Let’s make a deal: Study finds some caterpillars offer ants a ‘reward’ for protection

SÃO PAULO, Brazil — Just like a local shop owner forced to pay the mafia a “protection fee,” certain species of caterpillars ensure their survival by offering nearby ants a sugary treat in exchange for safe passage. In a sense, you could say the caterpillar makes the ants an offer they can’t refuse.

Brazilian researchers say that for local caterpillars surrounded by predatory ants, there are only two viable survival routes. Some caterpillars species are able to blend in quite well with nearby greenery and avoid detection. Other caterpillar species however, don’t have that luxury. For those unable to camouflage themselves, the only way to survive is to offer ants a caloric reward in exchange for safety.

The Federal University of Rio Grande do Sul (UFRGS) and the University of Campinas (UNICAMP) teamed up to make this discovery.

“Ants prey on many insects that live on plants and establish mutualist interactions that benefit both ants and plants. To live on plants that have ants, caterpillars develop strategies that enable them to coexist with the ants. There are many advantages to living near ants. Many ants are aggressive and limit the occurrence of certain organisms. So if any animal is able to live close to ants without being attacked by them, it may acquire an adaptive advantage,” explains principal study investigator Lucas Augusto Kaminski, a researcher in UFRGS’s Zoology Department, in a university release.

These revelations were made possible in part thanks to José Roberto Trigo, a professor who passed away in 2017. During the 2000’s, Trigo published a number of groundbreaking studies that showed cuticular hydrocarbons (CHCs) are used extensively by ants.

Why don’t ants attack the caterpillars?

CHCs are an outermost layer of cuticles on the surface of virtually all plants and insects. Each species usually has its own variety of CHCs and uses them as a waterproofing agent and communication signal.

Ants have very poor vision to begin with and interact with the world primarily through their feelers or antennae, so CHCs are very important for ants. Now, certain caterpillar and insect species have evolved to present the same CHCs as the plants they live on. Why? Protection. If a nearby predator can’t detect the presence of another insect on a plant, it won’t know to attack.

For this new research project, study authors assessed CHC composition for six caterpillar species, three plants, and two breeds of ants using mass spectrometry and gas chromatography. Most of the caterpillar and plant CHCs showed roughly 95% similarity. In these cases, it’s quite clear the caterpillars are blending in with the plants to avoid ant detection. A few caterpillar species however, only showed 34 to 55 percent similarity with the plant CHCs. These caterpillars would be very visible and detectable for nearby ants.

This left researchers with a nagging question. Why don’t ants constantly attack these visible caterpillars? Their subsequent investigation discovered that those same species have specialized organs that produce a sugary “nectar” that can be given to ants as a “caloric reward.”

“If a caterpillar stays hidden, it doesn’t need to give ants anything, but if it produces a reward it must be conspicuous. What’s involved here is the evolution of the caterpillar’s communication with the ant,” Barbosa concludes.

The study is published in Ecological Entomology.