A bad gut feeling

By Athayde Tonhasca

In July last year, a group of about 70 enthusiastic and hopeful Americans criss-crossed the slopes of Mt. Ashland, near the Oregon–California border, in the hope of making history: catching sight of the elusive Franklin’s bumble bee (Bombus franklini). Those biologists, landowners, students and other volunteers had a good day out, but came back empty-handed. As did the people engaged in previous searches at Mt. Ashland every year for over a decade.

A meadow near the California-Oregon border, Franklin’s bumble bee habitat © Pacific Southwest Region USFWS, Wikimedia Commons.

An American endemic, the Franklin’s bumble bee has always been rare, with only 94 sightings in 1998, when its monitoring started. All records have been restricted to an area of less than 34,000 km2 between southern Oregon and northern California in localities with altitudes between 160 and 2300 m; this territory represents one the narrowest bumble bee distributions in the world. And something bad happened after 1998, because numbers plunged: 20 bees were recorded in 1999, nine in 2000, one in 2001. After a hopeful recovery in 2002 (20 bees), the negative trend continued. Three bees were sighted in 2003, and one worker was last seen in 2006.

A Franklin’s bumble bee © Colleen Meidt, USDA Agricultural Research Service.

Considering the long run of unfruitful searches, could the Franklin’s bumble bee have gone extinct? Possibly, but rare species are a tricky problem. They are difficult to find because they are naturally scarce, so how can we be sure they are really gone when we look for them? It is unlikely that a species searched for as exhaustively as the Franklin’s bumble bee could remain undetected for so long, but it’s not a farfetched proposition. Among insects, there are many cases of ‘Lazarus species’, that is, species believed to be extinct only to ‘resurrect’ some time later thanks to a fortuitous observation. The downland furrow bee (Halictus eurygnathus) was not seen in Britain since 1946 until rediscovered in 2003; the bone skipper fly (Thyreophora cynophila) was unexpectedly found in Spain in 2010 after remaining unrecorded for 160 years throughout its native European range. Even the largest bee on Earth, the Wallace’s giant bee (Megachile pluto), was not seen for 37 years.

A downland furrow bee and a bone skipper: both species were believed to be extinct until rediscovered ©
Arnstein Staverløkk (L) and Panzer et al., Wikimedia Commons.

So the volunteers who dedicate their time in pursuit of the Franklin’s bumble bee have a reason to be cautiously optimistic. Who knows, one of these secretive bees may yet make its way to the bottom of a collector’s net.

Ecologists and conservationists are not surprised by the hard times that befell the Franklin’s bumble bee because this species is naturally rare, therefore particularly susceptible to environmental stresses. But the rusty-patched (B. affinis), the western (B. occidentalis), the yellow-banded (B. terricola) and the American (B. pensylvanicus) bumble bees tell a more alarming story. Once widely distributed in North America, these bees have disappeared from many sites where they were previously common. The decline of the rusty-patched bumble bee has been particularly dramatic: it suffered a range retraction of at least 87%.

A rusty-patched bumble bee © USGS Bee Inventory and Monitoring Lab, Wikimedia Commons.

The possible demise of the Franklin’s bumble bee and the decline of the other North American species must have a reason. Several have been proposed, including habitat degradation, wildfires, climate change, human encroachment of bees’ habitats, pesticides, competition with the European honey bee (Apis mellifera), and introduced parasites and diseases from commercially bred bumble bees. This last possibility came from deductive, detective work. Researchers have noticed that the sharp decline of those species coincided with the collapse of commercial western bumble bee colonies, which were then used to pollinate greenhouse crops. These commercial bees were severely debilitated by the microsporidium (a type of unicellular fungus) Nosema bombi, a gut parasite.

Occurrence records for the rusty-patched (A), yellow-banded (B), and American bumble bees (C), and all bumble bee species (D). Blue points are records from 2000 to 2010, and are overlaid on green points representing records from 1980 to 1990 © Szabo et al., 2012.

According to this scenario, western bumble bee queens shipped to rearing facilities in Belgium in the early 1990s became infected with N. bombi (a known problem in commercial production of bumble bees). When reared colonies resulting from those imported queens were brought to America and distributed to crop producers, the fungus made its way to wild populations of western bumble bees and the other species (National Research Council, 2007. Status of Pollinators in North America).

The evidence for introduced parasites (there are many others besides N. bombi) as the cause of bumble bees’ decline is circumstantial, as it is based on correlation of events. But the hypothesis is compelling. For example, studies have demonstrated high incidence of pathogens (parasites and diseases) in wild bumble bees collected near greenhouses where commercial bees are used – including the common eastern bumble bee (B. impatiens), which replaced the western honey bee in American greenhouses.

Nosema spores from dissected bumble bees. The smaller elongate spores measure 4.5 × 2.0 microns © Bushmann et al., 2012.

Parasites do have the potential for mayhem: the critically endangered Patagonian bumble bee (B. dahlbomii) is believed to be threatened by a protozoan gut parasite that was introduced by commercial bumble bees. But we don’t have a clear understanding of the impact of pathogens on wild bees because these agents are difficult to detect, and some infected individuals are asymptomatic. To complicate matters, susceptibility to N. bombi varies significantly between bumble bee species. So we can’t say with certainty that N. bombi or any other parasite is responsible for the decline of some American bumble bees, but they are likely to be one of the contributing factors. 

Several bumble bees in Britain and continental Europe are also declining. Losses on this side of the Atlantic have been attributed mainly to agriculture intensification and habitat degradation, but parasites and diseases are believed to play their part. And like in North America, commercial bumble bees are likely sources of infections to wild bumble bees, some solitary bees, hover flies and other insects. To make things worse, the European honey bee is also a reservoir of pests and diseases, and a competitor for resources to boot. 

Commercial bumble bees and honey bees make unquestionable contributions to crop production around the world, but they have a dark side. Despite the uncertainties, multitudes of local conditions and conflicting research results, data on the negative effect of these interlopers on wild bees and other pollinators are accumulating (e.g., Mallinger et al., 2017Iwasaki & Hogendoorn, 2022). Addressing the conflicts between crop production and conservation will not be easy, but address them we must for the sake of our wild pollinators.

A: A commercially-reared buff-tailed bumble bee (B. terrestris) with wings crippled by the Deformed Wing Virus; the yellow and white patches are anomalous pigmentation. B: a normal individual © Cilia et al., 2021