol agent may depend on postrelease evolution of beneficial life history traits. In this study, we measure the evolution of critical day length for diapause induction (day length at which 50% of the population enters dormancy), in a beetle (Diorhabda carinulata) introduced into North America from China to control an exotic shrub, Tamarix spp. Beetle populations were sampled from four sites in North America 7 years after introduction, and critical day length was shown to have declined, forming a cline over a latitudinal gradient At one field site, decreased critical day length was correlated with 16 additional days of reproductive activity, resulting in a closer match between beetle life history and the phenology of Tamarix. These findings indicate an enhanced efficacy and an increasingly wider range for D. carinulata in Tamarix control.
The genus Diorhabda (Coleoptera: Chrysomelidae) was recently revised, using morphological characters, into five tamarisk-feeding species, four of which have been used in the tamarisk (Tamarix spp.) biological control program in North America and are the subject of these studies. The taxonomic revision is here supported using molecular genetic and hybridization studies. Four Diorhabda species separated into five clades using cytochrome c oxidase subunit 1 sequence data with Diorhabda elongata separating into two clades. Amplified fragment length polymorphism (AFLP) analysis using genomic DNA revealed only four clades, which corresponded to the four morphospecies. Hybridization between the four species yielded viable eggs in F1 crosses but viability was significantly lower than achieved with intraspecific crosses. Crosses involving Diorhabda carinulata and the other three species resulted in low F2 egg viability, whereas crosses between D. elongata, Diorhabda sublineata and Diorhabda carinata resulted in > 40% F2 egg viability. Crosses between D. carinulata and the other three species resulted in high mortality of D. carinulata females due to genital mismatch. AFLP patterns combined with principal coordinates analysis enabled effective separation between D. elongata and D. sublineata, providing a method to measure genetic introgression in the field.
The northern tamarisk beetle (Diorhabda carinulata) was released in 2001 as a biocontrol agent for Tamarix spp., an invasive tree that dominates riparian ecosystems throughout the southwestern United States. The factors that influence its effectiveness at controlling Tamarix, and the effects of control on plant communities, are not well known. Here we report patterns of Tamarix dieback, mortality, and vegetation composition at ten of the early D. carinulata release sites in western Colorado. Across the ten release sites, 265 permanently marked Tamarix trees were measured over a six year period (2008-2014). Vegetation composition and woody debris adjacent to each of these trees were measured annually for four years (2010-2014). We examined relationships between site factors (soil properties, hydrology, and land use history), Tamarix dieback, and vegetation composition. Tamarix mortality was observed at seven of ten sites, where it ranged from 15-56% after six years. Overall, Tamarix crown cover decreased by more than half (54%) while crown volume decreased by 63% in the first two years of the study. Neither total plant cover nor fallen woody debris increased under Tamarix trees over the last four years of the study. Combined cover of classified noxious weeds and other non-native species was greater than native plant cover at eight of ten sites. D. carinulata proved to be effective in controlling the Tamarix invasion locally. However, the high cover of noxious weeds will continue to be a management problem, with or without Tamarix control by the northern tamarisk beetle.
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