There are more than 20,000 species of ants, including such noticables as picnic ants, sidewalk ants, carpenter ants, and so on. Less noted by most of us, there are also many mutualistic ants that have beneficial relationships with plants or insects.
Some mutualistic ants protect plants from herbivores. For example, gardeners often notice that ants are crawling over their peony buds. Those peony buds have extrafloral nectaries on the sepals that enclose the buds. The nectaries offer sugars, amino acids, lipids, and other nutrients to the ants. Once a scouting ant finds that nectar, she lays a scent trail back to her nest and recruits other ants to come to the feast. The nectar-feeding ants help protect the peony flower from aphids, thrips, and other pesty insects. The type of peony usually found in our gardens is probably some cultivar of Paeonia lactiflora, which originated in east Asia. (There are many wild species of peony, mostly in Eurasia plus two in North America, but I haven’t found any info about extrafloral nectaries on them).
There are numerous thorn trees in tropical and semi-tropical areas around the world that are associated with ants of many different kinds. Once upon a time, they were all classified in the genus Acacia, but taxonomists have discovered that the name Acacia actually covered several different lineages that are now separated into perhaps five different genera. One of these new genera is called Vachiella, with over a hundred species. At least some of these have mutualistic ants that are fed from extrafloral nectaries on the leaf stems and housed in swollen thorns that the ants hollow out. In Central America, the bullhorn acacia is guarded by mutualistic ants that, in addition to living in hollowed-out swollen thorns and feeding on nectar, feed on nutrient-rich food bodies produced at the leaf tips. Herbivore damage activates the ants, and herbivores such as cattle and leaf-cutter ants are deterred by the biting, stinging ants from foraging on these trees. Researchers found that the extrafloral nectar of one species contains both sugars and enzymes that inhibit most sugar digestion by the ants, but it also provides one special sugar that is digestible, and the ants can’t live without this, so they remain on the tree and defend it. Another species in the genus lives in Africa, housing different kinds of ants that feed at nectaries and guard the tree from herbivores. For one African species at least, the browsing of large mammals actually helps maintain the mutualism! When large mammals were excluded from browsing for 10 years, the thorn trees produced less nectar and less housing, and mortality of the trees increased.
An ant on a peony bud. (Photo by Pam Bergeson)
Some tropical pepper plants also have mutualistic associations with still-other kinds of ants, housing the ants in hollow stems and feeding them on special “pearl bodies” full of nutrients. The ants defend the plant against fungi and herbivorous caterpillars but are apparently subject to a specialized beetle that also lives in those hollow stems and feeds on the ants and their larvae.
Pollination by ants is rare, involving many kinds of ants but documented to occur in only a few diverse genera of plants, including some orchids in various parts of the world. They can be effective pollinators of some species of wild strawberry (but no data for our local one), especially where flying pollinators such as bees are uncommon. Note: ants may enter a flower for various reasons; not all are pollinating!.
The role of ants in dispersing seeds is somewhat better known, reportedly occurring in thousands of plant species and involving many kinds of ants. The seeds of these plants produce a nutrient-rich appendage (a food-body called an elaiosome) that ants like, usually taking the seed home to the nest, where they eat the appendage or feed it to their larvae and discard the seed in their rubbish heap, a fertile bed for seed germination and seedling growth and a place safe from many potential predators. In some cases, the seeds may be dropped on the way back to the nest, after the hungry ant eats the elaiosomes, or simply tossed out of the nest, thus at least accomplishing seed dispersal if not providing the germination bed. Examples include bleeding hearts and their close relatives, Trilliums, and at least some trout lilies (a.k.a. glacier lilies).
Many stick insects just drop their eggs onto the forest floor. These eggs look rather like seeds and they bear an appendage like an elaiosome. Ants pick up at least some of those dropped seeds and take them to their nests. But not just any ant: the appendage is biochemically more similar to insects than it is to seeds, so carnivorous and omnivorous ants are more likely to be interested than seed-eating ants in picking them up. That achieves seed dispersal and helps protect the eggs from parasitic wasps. When the eggs hatch in the ant nest, the nymphs have to get to the surface, and they usually do, provided that the nest is fairly shallow (less than about six centimeters deep). As they emerge, there might be a risk of predation by the ants, but that remains to be seen. It seems to be a rather chancy business.
Ants also tend the caterpillars of some lycaenid butterflies, the so-called blues. These caterpillars have a nectary on their backs, which draws the ants. They also emit pheromones that mimic the odor of the ants, which apparently makes them feel “at home” and less likely to desert the caterpillar. Ants usually defend the caterpillars from insect parasitoids and predatory flies. However, some of the blues have turned this trick around, somehow inducing the ants to carry them home to the nest, where they live as parasites: they are fed just as the ant larvae are and may even consume some of the ant larvae. Certain Australian ants are reported to tend the caterpillars of some blues but prey upon those of other species.
Aphids and ants interact in well-known, much-studied mutualisms. Aphids are herbivorous insects that excrete sugary fluids as honeydew, which ants consume. The foraging preferences of ants may vary with the kinds of sugars in the honeydew, and furthermore, the honeydew may also, at least in some cases, contain a specific molecule that increases the aggressiveness of the ants. The ants protect the aphid colonies from insect predators and fungal infections and in some cases may take the aphids into shelter from inclement weather. In a few cases the aphids have evolved a way to take advantage of the ants, inducing them to carry the aphids to the ant nest, where the aphids then feed on ant larvae.
And that’s just a sample of how those eternally busy ants are closely involved with many other species! There is much more complexity to all these examples than this space allows, and I encourage interested folks to dig a little deeper.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
