Most of us grow up thinking that animals either eat or ignore each other, but close observation reveals much more complex relationships, such as symbiosis, the long-term relationship of species that have evolved to live together.
Symbiosis can take several forms, depending on the costs and benefits for each species: With parasitism, one species benefits to the detriment of the other; with commensalism, one species benefits with no apparent benefit or detriment to the other; and with mutualism, both species benefit. These relationships can occur within or between kingdoms, such as between plants and animals, and some species can take on more than one symbiotic role in the course of their lives.
A well-known — but not completely understood — example of symbiosis in humans is our relationship with the microbes in our intestines. We have a mutualistic relationship with some, which help us process our food; other microbes and larger organisms living there may benefit from the food we provide but make us sick. New research is showing that the kind of microbes we have in our guts can go way beyond breaking down food and actually affect brain function.
One insect family — ants, of which there are more than 14,000 species — demonstrates all three forms of symbiosis. Leafcutter ants, for example, cut and process leaves that they bring to their nests to serve as a medium to grow fungus in “gardens.” New queens carry some residual fungus in a pocket in their bodies so they can start their own gardens.
In this mutualistic relationship, the ants help the fungus survive and spread, and the ants get nutrition from eating the resulting “crop.” This two-step farming strategy seems pretty complex for an insect with a decentralized nervous system, but somehow it got hard-wired into the genes of leafcutter ants millions of years ago.
Some ants grow insects that are to the ants what milk cows are to us. Aphids are the best example of these “livestock.” Instead of producing milk, they excrete a sugary substance known as honeydew through readily accessible tubes on their backs. The ants stroke the aphids to “milk” them for the honeydew.
And just like any good farmer, the ants protect their livestock from predators and even, in some cases, move them to more nutritious “pastures” so that both species prosper. Research shows that a substance on the ants’ feet helps calm the aphids and keeps their wings from developing, so they’re easier to manage.
Some ants farm other insects, as I found out this summer. I spotted some medium-sized ants, along with the eggs and tiny larvae (less than one-quarter inch) of treehoppers, on the underside of several leaves of the giant sunflowers I had planted next to my house.
At least one adult treehopper was on each leaf, too. They were harder to spot because of their appearance — they looked like jagged pieces of mottled-brown leaves. Treehoppers typically raise their young communally to share the parenting chore of guarding the eggs from predators.
For weeks, the ants hovered over and circled around the treehopper larvae but did not move them and didn’t seem to have other pronounced interactions with them. Although I assumed the ants were feeding off the excretions of the treehoppers, the larvae were so small that observing the actual transaction was another matter, even with the small magnifying glass I have. Any time I disturbed the leaf, however, as when I turned them over to photograph the “farm,” ants would charge out to meet the threat.
With more than 1,000 ant species in North America, identifying a particular species is difficult, although observing such a symbiotic relationship helps narrow down the possibilities. Different species of ants will farm different species of insects, with some ants obligated to only one species or genus of livestock. The ants on my sunflowers were obviously not aphid farmers, since they showed no interest in aphid larvae that showed up on another sunflower leaf at the end of the growing season.
After perusing some great ant references in the Rappahannock County Library’s conservation collection and numerous websites, I threw up my hands and took photos of the farming operation, along with some of other invertebrates I couldn’t ID, to Jim McNeil — the entomology professor at the Smithsonian-Mason School of Conservation. He has a great sense of humor, obvious passion for his chosen profession and the rare ability to explain the complex mysteries of the bug world in terms a non-expert like myself can understand.
After doing some research, Jim emailed me to say he was “pretty confident” that the treehopper is Entylia carinata (sometimes called a keeled treehopper) but wasn’t sure about the ant. The photos weren’t great, so I was happy to find a specimen of the ant, which I’d misplaced, and took it to Jim last week. Knowing the livestock helps in identifying the species of the farmer, since several ant species are associated with the treehoppers.
Jim emailed me the next day to say he had come up with “a pretty solid ID” of the specimen — a worker in the Camponotus castaneus species. Although in the carpenter-ant genus, these ants “have been documented tending aphids and treehoppers.” BugGuide.net lists one of this species’ common names as “reddish carpenter ant.”
I also asked Jim if the ants really were farming the larvae or just hanging around for some other reason, since I couldn’t see whether the ants were eating anything excreted by the larvae. He said that, unlike aphid larvae, which excrete large globes of honeydew, treehopper larvae excrete much less waste, so it’s hard to see the interaction without the proper equipment. These ants also don’t seem to milk the treehoppers, from what I’ve observed.
The remains of the sunflowers are now in my compost pile. I miss observing the amazing assortment of invertebrates that showed up on them — from pollinators to predators and parasites, to the farming ants and their livestock. It was a huge return for just throwing a few seeds in the ground.