Distribuição espaço-temporal de recursos florais utilizados por espécies de Xylocopa (Hymenoptera, Apidae) e interação com plantas do cerrado sentido restrito no Triângulo Mineiro

Abstract

The identification of floral resources used by bees is the first step towards the elaboration of management plans and conservation of populations of pollinators in the Cerrado. The reduction of natural areas and, consequently, nesting sites and floral resources used as food by adults and immatures, is the main factor acting directly on the decline of populations of pollinators. The Triângulo Mineiro is one of the largest producers of yellow passion fruit, and the low productivity may be related to the decline of populations of Xylocopa spp., which are the main pollinators of this crop in the region. In the absence of these large bees, producers are forced to use hand pollination procedures, which significantly increases production costs. In order to support management plans and conservation of natural areas and populations of native pollinators, we carried out an assessment of the availability of floral resources used by Xylocopa spp. in natural areas surrounding yellow passion fruit orchards. We studied four areas of cerrado stricto sensu, where we plotted transects covering a 2 ha area in each site. In these transects we recorded all blooming plant species during two years. We identified the main pollination systems and their distribution in the vertical stratification, and examined the distribution of floral resources used by Xylocopa spp. We identified the sources of food resources to these bee populations by pollen analysis from three subgroups of samples (feces, nesting cells and body of foraging bees) during the period from January 2006 to December 2007. A reference slide collection and a catalogue of pollen from native plants present in the transects were organized and were fundamental for pollen identification. In all areas, over 80% of the plants were visited by bees throughout the year, and even during periods with lower temperature and humidity, there were floral resources available to the bees. The Xylocopa spp. were found in activity throughout the year and used the pollen and nectar as food sources for adults and immature. The number of samples in each subgroup varied depending on the bionomics of these bees and the subgroup feces was the most representative (60.13% of the samples). The Xylocopa spp. used a total of 112 plant species, of which 72.32% of them were found within the transects, seven (6.25%) occurred in the Cerrado biome, but were not sampled in transects, seven were cultivated species, four (3.57%) were found only in vereda , two (1.79) were ornamental and others (9.82%) were indeterminate. It was possible to verify that the Xylocopa spp., although generalist, used more often a very small fraction of all species of native plants that had floral resources throughout the study period, which accounted for only 14.60% of all species found in those areas. This narrower effective niche is similar between the four bee species studied indicating strong niche overlapping. This set of plants is comprised mainly by flowers with poricidal anthers, yellow flowers or at least yellow anthers, medium-sized to large and zygomorphic flowers. Among the most important plant species for the maintenance of Xylocopa bee populations in the Triângulo Mineiro region were: Campomanesia adamantium, Caryocar brasiliense, Mimosa hirsutissima, Ouratea spectabilis, Rhynchanthera grandiflora, Senna obtusifolia, Senna rugosa, Senna sylvestris, Senna velutina, Solanum lycocarpum and Styrax ferrugineum. Most of these species are considered invasive by producers, and are often removed from areas surrounding the orchards. However, these species should be preserved or even replanted in the areas surrounding passion fruit orchards in the region. Since passion-fruit flowers are visited by Xylocopa spp. exclusively for nectar collection, the native plants are the main source of pollen during the year. Xylocopa bees need these two floral resources for the survival both of adults and immatures.