Threats
Native bees are facing a number of serious threats contributing to the decline of many species. In many cases, it is a combination of interacting factors, rather than a single factor, that poses threats to bees. In Minnesota, approximately 37% or 190 species have a NatureServe S-ranking between S1 critically imperiled (at very high risk of extirpation in the state) and S3 vulnerable (at moderate risk of extirpation from the state). An additional two species are ranked SX presumed extirpated and a third is ranked SH possibly extirpated; all three of these latter species are bumble bees (Bombus). For the majority of the remaining 319 species, 266 have a SU unrankable ranking, a status assigned when inadequate information exists to confidently assign a ranking to a species. The SNA not applicable ranking applies to non-native species.
NatureServe State Conservation Status Definitions
2
1
18
45
127
134
15
266
14
Number of Bee Species in Minnesota Occurring in Each Status Rank
Habitat Loss, Fragmentation, and Degradation
Bees face a number of challenges in the anthropocentric (human-centered) landscape; the primary factor impacting their survival is the continued loss, fragmentation, and degradation of habitat including diverse native landscapes. Large natural landscapes with a diversity of native plants flowering throughout the growing season and undisturbed places to nest provide ideal habitat for bees. However, these sites can become degraded from the lack of critical disturbances like fire or hydrological changes. The conversion of natural landscapes, whether on a small or large scale, can impact both common and rare bee species when there is a reduction in forage and nesting opportunities. Compounding the impact is the spread of non-native invasive plants and nutrient loading that can directly alter plant composition and favor plants that compete with native plant species. Expansion of urban development and conversion of land for conventional agriculture use or mining can eliminate bee habitat in large-scale landscapes.
The ongoing fragmentation caused by these land uses creates isolated, geographically distant habitats resulting in an increase in inbreeding in bee populations unable to move between habitats. This can lead to a reduced fitness in a population through the expression of harmful recessive genes, an increase in the production of sterile diploid male bees, or an imbalance in the sex ratio of offspring produced.
Private landowners are often responsible for small-scale changes: expanding the footprint of a lawn, removing habitat or nesting opportunities such as standing trees or logs on the ground, or changing the vegetation cover to favor plants that don't support bees. Moreover, introducing or continuing to cultivate invasive plants that outcompete native plants leads to the overall simplification of the number of flowering plants, are situations that make a landscape inhospitable for bees.
Invasive plants such as Rhamnus cathartica (European buckthorn) degrade natural areas resulting in a decline in the diversity of flowering plants that bees depend on.
Pesticides
The word pesticide is an umbrella term for all types of 'cides' or chemicals designed to kill organisms such as plants (herbicides), fungi (fungicides), mites (miticides), or insects (insecticides). Pesticides are one of the leading causes of bee decline. Bees are impacted by insecticides when they come into direct contact with the chemical or with plants treated systemically. Systemically-treated plants uptake the chemical and express it in leaf tissue as well as in pollen and nectar. One prominent group of systemic insecticides, neonicotinoids, are chemicals that target the nerve impulses of insects. They have been under review for their documented impacts on bees. Exposure by bees to these insecticides can be either lethal or sublethal, impacting a bee's behavior such as its ability to forage, fly, or return to its nest. Also, dust from neonicotinoid-treated agricultural seed including corn and soybean seed, can become airborne and pose a risk to bee populations foraging or nesting nearby. Neonicotinoids are now found in surface waters and at levels that can harm aquatic insects. Other organisms such as birds and mammals can also be harmed if they ingest treated seed.
Agriculture is not the only source of insecticides; many of them are available for homeowner use and contain neonicotinoids or other active ingredients toxic to bees. These insecticides are often applied at higher rates than in an agricultural setting. Anyone applying pesticides must follow the instructions on the label. The label is the law. Even insecticides such as pyrethrins, derived from naturally-occurring sources (plants), are highly toxic to bees. Similarly, pyrethroids, synthetic insecticides that are frequently used by commercial companies to spray homeowner yards for mosquitoes, are also highly toxic to all insects, including bees and beneficial insects.
Photo: Shutterstock ID 1493338982 Mabeline72
Although often considered to be a safer alternative to synthetic pesticides, many pesticides used in organic agriculture(biopesticides) may also have detrimental impacts on pollinators by reducing survival and reproductive capacity, and altering behavior. If the intent is to provide habitat and forage for bees, always ensure that the landscape is pesticide-free and safe for bees.
Pathogen Spillover From Commercial or Managed Hives
When bees are domesticated or bred for commercial hives or colonies, housing many bees together in one supplemental nest can have consequences for pathogen transmission within nests and to wild populations visiting shared floral resources. Numerous studies investigating pathogen spillover from Apis mellifera (European honey bees) have been published since 1960, with a significant increase in publications occurring after 2020 as molecular tools used to detect pathogens improved. Deformed wing virus (DWV), a pathogen that causes deformed wings in infected hosts, is the most common pathogen cited in spillover studies.
Many eusocial and solitary bees sold for pollination have been moved outside of their native range. In some cases, they are shipped all over the country and across international borders. Even when the bee species is native to a given region, importing bees raises concerns about the introduced species having non-local genotypes. Trade and transport of species also raises the risk of introducing new pests, pathogens, or diseases to new areas. This applies to both managed eusocial bees—Apis mellifera (honey bees) and Bombus (bumble bees)—as well as solitary species such as Osmia (orchard mason bees). Colla et al. (2006) found wild bumble bees near commerical colonies in greenhouses were more frequently infected with the gut pathogens Crithidia bombi and Nosema bombi. Although limited evidence exists, scientists suspect the spread of Nosema bombi from commercial to wild populations as a reason for the precipitous decline of four bumble bee species including the federally endangered Bombus affinis (rusty patched bumble bee). Commercial rearing of bees can include selective breeding for desirable traits, traits that benefit colonies managed for crop pollination but not for colony fitness in the local environment. If managed bees escape and mate with wild populations, they can pass on undesirable genes, ones that leave wild bee offspring with a limited ability to adapt.
A Bombus impatiens (common eastern bumble bee) worker returns from a foraging trip to a commercial colony housed in a cardboard box.
Resource Competition From Alien or Non-Native Bee Species
There are fourteen non-native bee species that occur in Minnesota, and approximately thirty in eastern North America. Most of these alien species were introduced accidentally, unknowingly transported in shipping containers or other cavities in wood, for example Anthidium manicatum (European wool carder bee) and Megachile sculpturalis (giant resin bee). A minority were purposefully introduced for the resources they produced, for example, the honey and wax produced by Apis mellifera (European or Western honey bee). Other non-native species, Megachile rotundata (alfalfa leafcutter bee) for instance, were purposefully introduced for crop pollination. Until recently, little attention has been paid to many of these non-native species to determine how these bees may be interacting or competing with native species. One reason for their inattention is any bee providing pollination services or an ecosystem service that benefits humans is perceived as beneficial. However, non-native species are cause for concern. They have the potential to compete with native bees for nesting sites and floral resources, provide pollination services to non-native or invasive plants, disrupt the pollination of native plants, and spread diseases such as deformed wing virus to native bee populations.
Since its introduction to eastern North America in the early 1990s, Megachile sculpturalis (giant resin bee) has been observed aggressively taking over nests of Xylocopa virginica (large carpenter bee). Giant resin bee females attack the nests, fighting then stinging the occupant(s). During a skirmish, resin gets transferred onto the victim. Large carpenter bees are often debilitated and unable to fly after being stung by the attacker and/or coated in resin. Territoriality in alien species can also drive off native bees from a patch of flowers. Anthidium manicatum (European wool carder bee) males chase away bees and other insects entering their territories, established in patches of flowers where Anthidium females forage. Anthidium males have spines on the end of their abdomen and, by curving their abdomen forward, they can tear the wings of the victim, rendering them flightless or sometimes even killing them.
Besides nest usurpation or territoriality in alien bees, another concern is the competition for floral resources, especially when most native bees are limited by the distance they can fly and often have an inadequate amount of habitat and available forage. The introduced Apis mellifera (European honey bee) can fly longer distances than solitary bees and also communicate with nest mates where good flower resources are located. Cane and Tependino (2016) calculated that a typical hive of honey bees collects the equivalent amount of provisions that 100,000 solitary bees would collect in three months, from June until August. The authors also noted particular bees vulnerable to competition from honey bees for resources: specialists dependent upon a narrow range of plants for pollen collection and unable to change forage plants, and bumble bee gynes and social sweat bees needing an adequate amount of forage to build fat reserves quickly before hibernating for the winter.
An Anthidium manicatum (European wool carder bee) male perches on a leaf guarding his established territory from intruders. Using the spines on the end of his abdomen, he combatively drives off other bees foraging in his territory.
Climate Change
Changes in temperature, hydrological cycles, phenology and distribution of plants, and weather patterns are some factors likely to impact bee populations. With rising temperatures and the earlier onset of spring, there are already documented changes in the phenologies of plants with most flowering earlier such as spring-flowering species that are temperature-sensitive, and a minority later than historical averages. The changes in the onset of flowering have consequences for bees, especially pollen specialist bees reliant upon a narrow range of plants. These changes may also impact plants dependent upon a few types of bees for pollination. The distributional changes of plants and bees is another potential mismatch or decoupling of critical bee-plant mutualisms. The decrease in suitable habitat with climate change and changing weather patterns will lead to a reduction in the distribution of bee species.
Direct mortality of bee populations from extreme weather events such as fires, floods, and droughts is also possible. In addition, prolonged periods of drought can negatively impact the quality and quantity of floral resources produced by plants. Also, a stressed plant may flower for a shorter period of time, limiting the amount of time that the floral resources are available to bees.
Bees are sensitve to temperature changes. With the trend to more record-breaking days of extreme heat, bumble bee nests can become overheated, reducing the reproductive success of the nest through the loss of larvae. Kevan et al (2024) noted that larval development and nest activity slow with increasing temperatures. For northern latitudes (above 50° north) where temperatures are rapidly increasing due to climate change, results of research by Hemberger and Williams (2024) suggest that bumble bees are unable to respond and adapt to the temperature changes.
Bombus terricola (yellow-banded bumble bee), is a bumble bee species in decline. Its range includes northern Minnesota, northeastern United States, and Canada. As temperatures warm rapidly, particularly in latitudes above 50° north, research demonstrates that northern species such as the yellow-banded bumble bee may not be able to adapt to the rising temperatures.
Explore Bee Families
Citations and Further Reading
Cane, J. H., & Tepedino, V. J. (2017). Gauging the effect of honey bee pollen collection on native bee communities. Conservation Letters, 10(2), 205-210.​
Colla, S.R., Otterstatter, M.C., Gegear, R., & Thomson, J.D. (2006). Plight of the bumble bee: Pathogen spillover from commercial to wild populations. Biological Conservation, 129, 461–467.
Evans, E. (2017). From humble bee to greenhouse pollination workhorse: Can we mitigate risks for bumble bees? Bee World, 94(2), 34-41.
Hemberger, J., & Williams, N. M. (2024). Warming summer temperatures are rapidly restructuring North American bumble bee communities. Ecology Letters, 27(8), e14492.
Kevan, P. G., Rasmont, P., & Martinet, B. (2024). Thermodynamics, thermal performance and climate change: temperature regimes for bumblebee (Bombus spp.) colonies as examples of superorganisms. Frontiers in Bee Science, 2, 1351616.
Portman, Z. M., Gardner, J., Lane, I. G., Gerjets, N., Petersen, J. D., Ascher, J. S., ... & Cariveau, D. P. (2023). A checklist of the bees (Hymenoptera: Apoidea) of Minnesota. Zootaxa, 5304(1), 1-95.
Wilson, J. S., & Messinger Carril, O. J. (2016). The bees in your backyard: a guide to North America's bees. Princeton University Press.