Last week, I wrote an Alert describing the "ETC Two-Step Control Method;" a very direct approach to eliminating Eastern Tent Caterpillars (ETC) (Malacosoma americanum). But what about more subtle "behind the scenes" ETC population regulators and where do extrafloral nectaries fit in?
What are Extrafloral Nectaries (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 elements provided by water and carbon dioxide. The recent rise in awareness of the importance of pollinators has highlighted this nectar-based economy. However, flowers are not the only place where nectar serves as coinage in exchange for services.
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."
You may be aware of the EFNs found on the leaf petioles of trees in the genus Prunus. Many of us first learned about these tiny glands in relation to their usefulness with identifying plants in this genus, particularly cherries. Indeed, the glands first served only as taxonomic features before research studies later revealed their ecological function.
The EFNs on cherries and other Prunus species 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, a lot of things look like donuts to me; I have a highly developed search image.
EFNs and Plant 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 ant protection extends to caterpillars on cherries in his paper, "Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation to Susceptibility of Caterpillars to Ant Predation" (Ecology, 59(4), pp. 686-692). The title says it all.
Tilman found that the EFNs on black cherry (P. serotina) commonly attract western thatching ants (Formica obscuripes) which are predaceous on many caterpillars including ETC. His research showed that the number of ants visiting the EFNs was directly correlated with the distance between ant colonies and cherry trees and ETC survivorship was positively related to those distances. He 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 activity. 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."
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. Obviously, many plants in the Prunus genus, including cherries, are myrmecophiles with their sweet love expressed through EFNs.