Macrotera portalis is a species of communal, ground nesting, partially bivoltine bees found in arid grasslands and desert regions. An oligolectic bee, M. portalis gathers pollen only from plants in the genus Sphaeralcea  and contains unique emergence characteristics. It is predicted that M. portalis has a patient technique of emergence to survive through the harsh conditions of the desert, resorting to bet hedging and timing emergence following the rainy season.
|Face of a large-headed, flightless male of Macrotera portalis. Specimen is deposited in the Snow Entomological Museum at the University of Kansas (accession #SMO290071). Yellow paint on top of head was used to mark the specimen during nesting studies.|
Perdita portalis Timberlake, 1954
Taxonomy and phylogeneticsEdit
Macrotera portalis is of the order Hymenoptera, family Andrenidae, and genus Macrotera. The family Andrenidae is specific to sand bees, andrenid bees, and mining bees. This species was formerly in Perdita, which contained 756 species and now contains around 600 species of bees which are located primarily in northern Mexico and the arid southwestern United States. Around 30 species removed from Perdita were placed in the genus Macrotera.
Description and identificationEdit
Macrotera portalis is a communal, ground nesting bee. As part of the family Andrenidae, M. portalis is a ground nesting bee. It has velvety patches between the eyes and the antennae base called facial fovea.
Macrotera portalis is biologically closely related to Macrotera texana. Both species are communal, females have provision over one cell each day, and intranest mating is common. Macrotera portalis, however, has more females in each nest. M. texana are not dimorphic, and their nests are usually not reused for too many generations.
Distribution and habitatEdit
Macrotera portalis are communal bees that are ground nesting, with many females often sharing the same nest while provisioning their own individual nest cells. The species is found in arid grassland and desert regions in northern Mexico, Arizona, and New Mexico. Macrotera portalis nests contain tunnels that consist of pollen and nectar in storage for development of larva. The species collect pollen only from plants in the genus Sphaeralcea. The nests are usually recycled for several generations and last for a long period of time. One study by Cornell professor Bryan Danforth demonstrated that nests were shallow, at most 15 cm deep, with some complex nests that contained more than 200 cells and an area with a 20 cm diameter. The main tunnels was absent of loose soil and stretched straight down with waterproof lining coated the cells. The nests were open during nesting period and did not have tumuli. Completed cells can range to a great number, with over 200 in some nests. Generally, there are about 2 to 29 adult females in one nest. Nests generally stay in the same location each year since nests are isolated, large and some prepupae are in diapause for more than a year. Further, since nests are used frequently, bees within a nests may be very closely related to each other.
There is partial bivoltinism seen in Macrotera portalis. Some larva are second generation adults because they develop and emerge in the same year. Larva would complete feeding, defecate, pupate, then eclose to adults. The period of development takes at least 21 days.M. portalis, known to be bivoltine, is estimated to have about 2 broods, perhaps 3, per year. The adults generally emerge in the spring or summer. Adult males and females usually mate following emergence from their cell. Females develop, lay eggs and continue to provide for and construct nests while larvae grow underground. In winter months, larvae remain in diapause, or resting condition, in their prepupal or pharate adult stage. Diapause continues until the next spring or summer when metamorphoses is complete and the bees may emerge as adults.
Sex and morph ratioEdit
Bryan Danforth, now a professor at Cornell University, performed a study and counted the number and percentage of females and males within the offspring of a colony. It was found that in 167 offspring 45% was female and 55% was male, reflecting no significant difference from a 1:1 sex ratio. Within the identified males, there was also not much of a difference between small headed males and large headed males, reflecting another 1:1 morph ratio (52.5% large headed male and 47.5% small headed male).
In Macrotera portalis, males are dimorphic, meaning that they have two distinct size classes. Male dimorphism has been found to be a derived state. The two distinctions in morph are dependent on characteristics of male shape and head size:
- 1. Small headed morph that is capable of flight. This morph bear a resemblance to other closely related male species. This morph is seen only on flowers and the small headed males are aggressive, combating with males of the same species as well as with male and female bees of different species. Further, small headed males regularly mate with foraging females.
- 2. Large headed morph that is incapable of flight. This morph has derived traits including flight muscles that are atrophied and indirect, enlarged facial foveae, and eyes of reduced compound. Large headed males are seen only in nests alongside females. Large headed males are also belligerent fighters with fights consisting of attacks with mandibles open, ending in the death of one male. Further, large headed males are very conscious of female reproductive behavior: during cell provisioning, large headed males devote a lot of time around open cells; large headed males only mate with the females when forming pollen and nectar balls before oviposition.
Female Macrotera portalis brood twice in a season. The female bees are in charge of cell construction, and in the communal nest each female makes one cell per evening, with no evidence of cooperativity during construction. If 2 or more females do share a lateral burrow, each seems to maintain the tunnel at the same level. Main and lateral burrows were kept open by packing soil into the side of the burrow walls by all females.
Female cell constructionEdit
Excavation of new tunnels or cells was performed by using her mandibles to loosen soil, pushing soil behind using her legs into the open lateral tunnel. The laterals of loose soil were cleared by walking backward and using their legs and metasoma to push the soil into a nearby lateral or main tunnel. Cell construction was performed by shaping the cells through repeated packaging using the female’s pygidial plate, adding soil when needed. When soil was smoothed and packed well enough, hydrophobic lining was added. The cell lining behavior consisted of brushing motions over the cell using the female’s metasoma.
Female cell provisioningEdit
Females performed provisioning, cell closure, cell construction, and oviposition every day. Females who foraged came back and formed a pollen ball by shaping loose groups of pollen into a sphere. If a male was present, the female would mate with the male during departure from the cell. If the nest did not have a male, the female would groom themselves and then return to the cell to continue making pollen balls. 1 or 2 copulations would occur before an egg was laid, then after the egg was laid the cell was closed by the female bee. The females then left the cell again for a feeding trip, not returning with any pollen. Female observation thus demonstrates that Macrotera portalis is communal as there is no observed cooperative or agonistic behavior. Further, females did not open cells to care for larvae after ovipositing or show proof of kleptoparasitism or robbing. Communality has been proven as an advantage due to nest defense for certain wasps and bees, yet the lack of observation of females at the nest entrance for defense demonstrates that defense is not associated with communality for M. portalis. Rather, communality is most likely due to the high cost of solitary nest development.
Smaller headed males tend to mate outside the nest whereas large-headed, flightless males mate within the nest. This projects into the concept of body size use within mating tactic, specifically fight or sneak, in order to maximize fitness. Larval diet and growth influence body size, thus determining which tactic to use. The flightless fighter phenotype that is characteristic of large headed morph are the males that mate within the nest. The flight capable, smaller headed phenotype that is characteristic of the small headed morph are predisposed to these conditions due to a smaller metamorphose, making these males tend to mate outside the nest. The factor that determines male larval size and thus the tactic of mating is maternal provisioning. It is hypothesized that larger male tactic confers a greater average fitness compared to the smaller male tactic, but this theory has little supporting evidence.
Bet hedging is characterized through individuals that minimize reproductive success of variance between years. Macrotera portalis practices bet hedging because only a certain portion of bees emerge in any given year, with many remaining in diapause. Although this leads to certain years of lower reproductive success, in years of low resources some offspring will survive and thus the advantages of lower variance may outweigh the disadvantages of reduced average fitness in diverse environments. Since M. portalis lives in a desert environment, of which conditions are extremely unpredictable, the between year variation is the greatest than any other biome with very minimal rainfall annually, it is likely that bet hedging is displayed within the species. Overwintering of desert bees have limited adult activity and reproduction during a short period of time after the desert rainy season. Larvae are small, exposed to high temperatures, low humidity, buried in the soil, subject to predation, desiccation, and pathogens.
It is theorized that desert, oligolectic bees such as Macrotera portalis’ emergence is triggered by rainfall. Induced emergence, in other words rainfall-triggered emergence, is observed within M. portalis. Museum specimen analysis demonstrated that M. portalis had a synchronous pattern of emergence consistent with the southwestern deserts late summer rains. M. portalis was further not found to be active in the spring. In the laboratory, It was found that the sex ratio of pre-pupae was female biased slightly, although initially the sex ratio is male biased. The morph ratio was shown to bias small headed males, but large headed males were found in many of the nests. Thus, there is evidence of slight protandry and small inclination for emergence of large headed males before small headed males. For a comparison of emerging and diapausing pre-pupae, it was found that the pre-pupae that emerged within the excavation year were much lighter compared to those who remained for an additional year in diapause. The difference was significant between males and females. This further demonstrates that if the pre-pupae contains sufficient reserves of resources to overwinter for another year it is carried out, thus the lighter bee emerge the current year. The lighter bees were further found deeper in the soil than those in diapause. Thus, emergence is influenced by larval condition as well as triggered by rainfall.
Resource based engagement sites are usually used by oligolectic bees, of which includes Macrotera portalis. Oligolectic species prey specifically on a small host of flowers. This oligolectic behavior is seen more in females as females more predictably crowd around host flowers of the same species in comparison to polylectic bees. M. portalis was found to have territorial male behavior in a female emergence site in a nesting area and a resource based rendezvous site that contains territorial males. The purpose of male rendezvous behavior is to secure a mate, specifically referring to the small headed males as they mate outside of the nest. Since females are considered a limiting resource, and males generally do not invest in offspring, males would maximize access to receptive females through selection for the purposes of reproductive success. Thus, oligolectic bees like M. portalis is observed to have territorial behavior in certain rendezvous sites, including female emergence site and territorial male behavior.
Interactions with other speciesEdit
Parasites and predatorsEdit
One study found two types of parasites and one type of predator found in observed Macrotera portalis nests. Pseudomethoca species near scaevolella was seen in a nest. The cells with defecated pupae were parasitized whereas those who had not were left alone. A meloid beetle was also seen in a nest. The beetle burrows between cells and devours bee larvae. Another nest that had no adults but only larvae and pupae was found to have been attacked by Solenopsis molesta, a type of ant, which kill pupae. Pseudomethoca perditrix has been known to parasitize larvae of Macrotera portalis. Pseudomethoca toumeyi is another known parasite of M.portalis.
Currently, there is no information on the conservation status of this bee species.
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