Challenges Facing Bees: Honey Bee Population Loss and More

News of honey bee population loss is becoming sadly familiar, but colony collapse isn't the only challenge bees face.

Varroa mite on honeybee

Varroa mites, like the one on the back of this honey bee (Apis mellifera), pose a serious threat to the health of bees and beekeeping, and may be one of the factors underlying colony collapse disorder.

Photo courtesy United States Department of Agriculture

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In The Bee (Princeton University Press and Ivy Press, Limited, 2014), Noah Wilson-Rich, Kelly Allin, Norman Carreck and Andrea Quigley provide a window into the vitally important role that bees play in the life of our planet. This richly illustrated natural history of the bee takes an incomparable look at the astounding diversity of bees, blending an engaging narrative with practical, hands-on discussions of such topics as beekeeping and bee health. The following excerpt is from chapter 7, "The Challenges Faced by Bees."

In recent years, the world’s media have reported that bees seem to be dying at an unprecedented rate. Certainly there have been significant losses of honey bees, but is this a new thing, and are the losses restricted to honey bees?

Historic Bee Losses

Calamitous losses of the western honey bee (Apis mellifera) have occurred at various times throughout history and throughout the world. For example, in the USA, significant losses were reported in Kentucky in 1868, then in many places in the early twentieth century, and again in the 1960s and 1970s.

More than a hundred years ago, substantial losses of bees in the UK were attributed to the “Isle of Wight disease.” This was first seen on the Isle of Wight, an island off the south coast of England, and some claimed that it nearly wiped out the native British black honey bee (A. mellifera mellifera), although the bees remained common elsewhere in Europe.

The leading entomologists of the early twentieth century worked to uncover what lay behind these bee losses, and in 1921 the cause was identified as acarine, or tracheal, mite (Acarapsis woodi). The symptoms caused by this mite, however, do not coincide with the bees’ symptoms as reported at the time. During the 1950s, Leslie Bailey at Rothamsted Experimental Station concluded that the Isle of Wight disease was almost certainly caused by chronic bee paralysis virus, a previously unknown infectious disease.

Recent Losses

The losses that have most hit the news are those of commercial honey bees, especially those used for pollination of almonds in California and apples in Pennsylvania. In 2006 many bee farmers returned to their hives to discover most of the bees were gone, and the few they found were dead. It was at this point that the term colony collapse disorder (CCD) was first used.

Although they rarely appear in the headlines, many of the world’s other bee species have also been in decline over the last fifty years, and particularly over the last twenty. As these bees live in the wild, and are not kept in hives, their disappearance has been less obvious—and they have been in decline for far longer than just the last few years. For example, the UK has lost three bumble bee species in the last 150 years. The apple humble bee (Bombus pomorum) has not been seen since 1864; Cullum’s bumble bee (B. cullumanus) was last recorded in 1941; and the short-haired bumble bee (B. subterraneus) vanished in 1988. However, all these species are found, sometimes commonly, outside of the UK.

The term bumble bee scarcity syndrome was coined in Europe in 2012 to describe situations where the number of bumble bees seen was abnormally lower than expected due to heat waves. These losses extend to the Americas and Asia. A number of unique North American bumble bee species are declining, with their ranges contracting, and four are scarce: the western bumble bee (B. occidentalis), the American bumble bee (B. pensylvanicus), the rusty-patched bumble bee (B. affinis), and the yellow-banded bumble bee (B. terricola). One species—Franklin’s bumble bee (B. franklini)—is believed to be extinct. In stark contrast, four other North American bumble bee species remain abundant and widespread, including the commercially important common eastern bumble bee (B. impatiens) as well as B. bifarius, B. vosnesenskii, and B. bimaculatus.

Worldwide, far less is known about the situation of solitary bees. Initial studies of North American solitary bees indicate that these species are declining at a slower rate of about 15 percent, compared with 30 percent for bumble bees, since the 1870s. A number of Hawaiian species of yellow-faced bees (Hylaeus spp.) are critically endangered, and some are potentially extinct.

Weather and Climate

As a potential cause for bee losses, the weather can have serious local impacts, but such a factor is outside human control. However, global climate change—which could lead to a myriad of changes affecting bee survival—may be within human influence.

The Impact of Weather on Bees

Since the start of the twenty-first century, winter weather in many countries has become very variable. This pattern seems to tie in with the pattern of honey bee losses in the USA, the UK, and Europe. For example, the US winter of 2011/12 was unusually mild, and it is significant that the loss of honey bee colonies was “only” 22.5 percent nationally, compared with 30.6 percent in the winter of 2012/13 when conditions were severe. Inclement weather has always been harmful to hive bees, and it is now recognized as an important factor in colony collapse disorder.

The negative impact of inclement weather on bee survival may be reversible as the weather becomes more favorable, but if a species is living at the edge of its climatic range or is rare, a run of several years of bad weather can be enough to lead to its local extinction.

Climate Change and Bees

Bumble bees, with their furry bodies, evolved in the cooler regions of the world. Within their normal habitat ranges, increasing temperatures—resulting in both warmer winters and hotter summers—could affect their ability to survive. One feature of climate change is erratic weather patterns. For example, during a day of unseasonably warm weather in midwinter, a queen might emerge from hibernation and fail to find food because it is too early in the year for the right flowers to bloom. Even if she has sufficient energy to return to her overwintering site, her reserves are depleted and she is less likely to survive to the spring.

Recent studies have shown that warm spring weather now starts earlier each year in temperate regions of Europe and North America. Concerns exist that if the bees emerge earlier than the flowers, neither will the flowers be pollinated nor will the bees find food. Few studies have provided evidence for these potential mismatches, however. For a few North American bee species, it was found that over 150 years they now emerge ten days earlier, and so do the flowers they visit. There is a possibility that both plants and bees might slowly adjust to long-term changes in the climate, but this is by no means certain.

In one study, the distribution of Colletes bees was compared with climate-based patterns of expected distribution across Europe. The ivy bee (C. hederae) has increased its range rapidly over the last twelve years, but it has not yet filled the range where the climate is already suitable in large parts of Italy, France, and Spain. It visits solely European ivy (Hedera helix), which is very climate-sensitive and flowers only in the warmer areas of its range. As the climate warms so this ivy is likely to flower farther north, enabling C. hederae to expand its range. It is expected that many plants and bee species will shift northward and to higher altitudes as the climate warms, disrupting the existing ecosystems.

Distribution models suggest that generalist bees will adapt better to climate change than the specialist species that are restricted to foraging on a single plant species or genus.

We know little about the direct effects of climate change on plants, bees, and their complex interrelationships. Higher temperatures affect floral scent, nectar, and pollen production in plants, changing their attractiveness to bees in terms of the lure (the scent) and the quality and abundance of the rewards (the nectar and pollen). Furthermore, an inverse relationship exists between temperature and the size of bees, with higher temperatures resulting in smaller bees. If bees become smaller through climate change, then, given that bigger insects carry more pollen and are able to forage over greater distances, the delicate balance between pollinator and plant could easily change.

Habitat Loss

Land-use changes are harmful to bees when habitats that were once suitable for them are lost or damaged. Once commonplace species have now become regionally extinct, and they may exist at low levels elsewhere.

Loss of Wild Habitats

Much land has been “reclaimed” for human use, often for farmland. Between 2006 and 2011, more than 500,000 hectares of grassland was reclaimed from the shallow wetlands of the Great Plains prairie pothole region of North America, including North Dakota, with an 80 percent loss of habitat for wildlife species. The impact of agriculture on the range and number of native bees, potential bee nesting sites, and suitable flowering plants is being assessed. Similar studies are also being carried out on non-native honey bees—because North Dakota produces more honey than any other US state (32.7 million tonnes in 2012).

Australia has a diverse and unique native bee population. Land clearance and agriculture, resulting in the loss of forage and nesting sites, has adversely affected many Australian bees. For example, grazing has severely reduced western myall trees in the southern arid regions, with the loss of nest sites used by the allodapine bee Exoneurella tridentata. Steps to conserve native Australian bees have been hampered by a lack of taxonomic expertise and suitable identification keys. In 2009, some 300–400 species remained undescribed.

Urbanization

Half of the world’s human population lives in towns and cities, and predicted population growth is likely to create ever greater tension between the competing demands for housing, agriculture, and preservation of the natural environment. Urbanization typically decreases biodiversity as it encroaches on rural habitats, and the remaining rural habitats are degraded through agricultural intensification.

Urbanization transforms the landscape, bringing habitat disturbance and loss. It eliminates nest sites and native forage, leading to fewer bee species. Of particular concern to ecologists is habitat fragmentation. Pockets or islands of suitable habitat are surrounded by inhospitable areas. These isolated habitat fragments are less likely to retain their native insect populations. Part of the solution lies in optimizing green spaces for bees. Bees can benefit from ribbons of pollinator-friendly habitat linking fragments of good habitat through reserves, parkland, and gardens.

Honey bees are able to gather supplies from a wide range of plants, so they thrive in cities, provided the number of colonies does not exceed what the local floral resources will support. Native bees, however, do not always fare so well. In the Polish city of Poznan, 104 native bee species were collected over three years—but one-third of the individuals belonged to just five species. Almost 90 percent of the individual bees (and three-quarters of the species) were generalists, with specialist bees accounting for less than 1 percent of the bees seen.


Planting for Bees

Urban areas often have a wider range of flowering plants than natural sites—and certainly more than intensively farmed fields—but comparatively little is known about the impact on the diversity and composition of bee populations. When the number of plant species was doubled in one Californian community garden, the number of native bee species rose from five to forty. The plants flowered successively from early spring to late fall, thus extending the bees’ foraging season. The project was shared with the public through education and outreach programs, thus spreading the words about helping bees through gardening.

Green roofs may have a significant role to play in bee conservation, and the number of projects across the world grew from 93 in 2000 to over 1500 in 2013. In Toronto, Canada, bee communities on green roofs were very similar to those at ground level, confirming the results of earlier studies in London, UK and Basel, Switzerland.


More from The Bee:

Why Are Bees Important — and What Makes Bees Different?


Reprinted with permission from The Bee: A Natural History by Noah Wilson-Rich, with contributions from Kelly Allin, Norman Carreck and Andrea Quigley, and published by Princeton University Press and Ivy Press, Limited, 2014.