The population response by individual bee species is likely as varied as the potential causes. Potential threats include: climate change, land use change, pesticides, parasites, invasive species, and more. Multiple stressors can occur simultaneously and in synergy too. Much uncertainty remains about the relative importance of each stressor and their interactions. Essential monitoring data on the rates, geographic scope, ecological aspects, and taxonomic nature of wild bee population trends are scant.

Stressors to VT bee populations

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Climate Change

Increasing temperature and extreme weather are among the greatest long-term threats to biodiversity, with bees being no exception. The possible implications are nearly infinite, ranging from phenological mis-match to reduced overwintering survival from increased metabolism as a result of warmer winters. Some species, particularly generalists with long flight periods, are likely to thrive with longer growing seasons and less severe winters and additional southern species are likely to migrate into Vermont in coming decades. On the other hand, northern species and those closely tied to specific plants are likely to suffer.


Climate change sensitivity of bees in Vermont. Each bar represents an individual species which are grouped by genus (bar at top). The height of each colored bar represents the importance of different climate factors in their distributions in the state.

Climate change will likely alter where bee species are found within the state. The graphic below illustrates how bee diversity may change given anticipated climatic conditions. We used climate envelope models to predict future distributions of 253 bee species using several greenhouse gas concentrations. Below, species richness predictions for 30-year intervals are shown assuming a moderate reduction from current greenhouse gas emissions (Representative Concentration Pathway; RCP 7.0). Use the sliding window below on the interactive graphic to explore how climate change may impact bee diversity across the state.

Interactive map illustrating how Bee species richness may change through time given anticipated climate change (RCP 7.0)
Link to larger interactive map

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Land Use Change

For the first time in over 150 years, the forest cover of New England has recently declined, largely from urban growth. Since bees are often described as solar-powered insects, and flower abundance is generally greater in open areas than mature forests, it's likely that reduced forest cover will be beneficial for some species. However, a distinct bee community inhabits forests and is easy to overlook, since much of the activity happens before leaf out in the spring. Forests are also important nesting sites for many species and may serve as buffers to the spread of invasive species and other anthropogenic threats. Forest fragmentation also has ripple effects with significant consequences for remaining vegetation. In particular, increased deer density and associated over-browsing is likely to reduce both the abundance and diversity of flowering plants. Several uncommon specialist bees (i.e. Azalea Miner Andrena cornelli, Yellow Loosestrife Bees Macropis, and Northern Dogwood Miner Andrena persimulata) are dependent on flowers favored by deer, and may decline as exurban deer densities increase.

Continued urban sprawl in Vermont, particularly in Chittenden County in areas of high bee diversity, is likely to be a major threat at both local and regional scales. One example happened during our surveys. Sandy soils generally support high bee diversity, but are often targeted for development. In Williston a sandy, vacant field that was a nesting site for a number of uncommon bee species was developed and is now covered with buildings, pavement, and turf grass. Similar sites around the state are being targeted for development, including for solar fields, which may be seeded with non-native grasses and mowed frequently. Direct mortality from vehicle strikes may be a significant source of mortality in some areas.

Agricultural intensification is a global conservation issue, with economic forces driving the consolidation and mechanization of agriculture, which generally reduces the biological diversity of farmland. In Vermont, one particular group of bees that is likely threatened by the industrialization of agriculture (and development of former agricultural land) are the four thistle and burdock associated species, including the Texas Mason (Osmia texana), a watchlist species.

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Introduced & Managed Species

About 95% of the state is within the foraging range of a registered Honey Bee hive

Invasive and introduced species are a major cause of biodiversity loss. For bees in Vermont, this threat can manifest through multiple pathways, including competition, pathogen spillover, habitat alteration, and direct predation.


Honey Bee (Apis mellifera) in Vermont. Locations of registered hives (triangles) and iNaturalist records (small dots) in Vermont. The underlying colors denote areas where Honey Bees are likely (dark green) and unlikely to occur (lightgreen)

The Western Honey Bee was introduced from Europe hundreds of years ago as an agricultural animal and is the most extensively managed bee species in the world. The vast majority of Western Honey Bees in Vermont live in managed hives, though they occasionally escape and establish colonies in natural settings or human structures. It is thought that very few of these feral hives are able to survive for multiple seasons. The effects of Western Honey Bees and commercial bumble bee hives on native species has been extensively studied with many finding significant negative impacts on native bee populations from competition and pathogen spillover.

Pathogen spread or 'spillover' can occur when infected domestic bee hosts interact with closely-related wild bee populations. For example, commercially produced bumble bee colonies used for pollination in greenhouses often have higher levels of pathogens than wild bumble bees. These pathogens may spread to wild bees when commercial bees forage outside of greenhouses and interact with local populations at nearby flowers. Additionally, evidence of RNA viruses spilling over from managed Western Honey Bees to wild bumble bees has been found in Vermont and other regions. There may also be synergistic effects between other stressors and pathogens. For example, a landscape-scale analysis found that greater usage of the fungicide chlorothalonil was a strong predictor of pathogen prevalence in four declining bumble bee species. This fungicide has been commonly used during the last 20 years on golf courses and ornamental plants in Vermont. To understand the extent of the threat to bumble bees and other wild bees, the prevalence and effects on fitness of these pathogens are in urgent need of further study.

Other non-native bees such as the Horn-faced Mason (Osmia cornifrons), Taurus Orchard Bee (Osmia taurus), and Alfalfa Leafcutter (Megachile rotundata), that have been historically managed for pollination services, have escaped and are likely affecting native bee populations. Since agricultural bee management practices present a possible significant risk to local and regional bee populations they should be implemented with great caution, especially since most of Vermont’s pollinator mediated agriculture is relatively small scale, where wild bees may be sufficient and produce higher quality crops.

Additionally, habitat alterations from invasive species, such as earthworms and exotic plants, is a threat to some native plant communities and nesting sites that bees rely on. The extent of these impacts on bee populations has yet to be studied in detail.


Proportion of non-native bee observations in Vermont. Species of Bombus and the Western Honey Bee (Apis mellifera) are not included.

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Systemic insecticides have detrimental effects on bumble bees and other pollinators

Bees may be threatened by exposure to pesticides when residues occur in pollen and nectar of their food plants. Of principal concern are insecticides applied to control insect herbivores, including topically applied compounds such as carbamates and organophosphates, as well as compounds applied below ground that are systemic within plants, such as neonicotinoids. Certain fungicides and herbicides are also toxic to bees. And, there is evidence for synergistic negative impacts of pesticide combinations on bees. This is a concern because bees are commonly exposed to residues of many pesticides simultaneously.

There is a very large body of peer-reviewed research now concerning detrimental effects of neonicotinoids on bumble bees and other pollinators. Sublethal effects from feeding on pollen and nectar include: lower reproduction rates, reduced navigation abilities, poor foraging behavior, less successful pollination, and reduced immune function. All of these may lead to lower reproductive output or failure.

Neonicotinoids can be sprayed onto foliage or applied as soil drenches, but they are predominantly used as seed treatments. When used in this manner, neonicotinoids are taken up by all parts of the plant as it grows. This means these systemic insecticides are present in pollen and nectar that pollinators can come in contact with when foraging. In addition, they have been found on nearby flowers, in waterways, and they persist in the soil. However, under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), seed coating gets a free pass with a regulatory loophole that allows seeds coated with chemicals to be considered ’treated articles’ rather than pesticides. The use of pesticides in this manner is not tracked. But some estimates find that in Vermont >99% of all neonicotinoid pesticide use is in the form of seed coating.

Additionally, many pesticides, including neonicotinoids, are used on nursery stock where they last for many years. This persistence means well-intended pollinator plantings may be exposing bees and other insects to toxins.

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