Once you take the time to wonder what creatures are doing, you wonder why they are doing it. While refilling the hummingbird feeder that hung on an apple tree in the backyard, the unusual number of ants clearly following a path as they walked up and down the tree trunk was a mystery. What were they doing and why were they doing it? The answers to the questions took some observations both at night and during daylight hours.
Following one ant closely to determine its destination led to a whole group of ants surrounding a mass of aphids on a newly sprouted leaf cluster on the tree. The ants were nuzzling, touching and stroking the aphids with their antenna but not eating or dragging the aphids back to their nest. In fact, the ants picked up, inspected, and returned the aphids safely to the group of aphids on the leaf.
The ants were the species Formica subsericea, a common ant in our watershed. Aphids, as all gardeners know and loathe, insert their mouthparts into the nutrient carrying tissue in the new growth of a plant. The pressure of the sap pushes the liquid through the aphids. The aphids digest some small portion of the liquid but pass out the majority. The ants harvested that excess liquid as a food source, transporting it to their nest in their storage stomach. These ants had entered into a trophobiosis relationship with the aphids, where one species feeds on the leftovers of another species.
During the day there would be 20 to 35 ants gathering food at any one time. As some left with distended storage stomachs, others with slim stomachs came to take their place. Temperature determined if ants were present at night; if the temperature were above 50, there would be a few ants collecting. If the temperature was below 50, if any ants were present they would be nearly comatose. At less than 48 degrees you could touch the ants and they would not scurry away, unusual given their flight response to larger animals. It seemed that any ants on the tree at dark were there for the night, like outrider cowboys watching over their herd.
Once the sun rose, activity increased on the tree. Even with the air temperature below 50, when the sun came up their dark colored bodies served to gather the warmth from the sun so the ants could get started on their daily work. Activity levels remained constant on the tree up through 90 degrees. Above 90 the numbers of ants harvesting fell almost to nighttime levels.
Ants are territorial using chemicals to mark their trails and territory and they use their antenna to "smell" their nest mate, although how they do this is still scientifically in question. An ant that was not a nest mate was an interloper and presumed to be stealing the food produced by the aphids. An aphid rustler caught stealing food faced severe chastisement.
If a confrontation with the rustler took place on a leaf, within a minute, several other home team ants would show up and all would join the attack. Since there was no other obvious means of communication, it is likely that chemical signals called in the helpmates. Once the interloper found itself surrounded it had two choices. One, be torn apart or two quickly depart by jumping off the leaf five feet to the ground. Most times interlopers chose to jump. If the confrontation took place on a trail along the trunk of the tree, the two ants could stay locked in a death grip for hours.
As tough as the ants were on each other, they just could not mount successful attacks against the likes of snap beetles or ladybugs. The ants attacked these rustlers, but even in large numbers they could not stop them from eating the aphids. They made some impact on the snap beetle by attacking its legs, but the ladybug legs were too well protected by her hard low slung body. At a ladybug consumption rate of two dozen aphids per hour, the ants soon had to find another food source.
Competitors were not the only reason the ants continued to explore the tree for new herds of aphids. As soon as another leaf had a healthy aphid herd, exploring ants found it and they tipped off their nest mates. Monitoring one of the new herds to see how long the aphids stayed healthy showed that within just two weeks most of the aphids had either grown wings and left or died on site leaving hundreds of tiny black carcasses on the leaf. Aphid mortality as well as rustling forced the ants to find new food sources.
No new insights into the life of the Formica subsericea ant were unearthed during these observations but the ants were fascinating to watch as they worked to feed their nest. Ant habits may be hard-wired instinctive activities but they do them well and with impressive determination.
David L. Deen is River Steward for the Connecticut River Watershed Council. CRWC is celebrating 60 years as a protector of the Connecticut River.