Associational effects – in which the vulnerability of a plant to herbivores is influenced by its neighbors – have been widely implicated in mediating plant-herbivore interactions. Much of this research focuses on either interspecific interactions or pest-crop dynamics in agricultural settings but given the degree to which defensive traits can vary within a single plant species, associational effects are likely to be important to intraspecific interactions between plants in natural populations. In this study, we observed hundreds of Datura wrightii plants – which vary with respect to their trichome phenotype – from over 30 dimorphic populations across California to determine if a relationship exists between the phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insect. We visited plants at three time points in order to assess how these effects vary both within and between growing seasons. We found that both the number and phenotype of neighbor plants impacted the likelihood of plants being damaged by two of the four herbivore species, but that these effects were only present early in the growing season (spring). Throughout the growing season (spring and summer) focal plant phenotype was more effective at predicting whether a plant was attacked by a given herbivore. We also found evidence that the focal plant phenotype influenced the likelihood of predatory arthropods (spiders and hemipterans) being present and of oviposition by Lema daturaphila beetles. Together these results show that associational effects between D. wrightii individuals can impact vulnerability to herbivores and emphasize the importance of within population trait variation for mediating interactions between plants and herbivorous insects.
Coauthors: Lynda Delph – Indiana University;Genevieve Pintel – University of Guelph;Sonya Sternlieb – Wesleyan University