Many species of stingless bees exhibit complex intraspecific and interspecific aggressive behavior towards each other when they meet on flowers or artificial baits. Such aggressive encounters significantly lower the amount of time that bees spend on food sources, as well as the amount of nectar or pollen which they can gather per visit. In addition, the intensity and duration of aggression at artificial baits rises sharply with increased sugar concentration. Different species vary markedly in inherent aggressiveness. Learning and recruitment appear to reinforce the effects of aggression on the spatial separation of foraging in competing colonies.

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We report on differences in foraging and recruitment patterns among 6 stingless bee species simultaneously exploiting a large grid of baits in a tract of tropical dry forest in Costa Rica. Comparative data for the species were obtained on the following parameters of foraging: time to initial bait discovery, rate of discovery of additional baits, rate of worker recruitment, time till attainment of approximate steady—state number of workers and visited baits, frequency of number of workers per bait, degree of site constancy of foraging within and between days, and responses to presence of intraspecific and interspecific rivals at baits. All species had unique aspects to their foraging, which reflected whether the species mark food sources and lay trails with pheromones, the degree to which workers forage solitarily, and their use of aggression. The behavior of Trigona fuscipennis, the pheromone—using group forager, was particularly distinctive. It found baits the most slowly, was the most site constant, and excluded all other bees from its baits. We hypothesize that in group foragers, extreme localization of workers and resultant slowness of discovery of new food sources represent a trade—off for the advantages of group defense of good sources. Other responses to rival bees included bullying tactics by lone, marauding workers of the large, facultative recruiter, Trigona silverstriana, and persistent, opportunistic insinuation by small, solitary foraging species. Spatial segregation of foraging location between the species increased over time on each day of the experiment, especially among interspecifically aggressive species. The observed diversity of tempo and mode of foraging reinforces the hypothesis that coexistence among these apparently food—limited social bees does not result from the simple partitioning of food resources by size or taxon, but from rather subtle partitioning based upon diversity in the timing, persistence, renewal rate, and spatial dispersion of their limited food resourves.

The density and dispersion patterns of nests of 5 stingless bee species are described for a tract of Costa Rican tropical dry forest. Features of suitable nest sites are analyzed, but it does not appear that nest site availability limits colony density or determines colony dispersion. Rather, food limitation is suggested by a linear relationship between the logarithm of colony biomass and the logarithm of foraging "home range." Colonies were uniformly dispersed intraspecifically in 4 species. The pattern in a 5th species could not be distinguished from a random dispersion. Partial nest counts in 3 more species suggested a clumped dispersion in at least 1 species. The 4 species with uniform nest spacing all use pheromones for recruitment in foraging, whereas the remaining 4 species do not. The 3 most uniformly dispersed species are also intraspecifically aggressive at food sources. We propose that colony spacing is accomplished in these species by aggressive prevention of new colony establishment. The mechanism of colony spacing we propose has as essential elements: pheromone marking of potential nest sites; recruitment of workers; and aggression between workers from rival nests. This mechanism has been observed in operation in at least 1 of the species.

The foraging patterns of two eusocial stingless bees, Trigona fuscipennis and Trigona fulviventris, were studied on a population of the shrub, Cassia biflora, in a tropical dry forest in Costa Rica. Trigona fuscipennis, which forages in large groups that monopolize plants, restricted its visits to large, dense clumps of Cassia. Visited plants had significantly more flowers than nonvisited plants. Trigona fulviventris, which forages as individuals of in small groups, visited more widely spaced or isolated plants. In the isolated plants visited by T. fulviventris, there was no significant difference in the number of flowers on visited vs. nonvisited plants. In clumps, however, the few plants visited by T. fulviventris had significantly fewer flowers than plants not visited. This observation and observations of interspecific aggression suggests that T. fulviventris is excluded from the better plants in clumps by T. fuscipennis. We believe the observed foraging patterns result from both evolved strategies and displacement due to short—term ecological competition. To explain the results, we hypothesize that two species may stably partition and coexist on a single resource, provided that (1) the resource has a highly variable spatial dispersion pattern, ranging from dense local patches to large regions of thinly spread resource, (2) the thinly spread resource is profitably harvested by only one of the species, (3) the dense patches are controlled by the second species, and (4) the resource is nonmobile such that not mixing of resource occurs between dense and sparse areas. We propose the terms low— and high—density specialists to describe such species.