Human activity makes the biological world less and less diverse. Global transport has enabled many plants and insects to expand their range and colonize new, hitherto unknown areas. Some of these "alien" organisms become invasive species. Especially when they are allowed into a paradise free of pathogens, parasites and predators, which allows them to create mass populations and disrupt ecological interactions among native local flora and fauna. The impact of non-native organisms, both plants and insects, on native bees varies from insignificant or at least undetectable to dramatic.

The main source of invasive plant species is the horticultural trade. Flora in various parts of the world is therefore dominated by alien species. Many imported plants produce showy and abundant inflorescences that are attractive to native bees.

“Foreign” plants can multiply floral resources by providing more pollen or nectar than other plant species, or by filling a hole in a hole. Such a shift in the amount and timing of the availability of floral resources may have a positive or negative impact on native bees. The wide spread of Impatiens glandulifera has resulted in a marked reduction in bee visits to native plants, such as: Stachys palustris

Fig.1 Glandular Impatiens in its native range in the Valley of Flowers National Park in India . Raghuram A. Wikimedia Commons.

Noteworthy are the examples of leafless tamarisk ( Tamarix aphylla ) and branched tamarisk ( Tamarix ramosissima ), whose native range includes some countries in Africa (Algeria, Egypt, Libya, Tunisia, Morocco, Kenya, Mauritania, Senegal) and Asia (Saudi Arabia, Yemen, Sinai Peninsula, Israel, Iran, Afghanistan, India). These plants inhabit riparian forests and are considered one of the most invasive species in western North America. Abundantly nectar-bearing tamarisk inflorescence fills the nectar gap between early spring and late summer flowering of native plants.

One of the negative effects of tamarisk infestation is allowing social bees to maintain colonies in places where they would normally not occur due to lack of food. Higher numbers of social bees increase the likelihood of competition with native solitary bees.

Non-native plants can have a major impact on solitary bee numbers if they encroach on undisturbed habitats and displace native plants. The woolly cornflower ( Centaurea solstitialis) appeared in North America at the beginning of the 19th century (in Poland it is known, for example, from the vicinity of Pińczów - Świętokrzyskie Province). Woolly cornflower is avoided by most native solitary bees, but attracts social species such as honey bees and alfalfa bees. The woolly cornflower not only displaces native host plants by competing for open space, water and nutrients, but also benefits the alfalfa grasshopper, which occupies active nests and exploits abandoned breeding grounds of native bee species.

Fig. 2. Mesierka alfalfa ( Megachile rotundata ), a male on an old man's flower ( Jacobaea vulgaris ), Bytom-Rozbark, ruderal thickets at the Rozbark coal mine. photo. Adrian Tync Wikimedia Commons.

Finally, invasive plants, strongly affecting the frequency and intensity of environmental changes, can profoundly affect solitary bees. Deserts around the world have been drastically altered by exotic grasses. A good example is the grasses Pennisetum ciliare , which are adapted to xerothermic conditions (areas that receive a lot of sunlight) and have been spread to warm deserts around the world to increase cattle forage. In Australia and North America, buffalo grass increases the incidence of spontaneous fires that kill native vegetation. Not only do buffer grasses offer no floral resources for wild bees, but the fires they foster also eliminate nesting resources such as resins, wood, and hollow stumps used by solitary bees.

Invasive bees

Most of the introduced bees were moved to new areas by accident, while almost 1/4 were introduced deliberately by humans (Russo 2016). Honey bees ( Apis mellifera ) have been introduced worldwide wherever pollinator-dependent crops exist. Other species that have been intentionally introduced to pollinate crops include the alfalfa moth ( Megachile rotundata) ; Red mason bee ( Osmia cornuta ), "blue bee" ( Osmia ribifloris ) , Alfalfa husk ( Nomia melander) and several species of bumblebees ( Bombus) . Most accidentally introduced species are terrestrial altricial (e.g. species in the genera Hylaeus , Osmia , Megachile and Ceratina ) or wood altricial (e.g. Lithurgus , Xylocopa ), which have spread with the international trade in timber and plant materials. There is even one brood-parasitic bee ( Coelioxys coturnix ) that has extended its range to the New World with its putative host ( Megachile rotundata ). Non-native bees potentially affect native species in many ways: competition for floral and nesting resources is the most obvious, but non-native bees can also transmit pathogens to native species.

Competition for floral resources

The most thoroughly studied interaction between native and non-native bees is competition for floral resources. If non-native and native bees depend on the same plant species, then native bees should suffer negative effects when resources are scarce. The European honey bee ( Apis mellifera ) is the most widespread and abundant bee on Earth. Colonies consist of tens of thousands of worker bees, and honey bees are highly polylectical, visiting virtually every flowering plant within their foraging range. They also consume huge amounts of pollen and nectar. It is estimated that over a period of three months, a single honeybee colony collects more than 650,000 pollen loads (10 kilograms of pollen) during the peak summer months (Winston 1987, Seeley 1995). Cane and Tepedino (2017) estimated the potential impact of honeybees on the average sized solitary bee in terms of lost reproductive potential. They estimated that a single honeybee colony consumes pollen equivalent to about 110,000 brood of solitary bees over a three-month period. A typical apiary (~40 bee colonies) uses enough pollen to feed about 4 million solitary bee offspring. This calculation is based on some simplifying assumptions (e.g. solitary bees and honeybees visit the exact same plants for pollen) but shows that the potential impact of honeybees on native solitary bees could be to drastically reduce fertility and population size

But do honey bees really have such a dramatic impact on solitary bee populations?

The literature is surprisingly ambiguous. Paini (2004) reviewed 28 studies on the effects of honey bees on native bees. These studies were conducted in many different locations, including Europe (where A. mellifera is native), but also Central and South America, Australia, New Zealand, India and Japan (where A. mellifera is not native). Most research has focused on what Paini called indirect influences, such as overlapping flower stocks or changes in flower visitation. Fewer studies have focused on measuring direct effects such as reductions in survival, fertility and population density. Indirect impacts are consistent with the idea of ​​competition between honeybees and wild pollinators, but resource overlap need not lead to reduced fertility or population size of native bees if they are able to change their foraging behavior in the presence of honeybees. In other words, indirect effects do not constitute conclusive proof that honey bees suppress native bee populations.

Fig. 3 A bee colony inhabiting a hive in Warsaw's Bielany. photo. Kamil Baj Beekeeping.

Most studies have found honey bees to have a negative impact on native solitary bees, but as Paini pointed out, many of these studies lacked enough replication to be convincing. Most studies, for example, were conducted in one or two locations or over a period of one year. Some studies also showed no effect. Roubik and Wolda (2001) monitored bee abundance in light traps on Barro Colorado Island, Panama, for 7 years before and 10 years after the arrival of the honey bees. They found no decline in the 15 most common native bee species after the emergence of honeybees. Paini et al. (2005) conducted a study that compared the fecundity of native Megachile sp. 323 before and after establishing apiaries in the Western Australian region. They found no significant changes in reproduction and fertility after the arrival of the honey bee colony.

Why is evidence of competition between native bees and honeybees so elusive, despite the apparent potential for competition for floral resources? One possibility is that native bees may change their foraging pattern in the presence of honey bees to avoid competition. Another possibility is that honeybees, because they communicate within the colony about the location of floral resources, can quickly switch to the most satisfying floral resources, even if those resources are far from the nest. This ability may allow honeybees to move quickly to the most nectar-producing floral resources (O'Neal and Waller 1984, Visscher and Seeley 1982) and thus leave some food for solitary bees.

Competition for nesting sites

While competition for floral resources has been the subject of much research, much less is known about competition for nest sites. Non-native altricials have the potential to compete with native species if their nesting site preferences are similar. Osmia cornifrons and Osmia taurus are two hollow-nesting bees that use exactly the same nest diameter as the eastern North American native Osmia lignaria . Competition for nesting sites is entirely possible, but we are not aware of any studies that have focused on the effects of this kind of resource overlap on native and non-native nesting bees in hollows. However, there is one case that seems to confirm this assumption. The rapid spread of Megachile sculpturalis across the eastern United States and Europe, and reports of usurpation of nests, suggest that this bee may significantly affect native bee populations over a vast geographic area. The long-term impact of Megachile sculpturalis on native bees could prove significant. The natural area of ​​occurrence of this mercury is eastern Asia. It was introduced by man to North America in 1994. In Europe, it was first observed in 2008. M. sculpturalis competes for nesting places, e.g. with zadrzechnia and masonry. It occupies breeding places in popular insect houses, therefore, in areas where it occurs as an alien species, it is not recommended to use nesting materials (reeds, bamboo, drilled wood) with a diameter of more than 1 cm in these areas.

Fig. 4 Large Meatball, Phot. John Baker Wikimedia Commons.

There are many unknowns regarding the impact of alien bees on native fauna. The lack of information on non-native species in natural and semi-natural habitats suggests that most non-native bee species remain closely associated with human-modified ecosystems and are naturalized rather than invasive. Our knowledge of the effects of introduced plants and bees on native bees comes from studies on a very small number of species. Demonstrating the clear impact of the widespread, resource-rich honey bee on native bees has been surprisingly difficult. Carefully designed, long-term experiments and community-level research would greatly improve our understanding of the effects of introduced plants and bees on native bees.

The spread of pathogens

One of the greatest threats posed by non-native bee species to native bee communities is the introduction of non-native pathogens - a phenomenon called pathogen transmission (Stout and Morales 2009). European honey bees acquired some of the most devastating pathogens and ectoparasites through contact with other closely related Apis species . Both the parasitic mite Varroa destructor , which is the main source of winter mortality in European honeybees, and the microsporidia Nosema ceranae were transmitted through contact with A. cerana , when A. mellifera was introduced to Asia (Van Engelsdorp and Meixner 2010). V. destructor now has a cosmopolitan range in both managed and feral colonies around the world (until recently Australia was the exception).

Fig. 5 Varroa destructor mite on the head of a honey bee pupa, Phot. Gilles San Martin , Wikimedia Commons.

European bee colonies are loaded with pathogens including fungi, bacteria, microsporidia, trypanosomes and viruses. In addition, there is increasing evidence that honeybees are a source of pathogens for wild bee species. Two studies (Fürst et al. 2014, McMahon et al. 2015) have shown that honeybee colonies share pathogens with nearby wild bumblebee colonies. Pathogen levels in honeybee colonies were positively correlated with pathogen levels in nearby bumblebee colonies, and analysis of pathogen sequence data suggests that honeybees are a source of pathogen infection for wild bumblebees. Similar implications apply to the impact of farmed bumblebees on their wild counterparts (Graystock et al. 2013).

The exchange of pathogens between honeybees and bumblebees is not surprising for researchers - both families are closely related apids and also belong to eusocial insects. However, are less related solitary bees also susceptible to infection with honeybee pathogens? We have very limited knowledge of the pathogens that infect solitary bees, but some of the viruses and fungi described in honeybees have also been detected in solitary bees (Singh et al. 2010, Evison et al. 2012). Ravoet and colleagues (2014) conducted a thorough study of pathogens infecting solitary bees ( Andrena ) and stump-nesting ( Osmia, Heriades ) in the vicinity of honeybee colonies in the Netherlands. Solitary bees were infected with the same Apicystis and Nosema species as honey bee colonies. This is the first study to document such extensive co-infection between honeybees and solitary bees. It is possible that these viruses have been circulating in wild and farmed bee populations for a long time.

How do pathogens spread? Growing evidence suggests that transmission of pathogens between farmed and wild bee species is via flower visits (Graystock et al. 2015). Flowers are the hubs where many species of pollinators interact. Many bee pathogens are transmitted through oral or fecal contact, and both pollen and nectar have been shown to harbor bee pathogens (Graystock et al. 2015).

We know very little about the symptoms and consequences of pathogen infection in wild bee pollinators. Therefore, it is difficult to state unequivocally whether the documented cases of pathogen infections really have an impact on bee longevity and reproductive success. It is still not known how strong the impact of pathogens on bees is. The assessment of risks and opportunities facing native bee species requires more specialized research, the results of which will shed new light on the existing interspecies relationships of bees.


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