Influences of environmental conditions and genetic status on dieback and mortality of Tamarix in response to biological control

Background/Question/Methods

The host-specific northern tamarisk beetle, Diorhabda carinulata (Chrysomelidae), was introduced into North America in the early 1990’s to suppress invasive Tamarix spp. in western riparian areas and promote recovery of native riparian vegetation. Where beetles initially established in northern states gradual dieback from repeated defoliation events led to substantial mortality after 4 or more years. As beetles have colonized southern regions, plant impacts appear to occur more rapidly, with mortality observed following two years, and even after a single year, of defoliation. As previously documented in northern areas, mortality is the product of incremental depletion of nutritional resources as indicated by soluble carbohydrate concentrations in root tissues. At our Virgin River study area, we are monitoring ca. 1000 trees across a gradient of elevation, time since initial beetle establishment, soil conditions (salinity, texture, pH) and climate variables, as well as genetic status with respect to T. ramosissima vs. T. chinensis contribution to the predominantly hybrid plants.

Results/Conclusions

Preliminary analyses indicate that soil salinity is positively but mildly correlated with host plant dieback, while we anticipate that genetic status is related to mortality but additional years are necessary to establish an unambiguous relationship with haplotype. A separate study at more northern latitude showed mortality was positively related to host plant growth rate as indicated by pre-beetle tree ring increment width, suggesting that nutritional allocation strategies may explain differential mortality patterns. More specifically (and as also postulated by W. Williams), plants which allocate metabolites to growth are at a disadvantage when exposed to repeated defoliation by Diorhabda as compared with plants that allocate nutrients preferentially to storage (represented by dominance of T. ramosissima genetic contribution) because they lack the reserves needed for regeneration from defoliation. If mortality is generally more rapid in southern regions, efforts to promote native vegetation should be accelerated to facilitate recovery of associated wildlife species that have declined regionally because of Tamarix invasion.