Biological control reduces growth, and alters water relations of the saltcedar tree (Tamarix spp.) in western Nevada, USA

We monitored the impacts of a biological control agent, the saltcedar leaf beetle (Diorhabda carinulata), on the saltcedar tree (Tamarix spp.) at two sites (Humboldt and Walker rivers) in Nevada, USA. At the Humboldt site trees that had experienced three to four defoliation events had more negative water potentials and lower foliar Δ13C than trees farther from the release site that had experienced only one defoliation event. We established paired trees (exposed to D. carinulata and sprayed with insecticide) at both sites and monitored impacts. Beetles reduced stem growth during the first year of defoliation at both sites but not in the second year at the Humboldt site. Defoliation did not affect midday water potentials, or leaf gas exchange during the first two years of defoliation of paired trees at either site. Furthermore there was no difference in foliar Δ13C in either year at the Humboldt site but defoliation during the first year lead to higher foliar Δ13C at the Walker site. These results suggest that initial defoliation by D. carinulata reduces growth but not water relations of saltcedar. However, repeated defoliation, potentially acting through reduced root growth, leads to an overall reduction in the water status of this invader.

Research highlights► Repeated defoliation by a biological control agent negatively impacts saltcedar water relations. ► Lighter levels of defoliation result in decreased stem growth but not water relations of trees. ► Impacts on saltcedar water relations are potentially associated with reduced root growth. ► Impacts are spatially and temporally variable. ► Impacts are dependent on biological control population dynamics and patterns of dispersal.