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Sphecodes

Sphecodes is a genus of cuckoo bees from the family Halictidae, the majority of which are black and red in colour and are colloquially known as blood bees. Sphecodes bees are kleptoparasitic on ground-nesting bees, especially bees in the genera Lasioglossum, Halictus and Andrena. The adults consume nectar, but because they use other bees' provisions to feed their offspring they do not collect pollen.

Distribution

Sphecodes is a cosmopolitan genus with species represented on every continent except Antarctica. The genus is also very species rich, with 21 species described from Siberia, 33 species from Central Europe, 17 species from the Indian region, 26 from the Arabian Peninsula and surrounding region, and 21 from Southeast Asia. In the Americas, there are 11 species described from the Neotropics and 63 from the Nearctic. Australia has only two native species, S. manskii and S. profugus, both of which are restricted to the northeast. The species Sphecodes albilabris is thought to have been introduced to both Australia and the United States by accident.

Taxonomy

The genus was erected in 1804 by Pierre Latreille for the species Sphecodes gibbus. That species was initially described by Carl Linnaeus as Sphex gibba. The fact that the type specimen was initially described as a wasp was not lost on Latreille as the name he chose for the genus, Sphecodes, means "like a wasp". In the late 18th and early 19th-centuries, the taxonomy of Apoidea was very unclear, and at various points Sphecodes species were described as the following genera: Sphex Linnaeus, 1758; Nomada Latrielle, 1802; Halictus Latrielle, 1804; Apis Linnaeus, 1758; Melitta Kirby, 1802; Andrena Fabricius, 1775; and Tiphia Fabricius, 1775.

The following genera are now considered synonyms of Sphecodes: Dichroa Illiger, 1806; Sabulicola Verhoeff 1890; Thrausmus Buysson 1900; Dialonia Robertson, 1903; Machaeris Robertson, 1903; Stelidium Robertson, 1903; and Sphegodes Mavromoustakis, 1949.

Charles Robertson was particularly prolific in describing genera that would be synonomized with Sphecodes later on, although the differences he noticed are now reflected in the subgenera. The following genera are now considered subgenera: Drepanium Robertson, 1903; Proteraner Robertson, 1903; and Sphecodium Robertson, 1903. Subgeneric classifications are currently debated, and whether they are truly monophyletic groups remains unknown.

Callosphecodes Friese, 1909 may either be a synonym or subgenus, but specimens that match its description are exceedingly rare.

Phylogeny

Relatively few phylogenetic studies have been completed for the genus, and none that are exhaustive or analyze biogeography. Sphecodes is a member of the family Halictidae and in the tribe Sphecodini. Sphecodini is sister to Halictini sensu stricto. Genus-level relationships within Sphecodini are still being determined, but recent phylogenies suggest that Sphecodes is sister to the genus Eupetersia. It is unknown exactly how many species there are worldwide, and even well-studied regions of the world such as the United States and Europe likely have many species to be described or synonymized.

A note on Halictinae tribes

There are two diverging approaches to tribe-level phylogenies within Halictinae. The first, used by taxonomists like Danforth, Gibbs, and Brady recognizes five distinct tribes: Augochlorini, Thrinchostomini, Caenohalictini, Halictini sensu stricto, and Sphecodini. This is also the approach that Wikipedia uses. The second, used by Ascher, Michener, Straka, and Engel among others recognizes only two: Augochlorini and Halictini sensu lato, with the rest of the tribes relegated to subtribes within Halictini s. l. This approach is also used by resources such as iNaturalist and BugGuide. Both are valid and represent the same evolutionary relationships.

Morphology

Sphecodes is in the family Halictidae, a group distinguished from other bee families by a highly arcuate basal vein. In North America, Sphecodes is the only genus of Sphecodini present.

Sphecodes are small-to-medium bees, usually with distinctive red abdomens although males are sometimes entirely black. The genus has somewhat oval, brachycephalic heads and thicker antennae compared to other bees. Most species have three submarginal cells, but some consistently have two, and others have either two or three. Their body size is correlated with that of their host: large Sphecodes parasitize larger hosts like Andrena, while small Sphecodes are more often associated with small hosts like Perdita. Like other kleptoparasitic bees, they are largely hairless and females lack scopae on their hind tibia. Both sexes have thickened and highly rugose cuticles. This adaptation allows them to deflect and block stings when fighting their hosts. In many other cuckoo bee genera, the larvae sport large mandibles used to kill the host. These are absent in the young of Sphecodes, where it is the adult that dispatches the host egg or larva.

Identification of these bees is difficult, in part due to lack of research and published keys. Important characters include the antennae, mandibles, propodeum, and male genitalia.

Natural history

Sphecodes are most commonly found hovering over the nest sites of their host bees. Females will either kill the hosts or wait until the nests are unguarded. She will then enter the nest, laying her own egg on the pollen provisions and killing the egg or larvae of the host. The larval bee will feed on the pollen provisions throughout the winter, emerging as adults in the spring or fall. Their phenology does not necessarily overlap with that of their host, making host associations sometimes difficult to determine.

There are few studies of parasitism within Sphecodes and much of what we know is inferred from a small number of observations. Due to the range of sociality of its hosts, particularly Halictus and Lasioglossum, Sphecodes species display a broad range of parasitism tactics.

For solitary hosts or social hosts in a solitary phase of their lifecycle, the female Sphecodes will wait until the host leaves. Then, she will enter the nest undetected, laying her eggs in recently closed nest cells. If the host bee comes back, the Sphecodes will fight with her, typically killing or parasitizing the host. Other cuckoo bee genera such as Nomada also sneak into unguarded nests, chemically mimicking their Andrena hosts to remain undetected. Surprisingly, studies of the cuticular hydrocarbons and Dufour's gland secretions suggest that Sphecodes do not mimic their host.

If the hosts are either communal or eusocial and thus always have some bees guarding their nests, Sphecodes will deploy a different tactic. In this scenario, the female Sphecodes drives the host out of the nest, possibly chemically. Augochlora have been reported leaving disoriented, or in quick flight.

Some species will also deploy a much more aggressive response, battling and killing every member of the colony. She will then spend several hours laying eggs in the provisioned brood cells. The propensity for Sphecodes to kill the adult hosts is unusual in kleptoparasitic bees, although they prefer to enter unguarded nests.

Hosts

Sphecodes species are most frequently associated with the closely related genera Lasioglossum and Halictus. However, they have also been known to parasitize Perdita, Andrena, Calliopsis, Augochlorella, Melitturga, Colletes, Agapostemon, and possibly Augochloropsis. Host associations of most Sphecodes species, particularly outside of Europe, are largely unknown.

Some species parasitize a wide range of hosts, while others are species or genus-specific. Within a generalist species, however, studies have shown that individual bees are typically specialists: a single female Sphecodes will visit the nests of a single host species, although other individuals of the same species of Sphecodes may use different hosts.

Host switching is widespread in the evolution of Sphecodes, with one paper finding it has evolved at least 17 times. This flexibility could be due to their destructive modes of parasitism or the high degree of host generalism.

Species

There are over 300 known species in the genus Sphecodes. As of 2015, there were 319 valid species described. The following list may be incomplete and does not reflect recent synonomies.

References