Knowledge gaps on Neotropical solitary bees *

Isabel Alves-dos-Santos^1*Maria Cristina Gaglianone²

• ¹Department of Ecology, University of São Paulo, São Paulo, Brazil
• ²Environmental Sciences Laboratory, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Campos, Brazil

Introduction Solitary bees represent the vast majority of Apiformes species (Michener 2007). With the exception of the subfamily Nomadinae (which are cleptoparasitic bees), the solitary lifestyle is present in all other Apiformes subfamilies, totaling approximately 15,500 species (Danforth et al. 2019). This represents about 77% of bee diversity. In solitary bees, there is no cooperation or intergenerational contact (Michener 1974). The female builds the nest alone, provisioning and defending it (Batra, 1984; Alves dos Santos, 2002). In each cell built, the female lays an egg. After hatching, the larva passes through four or five stages, consuming the food supplied by the mother before pupating. The adult then emerges and restarts the species cycle. These phases can last a few weeks or many months. In any case, offspring and mothers never meet. Some solitary bees build nests in pre-existing cavities, such as holes left by beetles in wood rotting tree trunks or various hollow branches. In this case, nests of these species can be obtained by offering artificial traps, such as bamboo segments or perforated woods (Krombein 1967; Garófalo et al. 2004). This technique greatly facilitates the collection of data on the biology of the species (Alves dos Santos et al. 2002) and enables their use on a larger scale for commercial purposes (e.g., Osmia cornuta and Megachile rotundata, both used for crop pollination) (Stephen 1961; Richards 1984; Bosch & Kemp 2002; Muniz et al. 2024). However, the vast majority of solitary bee species (three-quarters) nest in the ground, excavating their own nests (Danforth et al. 2019; Antoine & Forrest 2021). In this case, to study them, the first challenge is to find the nest, which is often just a small hole in the ground. Many species form aggregations, which are several nests in close proximity, and when the species is active, there is strong movement in the area, signaling the presence of nests (Cane 2024). In some species, these aggregations are permanent, meaning they become active annually and can increase significantly in size (Batra, 1999; Cane 2003). After finding a solitary bee nest on the ground, the second challenge is excavating it to reach the brood cells. Luckily, the nests are shallow (Celary 2004) or have few branches, making them easy to excavate. However, several species have deep nests in hard or sandy soil, requiring time-consuming and careful excavation (Bohart et al. 1972; Roberts 1973; Gaglianone 2005). These two steps make it difficult to obtain data on the biology of most solitary species. Access to the nest provides a unique opportunity to obtain data on the plants on which the species depends entirely, the potential enemies, seasonality of the species, development of imatures, nest architecture, and more. To illustrate the current knowledge gaps regarding solitary bees, we draw on data from two important groups of the Neotropical apifauna: the Eucerini tribe, which is well represented on all continents except Australia, and the subfamily Diphaglossinae, which is restricted to the Americas. These groups are commonly recorded in surveys of Neotropical bee fauna, although usually at low abundances, which further hampers the understanding of their biology. In this review, we examined literature published over the past ten decades addressing the biology of these bees as indexed in the Web of Science database. Knowledge of the Biology of Neotropical Eucerini The tribe Eucerini comprises 780 species (Michener 2007), with some very speciose genera, such as Eucera, from the northern hemisphere. Eucerine bees show particularly high generic diversity in the Western Hemisphere (Dorchin et al. 2018), with 31 genera, 16 subgenera, and 247 species (Urban et al. 2023), occurring from Quebec, Canada, to Chubut, Argentina. Representatives of this tribe are popularly known as long-horned bees, as the males have extremely long antennae (Figure 1), sometimes twice the size of females. There are reports of male roosting in flowers or branches, in aggregates of several species (Alcock 1998, Mahlmann et al. 2014, Silva & Andrade 2022). All Eucerini species nest on the ground. A compilation of research published over the last 100 years on nesting in Eucerini revealed that only about 4% of the species have been studied (32 species) (Table 1). The studies describe the nests, the number and arrangement of brood cells, and in many cases also provide data on the immatures, associated parasites and plants used. However, even among the 32 species studied, there are gaps in some of this information. Some of the most complete studies in terms of description about the species are those by Rozen (1964) on Svastra obliqua in Florida, by Parker et al. (1981) on Melissodes agilis in Utah, USA, and by Michelette et al. (2000) on Canephorula apiformis in San Juan, Argentina. Common to most Eucerini are the oval-shaped cells, vertical oriented, with a thin cell lining, eggs placed on top of the provision, pollen packed into the base of cells, liquid layer covering pollen masses. The mature larva places its feces against the cell cap, and then spins a thin cocoon, constructed of a number of course and fine layers of silk. Rozen (1991) compared the anatomical structures of the mature larvae of the Eucerini. Several species are polylectic, such as Eucera hamata (Miliczky 1985) and Thygater aethiops (Gonzalez & Ospina 2008), and others are oligolectic, such as the pumpkin specialist Peponapis and Xenoglossa (Hurd et al. 1971). Nests of three Peponapis species have been described. They form small nesting aggregations (6-8 nests) adjacent to Cucurbita fields, which they pollinate (Hurd et al. 1971). Usually, the brood cells reveal 100% pollen from Cucurbita (Krug et al. 2010). Of the 32 Eucerini species studied, 14 occur in the Neotropics, but there are numerous hiatuses. For example, there are no nest studies on the genus Florilegus (11 species) or Gaesischia (31 species), only one study on Alloscirtetica (44 species) and two studies on Melissoptila (54 species). Thus, even for the most common and specious genera in the Americas, we have no information on their biology, as their nests have never been located. The Brazilian Eucerini species are well-resolved systematically due to the dedication of Danúncia Urban with this tribe (Urban et al. 2023). Several species are oligolectic, such as Florilegus (specialist on Pontederia) or Gaesischia (specialist on Vernonia) (Schlindwein 1998). However, their nests remain undetected and may require more intensive and carefully field efforts to be found. Knowledge of the biology of Diphaglossinae The subfamily Diphaglossinae are large bees of the Colletidae family, restricted to the New World (Michener 2007). The subfamily is divided into three tribes, 12 genera, and approximately 130 species (Danforth et al. 2019). Like the Eucerini, all Diphaglossinae species are solitary and nest on the ground. The species are notable for their beautiful colors and abundant hairiness (Figure 1). A common characteristic of most species in this subfamily is their crepuscular habit, which makes locating their nests in the wild even more difficult, as the nest is closed all day. Females are active for only a brief window of time at twilight (Danforth et al. 2019). Investigations on nesting biology of Diphaglossine bees correspond to studies on 14 species, approximately 10% of their representatives (Table 1). Of these 14 studies, 11 were conducted on Neotropical species and the other 3 in Arizona (Linsley & Cazier 1970; Rozen 1984). In South America, the study conducted in Argentina and Chile with 5 species is noteworthy (Sarzetti et al. 2013). Recurrent parasites belong to the genera Triepeolus and Odyneropsis, both of the tribe Nomadini. Common to the studied species is the nest with a deep vertical tunnel, with laterals branches radiating from the main burrow in various directions, each ending in a single cell. The branches are filled with soil after oviposition. The brood cells are large to very large (corresponding to the large size of the bees), elongated, with a diameter somewhat larger than the diameter of the burrow, and circular in cross section. The brood cells are vertically oriented and curved at the top, which can reach 90 degrees or more (Ptiloglossa and Crawfordapis) (Rozen 1984; Sarzetti et al. 2013). The cells are lined with a cellophane-like layer and contain semi-liquid provisions. For some species, huge and permanent aggregations have been reported (Roberts 1971; Otis et al. 1983, Roubik & Michener 1984). A recent study of Ptiloglossa latecalcarata conducted in the Brazilian cerrado revealed a curious fact: the presence of monofloral pollen in the brood cells. In this case, the recorded pollen was of Caryocar brasiliense (Caryocaraceae), known as pequi, described as chiropterophilous plant, and visited by nocturnal bees at twilight (Araujo et al. 2020). The flowers of this species open in the evening and provide resources until dawn, supplying a significant amount of pollen for nocturnal bees. However, it is known that P. latecalcarata is not a specialist on Caryocar, but rather an opportunist for the plant that has nocturnal anthesis and is massing flowering around the nest. Similar behavior was observed on Campomonesia phaea (Myrtaceae) (Cordeiro et al. 2017). The females have a short window of time to forage and collect pollen and nectar for the offspring provision. Thus, the plant that is nearby can be the target pollen source. Discussion Using these two groups of Apiformes, Eucerini and Diphaglossinae, we tried to illustrate the gaps of knowledge on solitary bees in the Neotropics. However, we believe that similar deficits exist in other biogeographical regions in the world. We attribute these gaps to many factors, but we highlight one less discussed: descriptive or natural history papers are "out of fashion" and are not seen as high-impact results by journals or modern researchers. We disagree with this trend, as basic data on the biology of any species fuel discussions of "advanced papers" addressing evolutionary questions, phylogenetic relationships, population genetic, species interaction networks, among others. Classic studies of the biology of species can yield extensive insights into the group (Gaglianone 2005). The following two examples illustrate how such studies provide valuable information. With accumulated information on the immatures of many cleptoparasitic species and observations of the strategies of females in nests of all the tribes in the Neotropics, Rozen (2003) contributed substantially to the understanding of the evolution and phylogenetic relationships within the extensive cleptoparasitic subfamily, Nomadinae, the oldest clade of parasitic bees (Sless et al. 2021). Gathering such information was only possible through meticulous nest excavation and description of larvae anatomy of different lineages of cleptoparasitic bees, primarily from Neotropical, over many years. Through access to the nest of the European Andrena marginata, Stenmark (2013) was able to obtain a huge amount of data, on foraging behavior, pollen provision, pollen utilization, development, sex ratio and nest architecture, in Sweden. But the substantial highlight was that with his results, he was able to estimate the critical pollen resources needed for a nest and for an entire bee population, that is, predicting the carrying capacity (K-value) of bee populations in the habitat. Thus, he proposed a model with easily measured variables (the plants available in the area) that can be used as a tool in bee conservation planning. These two cases demonstrate the importance of information on species' biology obtained through field observation and experimentation. Bees, like all insects, face numerous threats. The fact of been less studied already constitutes a risk to solitary bees (Alves dos Santos et al. 2025). We cannot protect organisms if we do not know where they live and what they depend on. In Europe, the list of endangered species includes approximately 60% of bee species with insufficient data (Nieto et al. 2014). This deficiency prevents a conclusive assessment of the species' conservation status. For the Neotropical region, this percentage is certainly much higher. In conclusion, it would be very important for basic natural history studies to be given greater prestige by journals and the scientific community, taking into account the contribution they can make in several cutting-edge areas. Thereby, we encourage young scientists to leave the comfort of air conditioning and venture into fieldwork, where things happen. Soga & Glaston (2025) mention several negative impacts to science and education associated with the reduction in fieldwork experience. Furthermore, observing the activity of females building nests is very pedagogic, as well as very enjoyable. With a good protocol and some instruments, a wealth of data can be achieved.