Ants and caterpillars have a great working relationship, until it isn't

In the hidden corners of the world, caterpillars and ants have a remarkably specialized relationship. The caterpillars secrete a sweet, nectar-like substance that ants feed on, and the ants often protect the growing caterpillars from predators.

This interesting relationship, which exists between ants and the larvae of some species of the Riodinidae family (metalmark butterflies), is so particular that each species of caterpillar interacts with only one species of ant.

Scientists have now investigated the complex mechanisms of this mutualistic partnership.

They show that chemical signals and sweet secretions determine whether the ant response is one of protection or destruction.

Caterpillar protection from ants

In tropical forests, caterpillars provide an important food source to many insectivores. Caterpillar mortality is inevitably high, unless the adult butterfly lays its eggs on a plant that is inhabited by the correct species of ant.

The research was conducted by Luan Dias Lima during her postdoctoral studies at the University of São Paulo’s Ribeirão Preto School of Philosophy, Sciences, and Letters (FFCLRP-USP).

The ants and caterpillars were collected at the Serra das Araras Ecological Station in the Brazilian state of Mato Grosso.

The researchers analyzed the chemical signals produced by caterpillars. The goal was to understand how ants can identify specific types of caterpillars as those to protect or those to kill.

The findings suggest that this relationship is delicately balanced as each species of ant recognizes only one species of caterpillar as worthy of protection.

The ants will kill any other caterpillars, even if they secrete the sweet liquid that is believed to promote protection.

Chemical signals and sweet secretions

In the experiments, the researchers focused on caterpillars of the species Juditha molpe, which live in harmony with ants of the species Dolichoderus bispinosus.

The team also studied caterpillars of the species Nymphidium chione, which are found only where there are ants of the species Pheidole biconstricta.

They analyzed the cuticular hydrocarbons (CHCs) present on the ants, caterpillars and plants. CHCs are chemical compounds that are found in the coverings of plants and insects, and play a vital function in waterproofing, communication, and species recognition.

“We found insect and plant CHCs to be quite different from each other. In caterpillars, they signal food for ants. The caterpillars in our experiment offered a sweet secretion, allowing them to be recognized and protected by specific ant species,” explained Lima, the study’s first author.

This sugary secretion not only promotes recognition but also cements the partnership between these species by offering the ants a source of food.

In return for the nectar, the ants defend the caterpillars from predators, thus improving their chances of survival.

When ants turn hostile

However, when the researchers mixed up the ant-caterpillar pairings, the harmonious alliance turned hostile, and destroyed the delicate balance that sustains this symbiotic relationship.

At first, the unfamiliar ants showed little interest in the caterpillars, seemingly indifferent to their presence. They ignored them until they discovered the caterpillar organ that secretes the nectar, which momentarily piqued the ants’ curiosity.

Their curiosity soon turned to aggression, however, and they began to behave erratically, opening their mandibles and biting the larvae. What started as a potential new alliance quickly devolved into a deadly confrontation.

“Almost all of the caterpillars were killed during this pair-switching treatment. Soldiers of the species P. biconstricta even used their strong mandibles to cut J. molpe into pieces,” said Lima.

The importance of the right partnership

Why are caterpillars relying on ants when things can go so badly wrong? It turns out that ant protection is so important to the caterpillars that adult female butterflies lay eggs only on plant species colonized by the right ant species.

The life of the developing young caterpillars depends on their protection from ants. Without this protection, predators would destroy them.

In the future, the scientists aim to investigate whether CHCs also serve as a deterrent against other natural enemies like parasitoids.

Evolution of caterpillar-ant relationships

The exclusivity of these caterpillar-ant partnerships raises intriguing questions about their evolutionary origins.

Scientists suggest that these relationships may have developed through a long process of co-evolution, where both species adapted to each other over time.

One theory proposes that ancestral caterpillars, once vulnerable to predation, evolved the ability to secrete nectar as a survival strategy, and gradually refined their chemical signals to align perfectly with specific ant species.

In turn, ants learned to recognize these caterpillars as a valuable food source, strengthening the mutual dependence between them.

The evolution of such specialized interactions likely resulted from a long history of natural selection.

Caterpillars that failed to match their chemical profiles to local ants faced higher risks of mortality, while those that adapted found a niche where they could thrive.

By studying these ancient alliances, researchers hope to uncover broader patterns in insect evolution and symbiosis, and to shed light on how cooperation between different species shapes ecosystems.

Also contributing to the study were University of São Paulo, the Federal University of Rio Grande do Sul, and the Federal University of Alagoas.

The full study was published in the journal Insect Science.

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