Many plants respond to attack by secreting sugar-rich nectar—not for pollination—but for protection. Aggressive ants and other predators patrol leaf surfaces in response to extrafloral nectar (EFN) secretion, but how plants advertise this resource to draw predators to the plant has not been described. Scented floral nectar is well-known as a mode of plant-pollinator communication in plant ecology, yet volatile constituents of EFN as predator attractants have never been examined. To test for a recruitment signal from EFN itself that may directly alert and attract defensive ants, we conducted ant choice bioassays in headspace above nectar, sucrose, and water droplets, and analyzed the GC-TOF-MS profiles of EFN volatiles.
Here we show EFN from three different plant species (lima bean, Phaseolus lunatus; passion vine, Passiflora edulis; and cherry, Prunus avium) contain diverse suites of volatile constituents and that the ubiquitous black gardener ant (Lasius niger) responded to plant species-specific EFN volatile cues (analysis of similarities (ANOSIM): R= 0.6709, P= 0.001, stress value= 0.183). Ants consistently chose zones above EFN compared to sucrose and water controls. Prior experience on a particular host plant species caused ants to spend 2.6, 4.7, and 6.02 times longer above that species’ EFN compared to the second most attractive EFN, based only on differences in volatile cues. A volatile-based communication pathway in EFN-mediated defense represents an important missing mechanistic component underpinning mutually beneficial ant-plant symbioses. Similar to well-studied floral systems, scented EFN may enable stationary plants to further manipulate mobile organisms capable of providing defensive services, and likely represents a trait contributing to the maintenance and stabilization of defensive mutualisms in EFN-secreting plants.