<b>Reunification of an invasive plant with its specialist herbivore selects for rapid adaptive changes in defensive volatiles</b>

Reassociation of invasive plants with specialist herbivores can select for the restoration of relaxed defense syndromes, however, we know little about how volatiles in plant defenses respond to such reunions. We studied populations of invasive Ambrosia artemisiifolia with varying reassociation histories (0, 12, 16, and 20 years) with the specialist Ophraella communa and found that longer reassociations correlated with higher abundances and diversities of generalist herbivores in a common-garden. Plants with longer reassociation histories were less attractive to Ophraella, and this correlated with reduced emissions of limonene and α-pinene, in turn correlating with increased defense against Ophraella. In contrast, longer reassociated plants were more attractive to the generalist Spodoptera littoralis and the reduced limonene emission correlated with weaker defense against Spodoptera. Our results imply that a specialist may drive rapid evolution of volatile-related defenses, enhancing plant resistance to this specialist but at the cost of weaker defense against a generalist.