The susceptibility of plants to herbivores is known to influenced by a variety of factors including the species and phenotype of neighboring plants. These phenomena are known as associational effects and can either have a positive (associational resistance) or negative (associational susceptibility) effect on a given focal plant. We conducted a field observational study to determine if associational effects contribute to the susceptibility of dimorphic Datura wrightii to various specialist herbivores. This species varies with respect to trichome type with a sticky morph possessing glandular trichomes and a velvety morph with non-glandular trichomes. These two morphs coexist within populations across California, sometimes growing close enough to one another than their branches intertwine. Several species of herbivores – including Manduca sexta, Lema daturaphila, and Tupiochorus notatus – feed upon this plant leave a unique pattern of leaf damage. We estimated the leaf area damaged and counted the number and phenotype of neighboring plants (those within 1m of the focal individual) for hundreds of plants from over 30 populations of D. wrightii across California. We then analyzed data using a linear mixed effects model in R to determine if a correlation between herbivory and neighboring plant phenotypes was present.
Results/Conclusions
We observed plants at three time points summer (late July/early August) 2017, spring (late April/early May), and summer 2018. We found that the phenotype of a plant’s neighbors can influence its susceptibility to herbivores, but that this relationship varied over time. In the summer of 2017, we found that T. notatus damage correlated with a plant’s neighborhood such that soft plants with more sticky neighbors were more susceptible. This relationship also occurred in the spring of 2018 but did not in the summer of 2018. However, in the summer of 2018 we found a correlation between L. daturaphila damage and a plant’s neighborhood such that sticky plants with more sticky neighbors were resistant to this herbivore. Together these results show that associational effects between D. wrightii trichome morphs can mediate interactions between plants and their herbivores, but that these effects are not consistent over time and can vary from season to season and year to year.