Halictus sexcinctus

Halictus sexcinctus is part of the family Halictidae, which are commonly referred to as the sweat bees. Species in the genus Halictus are the most recently evolved in the Halictid family, and H. sexcinctus falls into the most recently evolved clade of the family, which is a eusocial taxa.[1] The family Halictidae has the most eusocial species of any bee family.[1] Study of the sociality of this family has been held back by a lack of understanding of how the species are phylogenetically related, and how closely related species within the family show very different levels of sociality.[1] Due to the different levels of sociality seen with the species, it was first thought possible that H. sexcinctus was actually a cryptic species. A genetic study of their mitochondrial DNA showed H. sexcinctus is in fact one species. However, that study was not definitive, and further studies of their nuclear genes are needed to be sure.[2] H. sexcinctus has been observed to be both solitary and eusocial. Because it belongs to a eusocial clade, its solitary behavior is an evolutionary reversion. However, H. sexcinctus is not alone in this, as there have been multiple losses of eusociality within the genus, such as in the species Halictus rubicundus.[1]

Halictus sexcinctus is a species of ground-nesting bees, previously thought to only exhibit solitary behavior.[3] They are relatively large compared to other sweat bees.[4] They are 14–16 mm in length, and have white felt bands on their abdomens.[5] Workers are typically smaller than foundresses, while foundresses will usually be the largest and most worn individuals in the nest.[4] Sterile and reproductive workers do not differ in size.[6]

Halictus sexcinctus are commonly found across Europe, and can be found as far north as southern Sweden[5] and as far east as Turkey and Iraq. They are solitary in the northern part of the range and social in their southern range.[4] They live in very large aggregations in central Europe, despite being solitary in this part of their range.[4] H. sexcinctus prefers to nest in sandy or loamy soil.[5] They use their mandibles to dig nests in the ground or even on vertical substrates if it is made of earth. Their nests consist of a main tunnel dug diagonally into the ground, with several short side chambers off of it leading to brood cells. One egg is laid per cell on top of a nectar-pollen mixture.[7] Females primarily collect pollen from plants of the family Asteraceae, and tend to forage in plant species with wide-open flowers.[5] Examples of plants they forage pollen and nectar from are asters, daisies, and sunflowers.[7]

It takes about 4.5–5 weeks for a newly laid H. sexcinctus egg to develop into an adult. Foundresses will on average lay 9.6 eggs in their first brood. The colony cycle lasts around 10–12 weeks, constrained on either side by a winter hibernation.[4]

There is a difference between communal and eusocial organization in halictine bees. Communal colonies do not have castes, but rather multiple females of the same generation work together with relatively little reproductive skew and aggression. These females may or may not be related. Eusocial colonies have a queen (the foundress) and reproductive castes with high reproductive skew and aggression, typically directed from the queen to workers. This type of society is formed when the queen’s progeny become workers.[9] H. sexcinctus has the strongest polymorphic social behavior discovered in sweat bees so far.[6] The social polymorphism they exhibit is one where either a solitary female founds a nest, and sociality emerges in the next generation of females, or several females from the same generation will cofound a new colony.[2] Solitary and communal behavior is seen in the northern range, while populations in the southern range exhibit communal and eusocial structure.[6] It is fairly unusual to find both communal and eusocial behavior exhibited by the same species outside of the halictine bees.[2] H. sexcinctus was the first example of a definite eusocial/communal polymorphism in halictine bees, and may possibly be the most extreme example of intraspecific social polymorphism among insects.[9]

Reproduction between foundresses is shared fairly equally in communal nests, and there is almost no aggression between the females.[2] In all socialites, foundresses tend to show higher levels of ovarian development than workers and are also more likely to mate. Sometimes, workers will actually have more developed ovaries than foundresses. Compared to other Halictus species, H. sexcinctus has a high rate of worker ovarian development. In eusocial colonies there is a reproductive skew favoring queens, but workers still have quite a lot of opportunity for mated reproduction. Unlike many bees, insemination status does not determine which females are queens and which females are workers, though of course only mated females can produce daughters, a necessary requirement of being a queen.[4]

The majority of offspring produced in the first brood are female, ranging from 74-81% of the offspring. Only about 50% of foundresses survive to the production of the second brood. This means that some unmated workers will produce offspring, which leads to more male offspring being produced in the second brood than in the first. Therefore, the ratio of males produced in the second brood most likely depends upon whether the foundress does or does not survive. Very rarely, foundresses will not mate and thus produce an all-male brood, which therefore means they will not produce a second brood. Less pollen is needed in order to produce males, because they are smaller than gynes. When resources are scarce, the sex ratio may be shifted towards males.[4]

Foundresses tend to show more wear than workers do on their mandibles and wings, suggesting they participate in more work, such as constructing brood cells, maintaining the nest, and provisioning the brood. Workers show “reproductive opportunism,” where their level of altruism can range from entirely altruistic to entirely selfish. It is unclear how exactly this flexibility is possible or how this individual behavior leads to spreads throughout the population.[4] Sterile workers typically show more wear on their wings and mandibles than reproductive workers. This is most likely due to sterile workers performing more work than reproductive workers. This suggests that infertile workers are more altruistic in order to gain inclusive fitness through kin selection, due to the fact that they are unable to produce their own offspring. Some reproductive workers are somewhat altruistic and will help to raise the broods of their sisters and mother. Some, however, are not altruistic and seem only to take care of their own brood, essentially making them parasites to the colony.[6] One reason that communal/eusocial polymorphisms are so rare could be because communal strategies are unstable due to their vulnerability to cheating.[9]

A new species of mites, Histiostoma halicticola, was discovered to parasitize H. sexcinctus in a study by Fain et al. In this study, the new species of mite was observed only in the deutonymphal stage. The bees harboring these mites were found in the Döberitzer Heide nature reserve, nearby Berlin, Germany. The mites were found on both males and females. On females, the mites are found in a deep, bristly furrow on the second tergite. This site protects the mites and makes them hard to remove. When a female H. sexcinctus had more than thirty mites on its body, the mites were then found in other areas lacking specific shelter. This shows that the furrow is the preferred spot, and the mites will only settle elsewhere if the furrow is already full. Males lack this particular furrow, so the mites are found on the smooth, concave ventral surface of the thorax between the coxae. H. sexcinctus was also found to harbor mites of the families Pygmephoridae and Scutacaridae.[3]