Mixed signals: Sex pheromone attraction and speciation in California Polyergus mexicanus kidnapper ants

Polyergus kidnapper ants exhibit a unique obligate parasitic behavior, raiding brood from nests of other ant species and exploiting the labor of host-workers that emerge from stolen brood. In California, Polyergus mexicanus comprises at least four genetically distinct lineages associated with different Formica hosts, suggesting incipient speciation. We investigated whether divergence in sex pheromone communication contributes to reproductive isolation among these lineages. Field assays across California tested attraction of male ants to synthetic pheromone blends modeled after two Polyergus species groups in North America: P. lucidus group (eastern) and P. breviceps group (western, including P. mexicanus). While both species groups share a major pheromone component, methyl 6-methylsalicylate, they differ in key minor components: an ester, methyl (R)-3-ethyl-4-methylpentanoate, in the P. lucidus group, and an alcohol, (R)-3-ethyl-4-methylpentan-1-ol, in the P. breviceps group. Surprisingly, California P. mexicanus males were more strongly attracted to the P. lucidus blend (salicylate + ester) than to their own P. breviceps group blend (salicylate + alcohol), contrasting with P. mexicanus from the eastern part of their range, which prefer the alcohol-containing blend. GC-MS analysis of P. mexicanus gyne mandibular gland extracts (the source of the pheromones) revealed a blend-ratio of 74:4:1 (salicylate:ester:alcohol), and male antennal responses (GC-EAD) mirrored this pattern. Ongoing mitochondrial COI analyses will clarify whether pheromone preference correlates with host-associated lineages. These findings suggest that divergence in minor pheromone components may promote reproductive isolation between host-associated lineages in P. mexicanus, offering new insight into the role of chemical communication in social parasite evolution.