Research: Caterpillar-Ant Bond Highly Specific

Fundação de Amparo à Pesquisa do Estado de São Paulo

Some families of caterpillars (larvae of butterflies and moths) have developed a specific kind of interaction with ants. One of these families (Riodinidae) includes two species that interact solely with two species of ants.

In a study published in the journal Insect Science, researchers in Brazil affiliated with the University of São Paulo (USP), the Federal University of Rio Grande do Sul (UFRGS) and the Federal University of Alagoas (UFAL) show that this interaction is highly specific in the sense that each caterpillar species interacts only with one ant species. The study was supported by FAPESP.

In an experiment performed by the researchers involving switched caterpillar-ant pairs, the ants attacked and killed the caterpillars even when the latter used specific organs to secrete a sugary liquid that should have assured protection by ants.

The researchers also analyzed the cuticular hydrocarbons (CHCs) that cover the outermost layer (cuticle) of virtually all plants and insects, serving as a waterproofing agent and as a communication signal, among other functions.

"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 that enabled them to be recognized and protected by certain ant species," said Luan Dias Lima , first author of the study, which was conducted as part of her postdoctoral research at the University of São Paulo's Ribeirão Preto School of Philosophy, Sciences and Letters (FFCLRP-USP), with funding from FAPESP .

In a previous study, the researchers demonstrated that other species that interact with ants may use a different strategy, hiding from the ants by releasing the same odor as the plant they inhabit (read more at: agencia.fapesp.br/34787 ).

This latest study is part of the project "The evolution of caste plasticity and caste dimorphism in insect societies" , supported by FAPESP and led by Fábio Santos do Nascimento , a professor at FFCLRP-USP. Another co-author, Diego Santana Assis , received a doctoral scholarship from FAPESP while studying at the same institution.

Caterpillar in pieces

In the experiment, the researchers collected caterpillars of the species Juditha molpe, which live in harmony with ants of the species Dolichoderus bispinosus, and caterpillars of the species Nymphidium chione, which are found only where there are ants of the species Pheidole biconstricta. The caterpillars were collected at the Serra das Araras Ecological Station in the Brazilian state of Mato Grosso.

In one group, the researchers switched only the plant caterpillars and kept the same ant species. In another, they switched the pairs, putting J. molpe caterpillars with P. biconstricta ants and N. chione caterpillars with D. bispinosusants, while combining all of them with the same plant genus (Inga spp).

In the first group, the interaction was the same as it had been with the previous plant, and the pairs interacted harmoniously. The ants touched the caterpillars with their feelers until they located two openings to specific organs for communication by the caterpillars with ants. The caterpillars secreted a nectar, which the ants consumed, and soon afterward the ants began protecting the caterpillars.

With different ants, however, the outcome was unfavorable to the caterpillars. In this context, the ants at first ignored the caterpillars until they found the opening to one of the specialized nectar secretion organs. After the ants touched the site with their feelers, the caterpillars secreted the nectar, which initially calmed the ants.

This effect was short-lived, however. It ended when a caterpillar was unable to go on producing the nectar, or when the ants touched the region near another organ used solely for interaction with ants, and the organ did not react. After inspecting the caterpillar a little longer, the ants began behaving aggressively, opening their mandibles and biting.

"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," Lima said.

Finding ants to pair with is so important for the caterpillars, he explained, that adult females lay eggs only on the plant species inhabited by the right ant species.

"When the eggs hatch, the ants will protect the caterpillars from predation by other animals. Without the ants' protection, they die before their time," he said.

The researchers now plan to find out whether the small amounts of CHC also ward off attacks by other natural enemies, such as predators and parasitoids that use chemical clues to locate prey and hosts.

About FAPESP

The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.

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