ABSTRACT Tropical forests are three-dimensional with the presence of numerous micro-environments formed by horizontal and vertical gradients. Such micro-environments can affect the nesting preference of organisms, including the trap-nesting Hymenoptera. Bees and wasps are key elements in ecosystems and are considered as sensitive to environmental changes, and trap-nests sampling methodology is widely used in their ecological and conservation studies. However, many uncertainties remain about nesting site preferences. From this perspective, our aim is to assess the diversity descriptors of trap-nesting Hymenoptera in different micro-environments. The sampling was conducted on three micro-environments (canopy, understory and treefall gaps) replicated on ten permanent plots within a Brazilian Atlantic forest hotspot. In each micro-environment, we installed trap-nest stations made by a set of twenty bamboo artificial nests. We found 762 brood cells from ten wasp and five bee species. The rarefaction curves indicate the lower species richness in understory stations, while treefall gaps and canopy stations were not significantly different. We analyzed abundance, mortality and parasitism rates using generalized linear models, but only abundance varies significantly among micro-environments. Our data indicates that trap-nesting Hymenoptera prefer to nest in micro-environments with higher exposure of sunlight. Canopy and treefall gap assemblages are consistently more abundant and diverse than understory probably due the higher temperature and lower humidity. On the other hand, mortality, parasitism rates, and the species composition were similar among environments. Our hypothesis is that the species composition was not affected as these species have a foraging range that encompasses nearby micro-environments.

Abstract Cavity-nesting bees are enigmatic because they are difficult to observe in the wild, hence trap-nests (man-made cavities) provide the means by which these bees may be studied. Trap-nests is an efficient methodology to study these bees and are common worldwide. These traps have been used for a variety of reasons, including inventories, to examine pollen load, to study habitat disturbance, and bee conservation. However Neotropical trap-nesting bees’ taxonomy and biology are still poorly known and here we provide a review about these subjects. We searched for trap-nest bee studies in the Neotropical Region using Google Scholar and ISI Web of Science at any time in the past to December 2017. We found 109 independent studies, most of which were from Brazil (87 studies), followed by Argentina (10 studies), and other countries had fewer than five studies each. A total of 140 species, 24 genera, 10 tribes and three subfamilies were reported in trap-nests. Nest architecture was described for only 49 species. Taxonomy is only well-known for 14 genera, somewhat known for seven and is essentially unavailable for three genera. Construction material, closing plug and cell shape are similar among species in the same tribes and genera. Vestibular and intercalary cells, and the preliminary plug are variable, even at the specific level. Apinae is the most studied group with available data for all genera recorded in trap-nests. Colletinae is the least-studied group and nothing is known for their nesting biology. Megachilinae is intermediate, with some studies of taxonomy and nesting. We suggest that further trap-nest studies should provide more detailed information on nest architecture and construction materials, including explicit mention of structures that are absent. All Neotropical bees need more taxonomic studies, but some, such as Hylaeus and Megachile, require more attention since Hylaeus is essentially unknown and Megachile is very common on trap-nests.

ABSTRACT Studies on the nesting biology of Liris are restricted to a few notes and observations on ground-nesting species. There are no studies of this kind about Brazilian species. We investigated and described the nesting biology of Liris sp. obtained by trap-nests that were installed at an area of Atlantic Forest vegetation (25°10'S, 48°18'W) in southern Brazil. The nests of Liris sp. are built with a variety of plant debris. They usually have one cell, but may have up to two. Nests do not show vestibular or intercalary cells. Immatures have a hard cocoon made with the silk they produce, mixed with the fine sand and sawdust left by the adult female at the bottom of the cell. No nest parasites were observed. The wasps did not go through diapause at the prepupal stage, and emerged within 36 to 46 days after nests were collected from the field. There was no emergence of male wasps. Even though Liris sp. nest in preexisting cavities, they resemble ground-nesting species of the same genus in their habits, nest architecture, and development characteristics. The absence of males in our samples might be related to nest diameter. The eggs from which males hatch can be laid in smaller burrows than those available at the present study. We believe that the hardiness of the cocoon is the species' main strategy against parasites, although it is complemented by the camouflage provided by the nest closure. We suggest that a broader comparison of the nesting biology of Liris Fabricius, 1804 should be carried out, leading to a better understanding of the evolution of nests in the genus.