Salinity driven interactions between plant growth and a biological control agent

Abiotic conditions can influence the effect that herbivores have on plant growth. Such biotic and abiotic interactions are of special interest in plant biological control programs because the goal of herbivore suppression of the target weed may not be reached in some abiotic settings. Additionally, target invasive plants typically occur across diverse landscapes raising the possibility that local adaptation to site-specific conditions leads to phenotypic variation that can affect herbivore responses. Here, we used Tamarix, an invasive plant, and its associated biological control agent, Diorhabda carinulata, to investigate how local variation in soil salinity and host plant origin influence interactions between the two taxa. To test if Tamarix was adapted to local conditions, we collected plants from sites with either low or high groundwater salinity, asexually propagated them through multiple generations, and then treated them with their home or reciprocal salinity levels. We found that plants accumulated the most biomass when grown at the salinities of their origin site. The biological control agent, D. carinulata preferred plants grown at source site salinity when given a choice against plants grown in the reciprocal salinity treatment. Although plants compensated for herbivory by regrowing foliage over three defoliation events and maintained similar leaf biomass through regrowth, they ultimately had a reduced basal area and 62% lower root biomass compared to the controls. Thus, herbivory caused a shift in plant allocation of resources from overall growth to compensation, reducing root and stem investment. Overall, D. carinulata caused a significantly greater reduction in total biomass in the high salinity plants than the low salinity ones when grown at their source salinity (averages of 63% and 32% respectively). Thus, the Tamarix biological control program may experience its greatest efficacy in high salinity areas where the impact of the agent is the greatest, likely due to increased water stress and reduced resources to enable regrowth.