Amped-Up Ants: Caterpillars’ Sugary Treats Earn Carpenter Ant Care
By Carolyn Bernhardt
Since the 1960s, researchers have observed ants providing protection to various insects, a phenomenon called myrmecophily. And by creating a mix of chemical, acoustic, and other cues, certain butterfly species have long found ways to become frequent beneficiaries of ants’ dedicated services.
In a recent study led by Geena M. Hill, a research biologist at the Florida Natural Areas Inventory at Florida State University, researchers tested whether the Florida carpenter ant (Camponotus floridanus) provides protection to the highly endangered butterfly Cyclargus thomasi bethunebakeri, often known as the Miami blue butterfly. Their results were published this week in the open-access Journal of Insect Science.
“We found that ants provide significant protection to Miami blue larvae, with later instar larvae receiving a higher level of protection due to differences in tending frequencies,” Hill says. The ants tended to the late instar larvae more than twice as much as they did early instar larvae.
Previous studies conducted by study co-author Matthew D. Trager, Ph.D., forest planner at the U.S. Forest Service, have shown that ant tending has sex-dependent benefits for female Miami blue butterflies. Ant-tended female Miami blue larvae grow into larger pupae and lay more eggs as adults. So, in this latest study, Hill and team conducted a series of timed observational trials in the lab to assess larval survival and ant protection from insect predators.
Ants were incredibly effective protectors for the larvae, but they seemed to provide greater protection over later instar larvae. Hill says she isn’t totally certain why this is, but she has a hunch that certain butterfly life stages influence ant tending behavior while some more vulnerable life stages do not as effectively stimulate ants to protect the larvae. She says she suspects this could be because each time the larvae molt they might develop and produce more secretions that signal to ants to help them as they increase in size.
At certain developmental stages, each caterpillar has a nectary organ tucked neatly into a slit in its back and flanked by tentacle-like organs on either side. When this organ emerges, it secretes a sugary substance packed with amino acids and other nutrients for ants. Hill and her colleagues watched in the lab as the ants’ energy levels soared whenever predators approached after the ants had ingested the substance. “They were all amped up!” she says. The carpenter ants would even drum their antennae on the back of the caterpillar to request a helping.
For the most part, Hill says she thinks the substance itself is enough motivation for the ants to protect the larvae from threats. However, it’s also possible that the chemical makeup of the secretions affects the dopamine levels in the ants. “We already know other species can trick the ants by using different chemicals,” she says. “So, it is interesting to think maybe they could have a chemical in the secretions that affects the dopamine levels [in the ants] and makes them more aggressive toward predators.”
Whatever the reason, this research shows that, for Miami blue butterfly caterpillars, Florida carpenter ants are effective protectors against attacks and help improve butterfly larvae survivorship. But while the Florida carpenter ant is the Miami blue’s most common associate, 16 other species of ants also tend the Miami blue caterpillars. Hill says she wants to test those other species to measure how effective each species is at promoting survival for the federally listed, critically endangered butterfly species. This information can help inform conservation efforts for rehabilitating the endangered Miami blue. “Hopefully with this work, we will see increases within the Miami blue population,” Hill says. “As our lab conducts butterfly releases, it will be important to select release sites that have Florida carpenter ants present.
Historically, the Miami blue was found throughout coastal Florida. But, with climate change and the effects of human-driven land development, the species’ numbers have long dwindled. “Predators are not the sole driver for the decline of the Miami blue, but with such low population numbers, they can have a big impact,” says Hill.
For a long time now, the Miami blue has been limited to a few islands in the Florida Keys. It mainly lives in the Key West National Wildlife Refuge. Still, over the last several decades, researchers have successfully reestablished the species into the ecosystem in Bahia Honda State Park—35 miles east of Key West and that much closer to mainland Florida. And work is underway to reintroduce the species in mainland Florida.
“Insect declines are being observed worldwide,” says Hill, “and it’s imperative to study these insects and their interactions so we best know how to protect them. There are so many species that will go extinct without us knowing. By protecting one species, we [could be] protecting others within the habitat as well.”
Journal of Insect Science