Specific odor resembles "chemical bar code" for bees, wasps and ants

Research shows that sex, age and colony function are among the types of information conveyed by the aromatic bouquets of social insects.

By Elton Alisson

Agência FAPESP – Similar to wine, flowers and perfume, social insects have a distinctive aromatic bouquet. However, for bees, wasps and ants, this bouquet varies according to species, sex, age, and function performed in the colony.

This specific odor can be compared to a “chemical ID”, facilitating identification by fellow insects, and can be used to label insects as members of a particular colony, as male or female, as young or old, or as a queen or worker.

The discovery was made by researchers at the University of São Paulo’s Ribeirão Preto School of Philosophy, Science and Letters (FFCLRP-USP) during a series of studies performed as part of the project “Behavioural mediation, chemical signalisation and physiological aspects regulating the social organisation in himenopterans,” supported by FAPESP within the ambit of its Young Investigators Grants program.

“We found that each insect has a specific odor that is comparable to a chemical bar code,” said Fábio Santos do Nascimento, a professor at FFCLRP-USP and coordinator of the project. “By reading this bar code, you can identify the insect’s species, genus, age and colony function.”

According to Nascimento, what gives social insects this chemical ID is a class of compounds called cuticular hydrocarbons, comprising linear carbon chains and hydrogen molecules (alkanes, alkenes and methyl-branched alkanes).

Found in the form of wax under the top layer of the cuticle, which is the insect body’s outer covering, these chemical compounds mainly serve to prevent water loss and dehydration and to protect the insect against microorganisms.

However, when the researchers analyzed cuticular hydrocarbons from different ant, wasp and bee species, they found that the chemical composition of these substances varied according to the insect’s species, sex and function and that this chemical variability assisted communication among nestmates.

In a study published in the journal Apidologie involving Melipona marginata, a Brazilian stingless bee found in São Paulo, Santa Catarina and Bahia (local name manduri), the researchers observed that older drones (males) and queens and workers (females) have different cuticular hydrocarbon profiles that can be sensed by other members of the colony.

Workers displayed a smaller percentage of alkanes (saturated wax) such as hentriacontanes and tetratriacontanes than drones, queens and bees of the caste that are destined for royalty. Although they are female, workers are normally sterile and incapable of becoming queens.

“Although the chemical structure of cuticular hydrocarbons is fairly stable, its composition in social insects varies according to the role the insects play in the colony,” Nascimento told Agência FAPESP. “Each colony also has a different chemical profile.”

Recognition

The researchers conducted an experiment to evaluate social insects’ ability to recognize members of their colonies by smell. In this case, the experiment involved Melipona asilvai, another stingless bee species (local names uruçu-mirim and rajada). Workers and foragers were placed at the entrance to a colony to which they did not belong to see how the guards reacted.

The results of the experiment, described in a paper published in the Journal of Chemical Ecology, showed that guard bees display flexible behavior in accordance with the degree of similarity of the cuticular hydrocarbon profiles between incoming foragers and themselves.

They were far less likely to ward off incoming bees with highly similar chemical profiles to their own. According to the researchers, this was probably because they confused the “invaders” with nestmates.

By contrast, they were more selective when the chemical profiles of incomers differed from their own and kept out more of these unrecognized bees. “A nest has many resources such as nectar in the form of honey, pollen and brood,” Nascimento said. “If the recognition system used by guards is flawed, bees from neighboring colonies can poach these resources.”

The researchers conducted a similar experiment with the giant ant species Dinoponera quadriceps (local name falsa tocandira). Foragers (workers that look for food) and nurses (workers that care for eggs) were placed at the entrance to a different colony from their own to see how long it took for them to be recognized as invaders by the guard ants of the alien colony.

The findings, published in the Journal of Insect Behavior, showed that guard ants took much longer to recognize nurses than workers as outsiders. Non-nestmate workers received significantly more bites and other attacks from guards than nurses. Moreover, ants guarding the alien colony took longer to react against nurses than against foraging workers.

A possible explanation for these differences in behavior, corroborated by analysis of the composition of cuticular hydrocarbons from the insects used in the study, is that nurses of the same species but from different colonies may share a larger number of chemical compounds.

“Specific receptors on their antenna tips, in the form of tiny hairs, capture chemical signals from other insects’ cuticular hydrocarbons, which we call contact pheromones,” Nascimento said.

“When in contact with another insect of the same species, they can identify the composition of its cuticular hydrocarbons and other chemical compounds to recognize whether it’s a nestmate.”

Profile change

The researchers also discovered that the profile of aromatic bouquets of social insects may change according to food intake, potentially hindering recognition by nestmates.

A study that the researchers described in the journal Insects involved a laboratory experiment in which they fed a group of worker ants from the leafcutter species Atta sexdens (local name saúva) with rose leaves and petals, feeding another group from the same colony with banaba or crapemyrtle leaves (Lagerstroemia sp.).

When the ants were placed close together, those fed with banaba or crapemyrtle rejected and attacked those fed with rose leaves and petals because of the change of odor.

“The saying that you are what you eat also appears to be valid for social insects,” Nascimento said.