Nest site and habitat preferences of Centris bees in the Costa Rican dry forest

A study of nest site and habitat preferences of five tree-hole nesting bee species of the genus Centris (family Anthophoridae) was conducted in 1986 at the Lomas Barbudal Biological Reserve in the Costa Rican dry forest. Centris bees, which are important pollinators of a large proportion of the dry forest flora, were lured to artificial traps in a habitat monitoring study and an experimental preference study, which tested for exposure to sun, nest hole diameter, and height above ground level. The habitat monitoring study demonstrated that C. bicornuta, C. vittata, and possibly C. trigonoides had an overall preference for the more shaded riparian and/or mesic forests. Centris bicornuta also showed some preference for oak forests, whereas C. nitida had a clear preference for this habitat. Centris analis had no obvious preference for particular habitats. The experimental study demonstrated that three Centris species had a strong preference for shaded sites and an 8 mm nest hole diameter; no preference was indicated for height above ground level. A preliminary study of temperature relationships suggested that temperatures around 40$^\circ$C were lethal to early instar bee larvae. The temperature studies also suggested that shaded habitats had more optimal temperatures for Centris development. Overall results of this investigation were considered within the framework of available habitat area. It was estimated that the preferred shaded habitats, riparian and mesic forests, accounted for only $\sim$13 percent of the total land area at Lomas Barbudal. The nonpreferred habitats, such as the dry deciduous, extreme dry, savanna and regenerative forest habitats accounted for $\sim$86 percent of the Reserve area. Thus, it appears that a relatively low percentage of the land is supporting most of the nesting habitat(s) for this specialized group of Centris bees.