Of all the different types of insects in the world, ants are the most numerous. They are incredibly widespread, living in almost every environment that plants do, except remote islands and the poles. Scientists believe that ants and flowering plants evolved around the same time, at least 140 million years ago, during the Cretaceous period. They’ve had a very long time to develop complex relationships, both antagonistic and symbiotic. Plants that engage in the more mutualistic end of this spectrum are called myrmecophytes, “ant-plants,” or, more romantically, myrmecophiles, “ant-lovers.” One of these is our local mayflower, which employs ants to disperse its seeds.
Some plants have even evolved new organs just for ants: extrafloral nectaries. Typically, a plant produces nectar only in the flower; a sugary treat to trick animals into carrying pollen, enabling reproduction. These structures, however, are extra-floral; they occur outside of the flowers. The nectar they produce is not for reproduction, but for violence! Ants, wasps, and other omnivorous insects will feed on the nectar, and on the way, hopefully snack on any pests that threaten the plant too. Peonies are the most well-known of these. An old garden legend states that ants are needed to open the flower’s buds, but they are really there to sip on nectar, and maybe snack on a bug.
Some of the gall-forming insects we met last month have another trick up their sleeve. In addition to creating the galls that house and feed their larvae, some species can induce galls to produce extrafloral nectaries. The hornets and ants that feed from these will deter other parasitic wasps, and maybe even birds, who are hunting the larvae within. Amazingly, these occur in oaks (pictured), which don’t otherwise produce extrafloral nectaries. Oaks don’t even produce nectar in their flowers, as they are wind-pollinated.
There may be another level of complexity at play. Ants have been practicing agriculture longer than humanity, farming insects in the order Hemiptera. These insects, such as aphids and scale insects, live on plant sap and produce a sugary waste product called honeydew. Enterprising ants will collect these bugs and amass them on certain plants in order to harvest the honeydew. The plants do not enjoy this relationship; sap-sucking insects are more damaging than chewing insects, and are more likely to transmit disease. Some studies suggest that extrafloral nectaries are not designed to attract ants at all, but to distract them; Given a steady nectar supply, they don’t need to engage in the labor of farming aphids. They are also kept out of the flowers, where they might bother pollinators.
Thousands of different plant species produce these glands, mostly in the legume, passionflower, and mallow families. While they can be on almost any plant part, ours are mostly at the leaf bases and stems. Not much to look at, these are small reddish bumps in symmetrical pairs, sometimes sporting a drop of nectar. Look for them locally on cherry, greenbriar, catalpa, poplar, and elderberry, or even some of your houseplants, like philodendron and hoya. Watch out for ants, though.
Written by Justin Cifello, Wareham Land Trust Member and Volunteer