Eastern carpenter bee

Description and identification

The bee is similar in size to bumblebees, but has a metallic, mostly black body with a slight purple tint. The X. virginica males and females have generally the same mass, but can be differentiated visually by the male's longer body and the female's wider head. The males also have a white spot on their face. Additionally, the males have larger thoracic volumes for given masses. Females of different social standing can also be told apart based on morphology. Primary females are larger than secondary or tertiary females, and additionally primary females have more mandibular and wing wear.

X. virginica have unique maxillae that are designed to perform perforations on corolla tubes to reach nectaries. Their maxillae are "sharp and wedge-shaped", allowing them to split the side of corolla tubes externally to access the nectar. The Eastern carpenter bees also have galae on their maxillae that are shaped like large, flat blades. Research has shown that individuals with sharp galae can use these to further aid in penetrating the corolla tubes.

Taxonomy and phylogeny

X. virginica belongs to the genus Xylocopa, which consists of over 400 species worldwide, in the subgenus Xylocopoides, which contains only 5 New World species, including Xylocopa californica, which also occurs in the U.S.

Distribution and habitat

The distribution of the genus Xylocopa stretches over most continents. Members of this genus are usually found in tropical and subtropical climates, while X. virginica is found only in the eastern portion of North America.

Nesting

X. virginica build their nests in wood, bamboo culms, agave stalks, and other comparable materials, but they prefer to nest in milled pine or cedar lumber. The nests are built by scraping wood shavings off of the wall. These shavings are then used to create partitions between nesting cells. The entrance cuts into the wood perpendicular to the grain, but they are built parallel beyond the entrance. These nests may be either social, containing groups of two to five females, or solitary. Social nests are more common, despite the fact that brood productivity is actually lower when females choose to nest together. Because X. virginica builds its nests in wood structures, it is common for them to nest in constructed furniture or buildings. X. virginica is the most common large carpenter bee in eastern North America, and they nest in small groups, so their nests are fairly commonly encountered.

The nests are usually round and have an average of 1-4 tunnels. They have multiple branches with each adult female living and laying eggs in a separate branch, but females share one common entrance. Because the nests are extremely costly to build, it is common for females to try to reuse old nests.

Colony cycle

In X. virginica, mating occurs only once a year, in the spring. Eggs are laid in July, and by about August and mid-September, all the pupae have become an adult. The juveniles begin the next mating cycle the following Spring, so one generation develops in a year. Females begin to exhibit the signs of old age around July. The indicative behavior includes resting in flowers, remaining in the nest, or even just falling to the ground from flight. The older individuals also crawl, avoid taking flight, and do not struggle when handled by humans. The old bees die by early August, the same time that juveniles emerge from brood cells. Due to the simultaneous nature of expiration of old bees and emergence of new ones, there is little overlap between generations.

Behavior

Carpenter bees are not solitary bees, but are not truly social either. The weak form of sociality they exhibit, with one female doing the majority of the work, and caring for her sisters, may be a transitional step in the evolution of sociality.

Dominance hierarchy

Female X. virginica can have solitary nests, but they usually nest in social groups. The social order of X. virginica is broken into three groups: primary, secondary, and tertiary. Primary females act as the dominant within a nest and are in charge of reproduction, providing food for the larvae, and laying all the eggs. This is different from many bee species in which there is a queen that focuses her energy solely on laying eggs while relying on provisions provide by subordinate bees. Secondary females may sometimes participate in oviposition, and they reinforce this potential role by helping provide for the larvae or performing nest maintenance. Tertiary females rely on the provisions provided by primary females and quietly await overwintering while remaining inactive.

Studies have shown that primary females are usually the bees that have overwintered twice, while tertiary bees have only overwintered once. Tertiary bees will most likely survive a second overwintering and develop further to become primary female the following year. Secondary bees may survive a second winter, but that is unlikely if they actively forage after their first overwintering.

Division of labor among the sexes

Not all females do the same work in a social nest. This is evident based on the varying levels of wear on the wings and mandibles of females of various social standing. Although many nests have more than one female, there is a division of labor between the older and younger females. During nesting time,only the older females are responsible for nesting duties such as digging, excavating the cells, lining the cells, collecting food, and ovipositing. Evidence of this activity can be found in their worn mandibles. Young females rarely leave the nest and guard the entrance while the older females work, resulting in unworn wings and mandibles in the younger females. Additionally, X. virginica is the only known species in which one-year-old females cohabit the nest with two-year-old females that do all the labor.

Males often have long period of hovering flying or fast pursuit of intruders while females flight activity is usually very directed, such as flights to flowers and food sites. Larger females have an advantage because they can carry larger amounts of pollen or nectar back to the nest and can fly longer distances.

Diet

X. virginica survive mostly on nectar and pollen. Newly emerged bees don’t have food stored in their nest, but they are occasionally brought nectar. As previously stated, X. virginica use their maxillae to penetrate the corolla of the plants and reach the nectar stores, known as nectar robbing behavior. This happens when the bee pierces the corolla of long tubed flowers, thus accessing nectar without making contact with the anthers - thus bypassing pollination. In some plants this reduces fruit production and seed number. In other plants, defensive mechanisms have allowed for pollination to occur despite corollary perforation.

Mating behavior

Each nest usually has one mated individual. Mating occurs in April and is often accompanied by a bobbing dance that involves about a dozen males and only a few females.

Males require female activity, specifically flight, in mating. Occasionally before mating, the couple will face each other and hover for a few minutes. When the male contacts the female, he mounts her back and attempts to push his abdomen under hers. Copulation occurs at this instant, and it is almost always followed by more mating attempts. If, during copulation, the female lands, the couple will disengage and the male will hover waiting for the female to take flight again; however, although the males almost always disengage and pause copulation when the female lands, there have been instances recorded in which the males will hold on to the female with all six legs and flap his wings in an attempt to lift her back into the air.

Larger males are usually more successful in mating. Because of their competitive advantage due to their size, males will likely claim a territory near female nest sites. Smaller males will stay at foraging sites or other areas they think females may pass so they can mate with reduced competition.

Kin selection

Research has shown that, regardless of sex, X. virginica show more aggression toward non-nestmates than nestmates, indicating that they can recognize each other. By living in social groups with inclusive fitness, offspring can be raised with the help of the nest community rather than in solitary.

The ability of X. virginica to recognize nestmates allows primaries and secondaries to exclude tertiary bees from their nests. Tertiary bees are a burden on resources because they don’t perform any useful activities, but they benefit from the food and shelter provided by the primary females.

Life cycle

Eggs are laid starting farthest from the exit hole. Researchers suggest that there is a mechanism that synchronizes the emergence time of young that are laid at different times by causing the younger eggs to develop faster. This mechanism prevents bees that would emerge sooner from removing their siblings and decreasing their potential competition.

Bees that have newly emerged have a soft cuticle and white wings. The wings later transition to brown, then to a bluish black. The larvae learn to fly only 3–4 days after emergence, but they remain in their nest for at least two weeks. These larvae eat honey but not pollen, and will develop within one month. Between the larvae and adult stage, there is a juvenile stage in which the bees remain in their nest and are usually found among siblings.

Territorial behavior in males

Males will establish territories near an active nest entrance to protect the colony and seek mating opportunities. For males that are near the nest entrance, their boundaries are usually linear and several meters long. For males that are farther from the exit, their boundaries are usually in the shape of a square and shorter in length. Males can stay in one territory for as long as two weeks. Although they do most of their foraging and resting during the night, they take small breaks throughout the day as well. After these breaks, they often have to fight off intruders that have taken advantage of their absence.

Flight near the nest are usually uniform and involve a lot of hovering. Flights protecting their territory could be as short as a few minutes, but may extend beyond an hour. Studies have shown a fascinating pattern in territory protection. Males won’t notice another individual unless they are flying at high speeds. When other individuals hover near the nest, it is highly unlikely that the male will pursue, whereas, if another male comes into a territory at a high speed, the territorial male will chase after him.

One study in particular gives a great illustration of this characteristic of X. virginica. In several experiments, males patrolling the entrance of a nest were confronted with either dead or living Eastern carpenter bees. The bee was suspended from a thread and dangled within the male's territory. Whether the bee was dead or alive, the male had no response when the bee was suspended and motionless. This is particularly interesting because we know that X. virginica are capable of recognizing other individuals of their species. However, when the suspended bee was released and allowed to fly in the male's territory or was swung through the territory on the thread, the territorial male pursued it.

Parasites

There is one common species of bombyliid flies known to parasitize the larvae of Xylocopa virginica: Xenox tigrinus.

Marking

Eastern carpenter bees have glands within the mandibular glands that are known to produce a marking chemical in X. hirsutissima that function as nest markers or female attraction. The glands are present in both males and females, but they produce no marking substance. However, X. virginica does have a Dufour's gland that is used to deposit a scent on a flower immediately following nectar collection. The scent, composed of hydrocarbons and esters, discourages X. virginica, as well as other bee species, from returning to that same flower.

Stinging

The male bee is unable to sting, though they will commonly approach human beings, especially if they wave or move parts of their body. The female, on the other hand, will sting if provoked While the pain level of these stings is not well-documented, researchers have testified that X. virginica will sting if handled.

Agriculture

X. virginica visits many different kinds of flowers in order to gather pollen and nectar to bring back to the nest for larvae. Most of the plants they visit are wild grown or grown for decorative value. Their active seasons are quite long, and the forage on a wide variety of plant species. Also, because the start of their activity season is dependent on temperature, it is very easy for greenhouse workers to manipulate the beginning of foraging activity. However, in comparison to species such as the honey bee, their smaller nest makes them less powerful as pollinators.

Destructive behavior

Because X. virginica builds its nests in various types of wood, it presents the disadvantage of weakening wood in manmade structures. They are also able to produce an excrement upon exiting their tunnels that may splash on the sides of buildings and negatively affect the aesthetic appeal of that structure. However, when weighed against the benefits X. virginica has as pollinators, the costs of its destructive behavior are insignificant. X. virginica offer benefits in the form of pollination for fruits, vegetables, legumes, and flower crops. Although the pollination strengths of X. virginica are secondary to that of the bumble and honey bees, the contribution is great enough to overlook destructive tendencies. Research has also shown that X. virginica avoid entrances that are stained white, which is a possible solution to keeping them out of unwanted areas.