Like many, my first introduction to extrafloral nectaries (EFNs) was in a dendrology class. The tiny structures were described as “glands” found on members of the Prunus genus. They appear on the leaf petiole or along the edge of the leaf blade near the base. I was told the glands are a sure-fire way to identify cherry trees. I was not told what the glands do for the plant, nor did I ask. My thirst for knowledge began and ended with passing the tree ID exam.
I eventually learned the glands are EFNs. I also learned EFNs are not confined to cherries and other members of Prunus. Indeed, according to a 2015 University of Florida fact sheet titled, "Many Plants Have Extrafloral Nectaries Helpful to Beneficials" (see "More Information" below), "… few people are aware of the extrafloral nectaries (EFN), nectar-producing glands physically apart from the flower, that have been identified in more than 2000 plant species in more than 64 families."
I’ve taken pictures of EFNs on everything from wild senna (Senna hebecarpa) to passionflower (Passiflora incarnata), one of my favorite plants because of its bizarre showy flowers. EFNs are also found on certain galls produced under the direction of cynipid wasps (family Cynipidae).
What Exactly are EFNs?
Nectar is the currency used by plants to pay insects and other animals to do their bidding. It costs plants nothing to photosynthesize this sweet medium of exchange using energy from sunlight to rearrange the chemical elements provided by water and carbon dioxide. The recent rise in awareness of the importance of pollinators has highlighted this nectar-based economy.
EFNs are exactly what their name sounds like. They are nectar-producing organs that are not found in flowers; they are extrafloral. The EFNs on Prunus can vary considerably in size and shape from species to species. Some appear as small red or green "bumps" or even "dots" on the petiole at the base of the blade while others look like well-defined pinkish-red to deep red donuts. Of course, I have a highly developed built-in search image for donuts.
EFNs and Payment for Protection
Sharp-eyed botanists and others have long known EFNs exuded nectar. Even at low magnification, nectar droplets can be seen oozing from some of the larger types of these plant structures. Some researchers also observed relationships between EFNs and ants. In 1910, William M. Wheeler reported in his publication "Ants: Their Structure, Development and Behavior" (Columbia University Press, New York, New York, USA) that ants appeared to benefit from EFNs.
In 1966, Daniel H. Janzen published direct evidence that ants attracted to EFNs significantly reduced herbivore damage to acacias ("Coevolution of Mutualism Between Ants and Acacias in Central America," Evolution, Vol. 20, No. 3. (Sept., 1966), pp. 249-275). University of Minnesota ecologist David Tilman showed that nectar brides can induce ants to protect cherry trees. The title of his paper says it all: "Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation to Susceptibility of Caterpillars to Ant Predation" (Ecology, 59(4), pp. 686-692).
Tilman found that the EFNs on black cherry (P. serotina) commonly attract western thatching ants (Formica obscuripes) which are predaceous on many caterpillars including eastern tent caterpillar (ETC) (Malacosoma americanum). His research showed that the number of ants visiting the EFNs was directly related to the distance between ant colonies and cherry trees. ETC survivorship was positively correlated to those distances: the greater the distance, the higher their survival.
Tilman also found that the highest number of ants visiting the trees occurred just after bud break and decreased as the number of active EFNs decreased. This time frame coincided with the development of ETC with caterpillars large enough to escape ant predation appearing after EFNs ceased their nectar payment for security services. He concluded that his data suggested, "… the ant-cherry relationship is a facultative mutualism and that nectar production is timed so as to maximize the chance of successful ant predation on tent-caterpillar colonies."
Galls produced under the direction of cynipid wasps are extraordinary plant structures. The wasps hijack meristematic (undifferentiated) plant cells by exuding phytohormones to direct plants to grow a bed and breakfast for their offspring. The gall provides both a home and food in the form of “nutritive tissue” surrounding the gall chamber. Imagine living in a room with walls made of pizzas!
Some gall-wasps even make security arrangements for their offspring by directing the inclusion of EFNs within the gall structure. Oak rough bulletgalls are produced under the direction of the cynipid wasp Disholcaspis quercusmamma. Nectar oozing from EFNs attracts a phalanx of gall-protectors from stingers like bald-faced hornets (Dolichovespula maculata) and yellowjackets (Vespula spp.) to biting insects such as carpenter ants (Camponotus spp.). Predators such as birds targeting the cynipid meat morsel within the gall stand little chance against the gall's security detail.
Final Trivial Pursuit
The term "myrmecophile" means "ant lover." It is derived from the Greek "myrmex" = ant, and "phlos" = loving. The term applies to the special relationship some plants and animals, including insects (e.g., aphids), have with ants. As shown by this Alert, many plants and some plant gall-makers are myrmecophiles with their sweet love expressed through EFNs.
Many Plants Have Extrafloral Nectaries Helpful to Beneficials