Geographical variation and gene flow in the eucalyptus defoliating beetle Chrysophtharta bimaculata (Coleoptera: Chrysomelidae)

1. Management of the eucalyptus defoliating beetle Chrysophtharta bimaculata is considered essential to the economic sustainability of timber production in Tasmanian eucalyptus plantations. This study examined population subdivision and host associations in Chrysophtharta bimaculata. We aimed to identify the spatial scale at which management practices should be implemented, and to aid in the development of long-term management strategies that avoid the build up of resistance to chemical or biological insecticides. 2. Genetic variation and gene flow among C. bimaculata subpopulations was examined using cellulose acetate gel electrophoresis. Subpopulations in central Tasmania were found to exchange relatively large numbers of immigrants, and could not be distinguished from a single large interbreeding population. 3. Restrictions to gene flow not related to geographical distance were observed among northern regional subpopulations. Our results are consistent with gene flow between northern subpopulations occurring primarily via movement into and out of central Tasmania along a limited number of gene how corridors. We suggest that the clearing of eucalypt ash forests for agriculture in the north-west, north-east and the 'Midlands' region may be partially responsible for the restricted pattern of gene flow observed among northern sites. 4. Adjacent subpopulations collected from different hosts at three locations show no evidence of genetic differentiation. Therefore, significant regional scale population subdivision does not occur on the basis of host species. 5. That moderate to high levels of gene flow occur among beetles from all districts and hosts, implies that resistance build up is unlikely to occur if control agents are used at low levels and against a limited number of subpopulations. However, any increase in the frequency of resistant genotypes at sites where control measures have been implemented implies that resistance is being exported to all other subpopulations. This may be a particular problem in northern regional subpopulations, where reduced immigration may facilitate the local build up of resistance under a significantly less intensive control regime.