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= Ptilothrix = Ptilothrix is a genus within the tribe Emphorini of the family Apidae. Bees of this genus are large and can range from 7 to 15 millimeters. Ptilothrix species are ground-dwelling and excavate nests in the ground. The genus is known for having long leg hairs, called scopae, to help gather pollen to provision their nests. It is well documented that the Ptilothrix genus specializes on specific families of plants for their pollen, including the Malvaceae, Convolvulaceae, Onagraceae, Cactaceae, Pontederiaceae and Asteraceae families. The genus is found in the new world, with species ranging from North to South America.

Life History
The Ptilothrix genus is made up of solitary bees in North and South America. Their life cycle is typical of other members of the Apidae family in that individuals start as an egg that forms into a larva, followed by a pupa stage, and finally an adult. Females oviposit eggs into ground nests, also known as brood cells, made of hardpacked soil in throughout summer and fall. Female Ptilothrix provision the brood cell with pollen bundles which provide nutrients to their offspring as they transition from larva to pupa. The adults emerge from the nest, which is sometimes covered with a layer of soil to prevent predation. Bees of this genus are large, and can range from 7 to 15 millimeters. They have long leg hairs, called scopa, which aid in gathering pollen (Michener 2007).

Bees of the Ptilothrix genus are solitary. Unlike honeybees or other eusocial bee species, the members of Ptilothrix do not form a hive or have division of labor of individuals. Females gather pollen from flowers and build individual nests for their offspring. Males wait for females at flowers where they attempt to mate. According to Schlindwein (2004), the male aggression and mating behavior at flowers may contribute more to pollination of their host flowers than female visitation. Additionally, Sampson (2016) found that while males accounted for 5% of pollen visits to Hibiscus flowers, they contributed over 20% of pollen grains deposited on anthers. Therefore, male aggression contributes to increased pollination and seed set among Hibiscus.

Range
Species of this genus are found in North and South America. As with most bee species, they are most abundant at latitudes outside of the tropics and concentrate in south and north America (Michener 2007).

Nest construction
Ptilothrix species are ground-dwelling and excavate nests in the ground. The nesting behavior seems to be conserved across the genus, however there are slight differences in interior construction. Nest construction is characteristic of the genus, and Rust (1980) and Martins et al. (1996) compare the nesting behavior of P. bombiformis and P. plumata to other members of the genus, respectively. Once a site is chosen, the female transport water to her nesting site to soften the soil for excavation (Rust 1980). The female bee then digs a single celled or multi celled burrow, and lines the interior with wax or feces for support (Rust 1980). A small chimney or turret can be seen on the soil surface of most species’ nests (Rust 1980).

Among P. plumata, found mainly in Brazil, there is a high nest mortality rate of approximately 88% among nests that are completed in a season. Predation by fire ants and other parasites are cited as the most likely causes. Females can make multiple nests and tend to avoid making nests in the rainy season (Martins et al. 1996). P. plumata showed a delayed dormancy in nests, with there being a bimodal presence of hatching in nests. This dormancy is in response to environmental conditions. If a female noticed parasites or predators while constructing or visiting a nest, she would cover the nest or shake her body at the entrance to protect it (Martins et al. 1996).

Similar to P. plumata, P. bombiformis nests on hard-packed clay sites with little vegetation and uses water to soften the soil. For P. bombiformis populations observed in Rust (1980), nest survivorship was higher than that of P. plumata, at a rate of about 57%. The major causes of mortality in these populations appeared to be fungal pathogens or failure of the egg to hatch.

Pollination
Ptilothrix species are generally accepted to be oligoleges, or pollen specialists. Oligolecty was first defined by Robertson in 1925 to refer to those species that specialize on a family or group of closely related plant species. The alternatives to oligolecty are polylecty, generalization on a diverse range of plant species, and monolecty, which is strict specialization to a single plant species. Cane and Sipes (2006) introduced another modification of the term that is added is eclectic oligolecty, for those bees that will specialize on plants of unrelated clades. They introduced the term mesolectic to describe bees that gather pollen from large groups of related plants. It is different than eclectic oligolecty in that the plants are more closely related.

Bees exhibit a variety of behaviors for gathering pollen, with oligolecty being the behavior of Ptilothrix. There are benefits and disadvantages for this strategy, and there is also plasticity to oligolecty. Cane and Sipes (2006) found that when the oligolectic bees are not given a choice of their preferred genus of host pollen plant, some can be opportunistic and forage on available plants. Others are strict in their pollen diet choice and will not visit the flowers outside of their preferred pollen hosts (Sipes 2006). This brings up the question of whether the bees are choosing not to visit the flowers because of factors related to incompatibility with the pollen, or do they simply not recognize the flowers as being sources of pollen?

P. plumata is oligolectic towards pollen from plants in the Malvaceae family. According to a study done by Schlindwein et al. 2009, approximately 90% of pollen grains found in P. plumata nests were from Pavonia species ''(Malvaceae). ''They averaged three to four different flower species to make up most of the pollen provision, indicating they are oligolectic and not monolectic when given the choice of multiple preferred flowers. P. plumata morphology of long scopa on hind legs also indicates that they could have a greater tendency towards Pavonia species, since the pollen grains are exceptionally large in Pavonia.

Telleria (2003) studied the oligolecty of P. relata in the Argentine pampas. He found fourteen different types of pollen in the bundles provisioned in nests. The conclusion made in this study is that the species should be seen as narrowly polylectic instead of oligolectic, since the pollen came from groups of plant species are not closely related. However, the study only included 13 nests due to an infestation of assassin flies in the area.

It is important to note that just because a bee is oligolectic to a particular plant species, it does not mean that it is the plant’s main pollinator. In a study done by Telleria (2003), P. relata is oligolectic to Malvaceae family flowers, however they are not the only pollinator of the plant family. In fact, they are not even the main pollinator according to the study. It is a distinction that brings up questions on the importance of specialists to pollination if generalists are filling the same role in pollination. Not surprisingly, the effectiveness of oligolecty depends on the species. In some cases where pollen grains are large, they are more effectively carried by the specialist oligolectic bees of Ptilothrix (Schlindwein 2004). Other cases, generalist bees are just as, if not more, effective. As previously mentioned, pollination effectiveness may have less to do with female pollen provisioning and more to do with male mating competition (Schlindwein 2004). Additionally, Alarcon et al (2008) found that the role of pollinators and plants as specialists or generalists within the community can change over seasons. They cite a study that showed that polylectic Bombus species are more effective pollinators of Malvaceae flowers than the oligolectic P. bombiformis (Sampson 2006). Also, it may not be in the specialist’s best interest to be an efficient pollinator since they typically want to provision as much pollen as possible in their nests and not lose it brushing up against a flower.