Most folks know that flowers often produce nectar to attract pollinators — animals that carry pollen from one flower to another, enabling seed development. Less well known is the widespread occurrence of nectar outside of flowers, produced by “extrafloral nectaries.” All nectars typically contain simple sugars such as glucose, fructose and sucrose, amino acids, some defensive compounds to deter microbes, and various micronutrients.
Extrafloral nectaries are found on many kinds of flowering plants in more than 90 taxonomic families, and even on some ferns. They occur on leaves, bud coverings, fruits, or shoots. The structure of such nectaries varies enormously, from pits and slits to tiny cups, to scales and hairs and thorns, to no special structure at all.
There was much debate over the function of these nectar-producing structures. Some biologists claimed that the plants were just excreting unwanted molecules. But that suggestion did not account for many facts about these extrafloral nectaries, including why they are located where they are, why the plant would want to get rid of sugars and amino acids that are needed for growth, or why — in most cases — ants and sometimes wasps or flies could often be seen sipping nectar from these sources. Back in the Midwest, I remember seeing dozens of ants crawling over the buds of my mother’s peonies and wondering what they were doing there.
Eventually, it became clear from extensive research that extrafloral nectaries usually attract ants, parasitic wasps, and even predatory mites and spiders that help protect the plants from herbivores. In a few cases, extrafloral nectaries attract insect-eating birds, which may then consume harmful caterpillars. The most common protective insects are ants that patrol the plant and attack insects that would eat leaves or flowers. Some adult wasps feed on nectar and lay their eggs on plant-eating caterpillars; when the eggs hatch, the larvae eat their caterpillar hosts. When a plant is swarming with aggressive ants or attended by stinging wasps, it may even be protected from grazing and browsing mammals.
It turns out that some kinds of plants nearly always produce extrafloral nectaries, but others produce them mainly or only when they are attacked by herbivores. If one part of such a plant is attacked, nectaries may be induced all over the plant and even on the plant’s neighbors (of the same species). That is, the volatile chemicals released when an herbivore attacks a leaf induce a defensive response on other leaves, even on other plants. (This kind of chemical communication is now known among a variety of plants.) It is cheaper, metabolically, to produce defenses only when needed, especially against intermittent herbivores, rather than maintaining them permanently.
Extrafloral nectaries are most common among lowland and mid elevation tropical plants, but many temperate-zone plants have them too. Examples include some members of groups that are represented here, such as elderberries, viburnums, iris, and impatiens, but whether or not our local representatives of these groups have these nectaries is not determined. The farther north one goes, and the higher the elevation, the fewer are the species that bear these nectaries. This geographic pattern is associated with a lower availability of ants at higher latitudes and elevations, where air temperatures are cool; they are also uncommon where soils are wet. If there are few or no protective ants (or other protectors) around, why spend energy on producing extrafloral nectaries and nectar?
Nevertheless, some plants that can develop extrafloral nectaries occur in our region, despite the high latitude (and in Southeast, the wet soils) and the paucity of ants. Members of the genus Populus (cottonwoods and aspens) are known to produce extrafloral nectaries on their leaves; indeed, there is a fossil leaf of a Populus species that possessed extrafloral nectaries on its leaves, over thirty-five million years ago. Some Populus species occur in Alaska. Quaking aspen, a tree of the interior, has extrafloral nectaries on young leaves. On black cottonwood, which grows in Southeast, the activities of sucking (but not chewing) insects can induce extrafloral nectaries on the leaves, at least on young trees. Still to be determined is whether or not bodyguards recruit to this food source and reduce herbivory on these trees.
Surprisingly, perhaps, nectaries also occur on some ferns, including bracken fern, which can be found in some places in Southeast. Ferns don’t produce flowers, so by definition, these are extrafloral structures. They appear primarily early in the season, while the fern frond is expanding. Ants and spiders sometimes attend these nectaries and cruise around the fern, attacking small herbivores.
Spring is coming, and there is something to look for, next time you are out and about, with nothing else special to do! Look at young cottonwood leaves and at young fronds of bracken fern for small glandular structures and for any insects that might be visiting these nectaries. More eyes looking would actually be very useful!
• Mary F. Willson is a retired professor of ecology.