Trail Pheromone of the Argentine Ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae)

Abstract

The Argentine ant (Linepithema humile) is recognized as one of the world's most damaging invasive species. One reason for the ecological dominance of introduced Argentine ant populations is their ability to dominate food and habitat resources through the rapid mobilization and recruitment of thousands of workers. More than 30 years ago, studies showed that (Z)-9-hexadecenal strongly attracted Argentine ant workers in a multi-choice olfactometer, suggesting that (Z)-9-hexadecenal might be the trail pheromone, or a component of a trail pheromone mixture. Since then, numerous studies have considered (Z)-9-hexadecenal as the key component of the Argentine ant trails. Here, we report the first chemical analyses of the trails laid by living Argentine ants and find that (Z)-9-hexadecenal is not present in a detectible quantity. Instead, two iridoids, dolichodial and iridomyrmecin, appear to be the primary chemical constituents of the trails. Laboratory choice tests confirmed that Argentine ants were attracted to artificial trails comprised of these two chemicals significantly more often than control trails. Although (Z)-9-hexadecenal was not detected in natural trails, supplementation of artificial dolichodial+iridomyrmecin trails with an extremely low concentraion of (Z)-9-hexadecenal did increase the efficacy of the trail-following behavior. In stark contrast with previous dogma, our study suggests that dolichodial and iridomyrmecin are major components of the Argentine ant trail pheromone. (Z)-9-hexadecenal may act in an additive manner with these iridoids, but it does not occur in detectable quantities in Argentine ant recruitment trails.

Pheromone communication in ants: a detailed analysis of concentration-dependent decisions in three species

“…(Z)-9-Hexadecenal, a molecule produced in the Pavan's gland, has been suggested as a main component of the trail pheromone complex (Cavill et al 1979(Cavill et al , 1980Van Vorhis Key and Baker 1982). However, (Z)-9-hexadecenal was analytically not detectable in natural trails, whereas two components of the pygidial gland (dolichodial and iridomyrmecin) were detectable and showed a trail following effect when the mixture was provided as a linear trail (Choe et al 2012). We therefore included pygidial gland extract in one of our experiments with L. humile.…”

“…We therefore included pygidial gland extract in one of our experiments with L. humile. In our analysis we also included the data of a behavioral assay done with ants from California (USA) using synthetic (Z)-9-hexadecenal (Choe et al 2012).…”

“…All pheromone concentrations we report are given relative to the base concentration with 1 bc as unit. Note that we expect the pheromone concentration on natural trails to vary greatly within species (see, e.g., Beckers et al 1992;Choe et al 2012) and the bc is only an estimation that can be expected to be close to naturally occurring pheromone concentrations. All our experiments go far below and above the bc, and they are designed in a way that the results are independent of the absolute pheromone concentration of the bc.…”

“…The 75 % detection threshold of the PF is the only exception since it is given in bc units. For (Z)-9-hexadecenal that had been previously investigated (Choe et al 2012), we were able to give absolute quantities of pheromone per cm of trail.…”

“…Thus, to understand the social behavior of ants, it is important to understand how information is encoded and transmitted by pheromones and how this information is translated into specific behavior. It has been shown that ants, confronted with a binary choice of trails, prefer the trail with the higher pheromone concentration over that of lower concentration and that trail following fidelity increases with higher pheromone concentrations (Hangartner 1969;Van Vorhis Key and Baker 1982;Choe et al 2012). This behavior resembles an analog communication system, which has the potential to transmit information about different states of the environment.…”