Immigrant bees that colonized North America

While humans were busy squabbling over the border between the United States and Mexico, a tiny black immigrant bee was discreetly homesteading in California. A new sighting of a Central American native bee, a member of the genus Plebeia, was recently reported in a genteel area of Palo Alto, some 500 miles to the north of Mexico.

Apparently, the manager of the Elizabeth Gamble Garden, an iconic public park, contacted a company for help in removing a bee nest from the premises. On seeing the nest, however, the exterminator sent a specimen to an entomologist who recognized the bee as Plebeia. Plebeia is one of many genera belonging to the tribe Meliponini, commonly known as the stingless honey bees.

Until this sighting, only one stingless bee colony was known to exist north of the Mexican border, a nest that was first discovered in a Palo Alto backyard in 2013 and was being monitored by the State of California. Plebeia is a small genus of heat-loving bees native to southern Mexico and Central America that ranges as far south as Argentina. Since the first sighting in California, at least three other photos of Plebeia have shown up on the citizen science site iNaturalist.org, all within a short distance of the original nest. These recent sightings are most likely descendants of the 2013 colony.

No one knows where they crossed the border or how they got so far north. Someone could have smuggled them in, or perhaps they hitched a ride in a shipment of goods. It is also possible, though highly unlikely, they traveled on their own over the course of many years. In any case, higher than average annual temperatures no doubt played a role in their survival.

From here and beyond

Here in North America, we tend to pigeonhole bees into two classes, honey bees and native bees. But that division is not accurate due to the many other non-native species that also live here. Lists have been assembled by various organizations, and depending on where you look and how you count, it is fairly easy to find the names of 45 to 70 bee species that have arrived on our shores since colonial times.

Based on the mathematics of reproduction, I suspect many more species have established populations we haven’t yet discovered. Like an invasive plant, a newly introduced bee species may increase by only a few individuals per year. Many years may pass before the population is large enough to spread. Often, by the time it is recognized as something unusual, the organism may have colonized a large area.

Terminology of introductions

Species that move into new areas are known as adventive, but the word has several shades of meaning. Some scientists include deliberately introduced species, but others include only those that arrived on their own or by accident.

Some writers use adventive to describe species that are not self-sustaining, but need an occasional population boost from their homeland. If an adventive species becomes self-sustaining in its new geographic area, it is then said to be naturalized. Other words with variable meanings such as acclimatized, immigrant, and invasive make the subject even more confusing.

Stingless bees are not alone

Whether the Palo Alto Plebeia bees will naturalize is unknown, but many bee species that arrive in North America are happy to call it home. The extreme example is the European honey bee, deliberately introduced into Jamestown, Virginia in the 1620s. Although it spread easily, the original population was supplemented frequently from its homeland until the Honey Bee Act of 1922 put a halt to additional imports in an effort to control the spread of diseases and parasites.

After that, little was added to the honey bee gene pool in the Americas until the accidental release of a small number of Apis mellifera scutellata occurred in Brazil in 1958. Those rogues immediately mated with the South American stock of Apis mellifera, giving rise to so-called Africanized honey bees. Although frequently called “scuts,” these bees are not pure A. m. scutellata, but a cross. Ironically, the cross appears to be more scrappy than the real thing.

The Africanized honey bee continues to spread farther and farther north aided by warmer temperatures, but also by shipping of queens and packages which likely contain genes from Africanized stock.

More recent interlopers

Another adventive bee from the south has settled in southern Florida and is a popular subject with photographers. Euglossa dilemma is a member of the orchid bee tribe, Euglossini. These bees, native to Central and South America, are a show-stopping iridescent green with tongues two-thirds as long as their bodies.

While the females have pollen baskets on their rear tibiae much like honey bees, the males have enlarged hollow tibiae with small openings in the side. Like bota bottles for bees, these containers are used to store plant oils that will later be deployed to attract females. Commonly known as the green orchid bee, Euglossa dilemma was first spotted in Broward County in 2003 by USDA scientists who speculate a nest was accidentally imported in a wooden object such as a crate or pallet.

Florida has also been colonized by Centris nitida, a large stocky bee known as the shining oil-digger. Unlike most Centris bees in North America, this one nests in dead wood, which is probably how it got to Florida. Although not considered a pest, it can be alarming because of its sheer size and speed.

A bull by the horns

USDA scientists have been sharply criticized for deliberately importing a species of mason bee, Osmia cornifrons, back in 1958. Although these bees look different than our domestic mason bees due to a pair of horns on the face, they pollinate basically the same crops as our own Osmia lignaria. So why import them and displace our natives?

But that’s only half the story. During the introduction phase, our intrepid USDA members failed to notice they were handling not one but two species of Osmia. While releasing Osmia cornifrons (the horn-faced bee) into North America, they were simultaneously but unknowingly introducing Osmia taurus (the taurus mason bee). Oops.

It turns out that both species have facial horns, both pollinate the same plants, and both are spreading rapidly. Ironically, the taurus mason bee — the accidental one — seems to be naturalizing faster and spreading farther than the deliberate introduction. Oops times two.

Digging deeper

While I’m tattling on the USDA, I should mention another intentional introduction, Anthophora plumipes, the hairy-footed flower bee. This species from Europe and southern China was introduced into the United States at the USDA honey bee lab in 1980. Although it spread slowly at first, melittologists now speculate this digger bee could spread throughout most of North America

An almost identical bee, Anthophora villosula, the Asian shaggy digger bee, was also introduced at Beltsville, although more recently. Both of these Anthophora species are comfortable in urban areas where they live near homes and gardens and are particularly fond of azaleas.

A ticket to North America

Although a substantial number of bee species have been deliberately introduced to pollinate particular crops, many of our non-native bees arrived accidentally. An early introduction, one of the few ground-nesting transplants, is Andrena wilkella, known as Wilke’s mining bee. This bee arrived in the early 1900s, probably by ship, when soil was sometimes used as ballast. This bee is well-established in northeastern and northcentral areas of the United States and southeastern Canada.

When you look at a list of introductions, you can see that ground-nesting bees don’t transport nearly as easily as those that nest in cavities. Items of commerce such as bamboo, wooden furniture, pallets, shipping crates, and ornamental plants have all contained stowaways.

The family Megachilidae comprises nearly all cavity nesting bees and represents the majority of introductions. On the other hand, the Andrenidae, nearly all of which are ground dwellers, rarely arrive in commerce. The Colletidae, Halictidae, and Apidae families are represented by both types of bees and have produced an intermediate number of introductions. Migration definitely runs in the family.

Foreign bees and the environment

How do all these foreign bees affect us? Although their interactions with the environment are not well studied, most entomologist agree that their impact can be enormous.

On the positive side, many of the deliberately introduced species do a great job of working the crops they were imported to pollinate. The alfalfa leafcutting bee, for example, is an excellent pollinator of imported alfalfa. The bee was introduced because honey bees are known to be poor pollinators of alfalfa. Simply put, honey bees don’t like getting bopped on the head by the trip mechanism in the alfalfa flower.

 After several irritating encounters, honey bees learn to avoid the hit by approaching the backside of the flower and reaching between the petals to find the sweets. This move, known as nectar robbing, avoids the stamens and results in very little pollination. The tiny alfalfa leafcutter, however, seems unperturbed by aggressive flowers. A few bumps is a small price to pay for all that pollen.

Other crop-specific introductions, including the Osmia bees mentioned above, do an excellent job with fruit trees and berries. Many of the introduced species are polylectic, meaning they pollinate many different crops, so are thought to be an overall boon to agriculture.

The fallout from influx

But all the news is not good. The most widely recognized downside from foreign bees is competition with native bees for floral resources. In second place is their potential ability to spread pathogens and parasites to native bees that have no natural immunity to these novel afflictions. In addition, some of the introduced bees specialize on what we consider invasive weeds, resulting in increased seed set and accelerated distribution of unwanted plants. Other worries include possible hybridization with native bee species that could cause a shift in pollination preferences, and possible shifts in well-established pollinator networks.

A good example of an unfortunate introduction is Lithurgus chrysurus. This bee, native to regions of Europe, the Near East, and North Africa, was discovered in 1970 in New Jersey. This bee has two irritating habits: It specializes on invasive spotted knapweed and, like the carpenter bees, tunnels into wooden structures to build its nest.

Massive structural damage has been reported to barns, garages, wooden bridges, and outdoor furniture. It is also known to colonize stacked firewood, wooden shingles, sheds, and picnic tables. Some entomologists fear that it has the potential to be much more damaging to wooden structures then our native carpenter bees.

Some other adventive bees have earned bad reputations due to a lack of social grace. The sculptured resin bee is known to attack native carpenter bees, coat them with plant resin, and then steal their nests. Anthidium manicatum males are extremely territorial and can often be seen attacking—or even killing—native bees or managed species like honey bees. And others, such as Apis mellifera, just seem to take more than their share.

Summer in the city

Another characteristic common among introduced species is their ability to thrive in highly populous urban areas. Accidental introductions are likely to occur at international ports in busy industrial areas. Once they arrive, the bees that can survive in those environments do, and the rest die, which leaves a self-selected assortment of urban-adjusted bees.

Another possible reason for urban settling may relate to the vast number of introduced plants that occur in highly-populated areas. Compared to forest, prairie, or even suburbs, cities host ornamental plants and weeds from all over the world where they grow from balconies, rooftop gardens, cracks in the pavement, and desiccated ballfields. The plants are eclectic and so are their pollinators.

In addition, recall that most introduced bees are cavity dwellers rather than ground dwellers. As it happens, cities are famously short on open bare ground, but are teeming with cavities, such as holes in wooden buildings, cracks between bricks, loose mortar, disused equipment, and overfull storage buildings. These tiny crevices afford endless nesting opportunities for city dwelling bees while they dine in irrigated flower boxes and city-maintained trees.

Thine enemies

While some bees have arrived with baggage — their homeland diseases and parasites — some have come unencumbered. Like invasive organisms of any type, if an exotic bee can escape enemies such as predators, pathogens, competitors, or famines, it can flourish and spread rapidly. In addition, predators living in the new land may reject the newcomer as a food source, simply because it is unfamiliar and not part of the usual menu.

A bee of a different color

Imported bees — no matter how they got here — are often easy to identify among all the natives. They often appear just a little “off,” a bit different from the others, even those in the same genus. The introduced Osmia bees I mentioned have those horns, the Anthophora have long black hair on the middle legs, and some of the introduced Megachilidae have unusual shapes or coloring. The sculptured resin bee, for example, looks different from most American Megachile and so does the alfalfa leafcutter. And Apis mellifera have wing veins unlike anything else in the entire hemisphere.

The differences make sense from an evolutionary perspective. Even though these bees are closely related, thousands of years of physical separation have allowed their gene pools to shift in slightly different directions. In the end, they are the same … only different.

In recent years, interest in “alternative” bees has blossomed such that previously little-known species are taking center stage. All the interest means more people are looking and discovering. As both professional and amateur bee stalkers continue to comb fields, meadows, and city parks, other adventive species will surely be found.

Acknowledgements

Special thanks to Colin Purrington of Swarthmore, Pennsylvania for his photos of Osmia cornifrons and Osmia taurus. You can see more of his nature photos at https://colinpurrington.com/

Thanks also to selwynq of Palo Alto, California, Ty Sharrow of Lehighton, Pennsylvania, and Judy Gallagher of Woodbridge, Virginia for their photos of Plebeia, Lithurgus, and Euglossa respectively.

Notes

For a worldwide list of bee introductions see Russo L. 2016. Positive and negative impacts of non-native bee species around the world Insects, 7(4), 69. doi:10.3390/insects7040069.

Bees Introduced into North America

The following list, compiled from various sources, shows bees introduced into North America since the 1600s. It does not include bees introduced into new areas within the United States (such as from the mainland to Hawaii) nor does it include bees introduced from one territory to another.

• Family Apidae
  • Anthophora plumipes
  • Anthophora villosula
  • Apis mellifera
  • Centris nitida
  • Ceratina cobaltina
  • Ceratina dallatorreana
  • Ceratina dentipes
  • Ceratina smaragdula
  • Euglossa dilemma
  • Peponapis pruinosa
  • Plebeia frontalis
  • Triepeolus remigatus
  • Xenoglossa strenua
  • Xylocopa appendiculata
  • Xylocopa tabaniformis parkinsoniae
• Family Andrenidae
  • Andrena wilkella
• Family Colletidae
  • Hylaeus albonitens
  • Hylaeus hyalinatus
  • Hylaeus leptocephalus
  • Hylaeus (Prosopis) variegates
  • Hylaeus punctatus
  • Hylaeus strenuus
• Family Halictidae
  • Halictus tectus
  • Lasioglossum eleutherense
  • Lasioglossum leucozonium
  • Lasioglossum zonulum
• Family Megachilidae
  • Anthidium florentinium
  • Anthidium manicatum
  • Anthidium oblongatum
  • Chelostoma campanularum
  • Chelostoma rapunculi
  • Coelioxys coturnix
  • Heriades truncorum
  • Hoplitis anthocopoides
  • Lithurgus chrysurus
  • Megachile apicalis
  • Megachile concinna
  • Megachile ericetorum
  • Megachile lanata
  • Megachile rotundata
  • Megachile sculpturalis
  • Osmia caerulescens
  • Osmia cornifrons
  • Osmia cornuta
  • Osmia taurus
  • Pseudoanthidium nana