Of the more than 8,000 species of ants worldwide—inhabiting virtually every climate zone (with the exception of arctic regions)—about 5 percent are found in North America, and many, but not all species, enjoy symbiotic relationships with aphids. Fossil records indicate that ants and aphids are little changed from when they first appeared during the late Cretaceous Period more than 60 million years ago. The relationship between species is so close that some scientists believe that ants are responsible for maintaining the survival of some aphid species. Not all aphids are tended by ants, but many species have become the domesticated “primary producers” of ants.
Aphids on plants play the role of range animals, like cows, in agriculture. From as early as the middle of the 19th century, the aphid-herding behaviors of ants have been described, and are now well known to science. Aphids penetrate the phloem tissues of plants with a stylet-like mouthpart. They obtain raw sugars from photosynthates by passively relying on the positive pressure within the plant’s vascular system.
Whether or not aphids are capable of transforming the sugars in some fashion is not well known, but the secretions that pass from the aphid, presumably as byproducts, are still high in sugar content. Known as honeydew, it is these secretions that ants feed on. Some aphids that are actively tended by ants will excrete honeydew in a way that makes it easier for ants to gather, and some aphids secrete only when stimulated by ants to do so. Some species of ants have even been known to clip the wings of aphids to prevent them from “stampeding,” and other ant species actually secrete a hormone that prevents wing development.
More recently, scientists have discovered that some aphid-herding species of ants actually secrete a chemical from their feet that calms the aphids, possibly enslaving them to a narcotic-like compound that encourages them to stay together. In exchange for the seemingly passive activities of ants as they gather honeydew, aphids are protected from predators (since many domesticated animals tend to lose their natural defenses). By utilizing the excrement of aphids, ants remove any pathogens that may be harmful to aphids, organisms that are amplified within the aphid’s digestive tract. Just like a sewage treatment plant, ants—by being able to use the excrement of aphids—are able to keep things clean (and in doing so they also discourage other insects that might be attracted to the honeydew).
Even though the behaviors of aphids and ants are rarely damaging to landscape plants, when an aphid punctures the vascular system, it becomes an infection court for various fungi, bacteria and especially viruses. In early spring, while leaves are expanding, an aphid infestation can cause leaf curling. Extensive infestations can lead to some defoliation, but the effects are rarely pathogenic. Probably the most potential for aphid damage is in fruit orchards, where feeding habits can lead to damaged fruit and secondary infections with other organisms introduced by aphids.
One of the most common maladies caused by aphid feeding is sooty mold. As the name implies, sooty mold is a surficial infection by a mold-forming fungus that has the appearance of soot. The mold uses the sugary residue excreted by aphids as its substrate, so when aphids are actively tended by ants there is rarely sufficient substrate to support a population of sooty mold.
Although the appearance of sooty mold can be disturbing, it is never pathogenic. Since the fungus is not actually infecting plant tissues, there is almost never cause for concern. When conditions are dry, the prevalence of sooty mold may be more extensive. Sooty mold is easily controlled with a fungicidal soap in those circumstances. If aphid populations are out of control, there are a number of ways to deal with the situation. Both contact and systemic chemicals are used to control aphids in commercial plantings. Some of the most common systemic compounds include dimethoate, menazon and disulfoton.
Ants, by themselves, rarely damage or kill trees, and usually only when the trees are affecting the habitat of the nest. By injecting formic acid in the rhizosphere, and sometimes directly into root tissues, ants can cause crown dieback and mortality. Evidence seems to suggest that ants kill roots only when offending plants are affecting the nest, otherwise they tend to leave trees and shrubs alone.
The Allegheny mound ant is probably the most commonly implicated species associated with tree and shrub mortality. It can form large mounds, supporting many thousands of individuals, mostly in shaded areas along field borders, in plantations and around homes. The ant ranges from northern New England to Georgia, mostly east of the Mississippi River. Where populations are located near residences, homeowners should consider eliminating mounds, since the ants will defend a disturbed nest and can inflict life-threatening bites to a small child.
Cypermethrin used as a soil drench directly over active mounds is the preferred method of controlling these insects. This compound is the active ingredient in a number of products that are fairly safe to use in residential landscapes.
When populations of ants and aphids are at normal levels, there is usually no need to use chemical controls. After all, ants have been “farming” aphids for millions of years before humans appeared on the scene, and evidence suggests they will continue to be successful, regardless of efforts to control them.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.