Impacts of paternal latitudinal origin on herbivore response strategies in Lythrum salicaria

Plants have evolved different strategies to withstand and respond to herbivore stress. The two main strategies plants employ under herbivore stress are known as tolerance and induced resistance. Tolerance is the ability to maintain high fitness despite experiencing high levels of herbivore damage. While induced resistance typically involves changing the concentrations of internal secondary metabolites as a response to initial herbivory and to prevent further damage. These strategies can lead to fitness differences between populations that vary by latitudinal origin. Previous studies have focused on the occurrence of herbivore response in general, whereas this study will make a direct comparison between response types and their fitness implications. Our study investigates whether eastern North American populations of Lythrum salicaria respond differently to herbivore stress. F1 crosses between L. salicaria individuals with different parental latitudinal ranges were grown in a common garden under two experimental herbivory treatments (pesticide sprayed vs. no pesticide) at the Queen’s University Biological Station. To identify differences in herbivore tolerance, we measured plant growth and development over 13 weeks and quantified herbivore damage, caused by the biocontrol beetle Galerucella calmariensis. A 48-hour feeding assay was then conducted to identify differences in herbivore resistance between populations. This assay provided beetles with a choice between young and old L. salicaria leaves from each experimental treatment. Our preliminary analysis of the feeding assay showed differences in leaf damage between parental lines. The initial analysis also showed that older leaves generally experienced more damage than younger leaves over the course of the 48-hour period. These results indicate population differences in herbivory stress response and differences in stress response effectiveness that varies with plant age. This experiment showed that L. salicaria individuals have varying degrees of herbivore stress response and effectiveness. However, further analysis that includes reproductive outputs and developmental differences will be completed to clearly identify the type of strategies L. salicaria populations with different parental latitudinal origins employ in response to herbivore damage.