Heterospecific recruitment pheromones facilitate efficient foraging by keystone Neotropical pollinators

Background/Question/Methods Chemical information can mediate indirect interactions between competitors in plant-animal mutualistic networks. Eavesdropping, exploiting evolved signals aimed at others, has the potential to strongly affect dynamics within a foraging guild. Stingless bees (Hymenoptera: Apidae, Meliponini), critical tropical pollinators, provide a good system to study eavesdropping with a trophic level. Several genera recruit nestmates to rich food sources with pheromone trails. Stingless bees frequently live in close proximity to multiple related species with similar diets, thus exploiting non-nestmate trails should increase search efficiency and ultimately colony fitness. Here we investigated rules underlying heterospecific eavesdropping by stingless bees. Using preference tests, we determined how the dominant Trigona hyalinata responds to various concentrations of the subordinate T. spinipes' trail pheromone. We then quantified the T. spinipes recruitment process to determine whether pheromone concentrations correlate with the effort required for successful resource takeover by eavesdroppers. Finally, we implemented a decision analysis model to test if eavesdropping decisions that are sensitive to both reduced search effort and potentially high costs of gaining access can maximize colony fitness.

Results/Conclusions Eavesdropping T. hyalinata showed a non-linear response across the T. spinipes pheromone concentration gradient. They were attracted to low concentrations, but avoided the higher concentrations left by an actively foraging T. spinipes colony. Their behavior matched the T. spinipes activity level indicated by pheromone concentration. Model results matched empirical eavesdropping behavior of three stingless bee species, and confirmed that assessing takeover effort maximizes net benefit to the focal colony. Additional model analyses predict how environmental factors and bee traits affect eavesdropping behavior. This research significantly improves understanding of interspecific dynamics and decision-making within a trophic level. Eavesdropping may be a key mechanisms increasing patch switching by foragers, and thus increasing plant outcrossing potential.