Are Bees Declining and, If So, Why?

Dr. Ralph Cartar Dr. Ralph Cartar, Associate Professor, Department of Biological Sciences and the Natural Science Program, University of Calgary

[A talk given to the November 12, 2008 meeting of the U of C Emeritus Association]

Bees are important natural and commercial pollinators in the northern hemisphere. Based on calculations in the late 1990s, the annual value of U.S. honey bee crop pollination was estimated at $15 billion. They pollinated around 100 crops, which made up about one-third of the U.S. diet. (Morse & Calderone, 2000) Pollination is an essential “ecosystem service” provided by several species of bees, including honey bees and bumble bees. (Constanza et al., 1997)

Factors contributing to honey bee decline

Honey bees, previously and now, have been afflicted by:

• Nosema Apis, a mid-gut parasite that shortens worker bee life span by 22-44% and depresses foraging
• Varroa destructor, a mite that kills male larvae and workers and prevents queen from breeding, usually causing death of colony within 2 years of infection
• Acarapis woodi, a mite found in the trachea
• Paenibacillus larvae, a bacterium that doesn’t affect adult bees but kills bee larvae (Johnson, 2007)
  
  

A new problem, colony collapse disorder (CCD), has arisen in the United States. Roughly half of the U.S. honey bee colonies were lost to this syndrome in the winter of 2006. The symptoms of CCD include: a rapid loss of adult bees but none dead in or near hive; excess brood, in all stages, abandoned in the hive; and low levels of Varroa mites. This virus may have spread from Ausralian imports of bee colonies. (Cox-Foster et al., 2007, 2008)

CCD has not occurred in Alberta yet. The biggest problem for Alberta bee keepers appears to be Varroa destructor, which has caused losses of 30% of the honey bee colonies over each of the past two winters. This matters because Alberta is the centre of Canada’s honey bee industry.

Native pollinator problems

However, honey bees themselves are problematic because they are not native to North America and in many ecological contexts may have a negative impact on native pollinators like bumble bees.
Native pollinators–almost exclusively bees–are responsible for nearly $3.1 billion worth of fruits and vegetables produced annually in the U.S. (Losey & Vaughn, 2006)

Interactions with wild bees enhance the pollination efficiency of honey bees. (Greenleaf & Kremen, 2006) Bumble bees deposit more pollen n the stigmas of apple flowers than honey bees do. (Thomson & Goodell, 2001)

Significant declines (67%) of bumble bees have been seen in the United Kingdom, with a habitat loss most likely due to agriculture and cities. (Biesmeijer et al, 2006).

Competition from honey bees has negatively affected bumble bee reproduction and foraging. (Thomson, 2004)

Pesticides are another potential agent of decline which is likely to affect bumble bees more than honey bees because of differences in foraging patterns and colony size. (Thompson, 2001; Thompson & Hunt, 1999) Proximity to habitats sprayed with insecticides decreases abundance and diversity of native bees. (Kevan, 1975) Insecticides can show up quickly in plant tissues (like nectar) where they can be ingested by visiting bumble bees. (Davis et al., 1988)

Commercial bumble bee colonies have become widely available in North America for pollinating greenhouse crops like tomatoes and sweet peppers. Once installed in a greenhouse, bees can move outside and collect significant amounts of pollen from plants outside the greenhouse. (Widdington et al., 2004) Since all of the commercial colonies are raised in high-density conditions, they are loaded with non-native parasites from Europe. These parasites might be horizontally transmitted to native bee populations when commercial bees feed outside of greenhouses.

Commercial bumble bee colonies are a likely source for transmittting honey bee diseases to wild bumble bees. (Genersch et al., 2006)

Agriculture and forestry are other agents of bee decline. Widespread replacement of hay with with silage (mostly grasses) has played a major role in bumble bee declines. (Fitzpatrick et al., 2007) Intensification of agricultural practices often result in “unimproved” landscapes being converted to cropland lacking bee-visited flowers. (Morandin et al., 2007) Heavy grazing (especially by cattle or sheep) can shift meadows from being good for bumble bees to being grass-dominated (bad for bumble bees). (Edwards & Williams, 2004) Pesticide drift from crop fields to the margins occupied by bumble bees can contribute to their decline.

Protecting our pollinators

Bumble bees can be protected in an agricultural landscape by ensuring that the share of farmland doesn’t exceed three-quarters. with the rest made up of bee refuge habitats. (Banaszak, 1992)

Parks, urban and rural, can help protect bumble bees if they contain natural habitats. (McFrederick & LeBuhn, 2006)

Diversifiicatin of honey bee farming practices could encourage natural selection and honey bee types more resistant to disesases. “. . . over-reliance on one managed non-native species is inherently unstable.” (Berenbaum, 2007: testimony to U.S. Congress)

Encouragement of native pollinators like bumble bees, especially in agricultural and silvicultural (forest management) landscapes, could result from allowing more set-aside land in the agricultural matrix, and more continuous unlogged habitat.

Studying the ecology and taxonomy of native pollinators, could lead to understanding which species we now have, and which ones might benefit from agriculture and silviculture.

Requiring that commercial bumble bee colonies are parasite-free, and that their worker-bees not be allowed to leave greenhouses in which their colonies are installed could address a number of the problems currently ailing bees.

Reference List

Banaszak, J. 1992. Strategy for conservation of wild bees in an agricultural landscape. Agriculture, Ecosystems & Environment 40:179-192.

Berenbaum, M.R. 2007. Colony collapse disorder and pollinator decline. Statement to Congress of U.S.A. 29 March 2007.

Biesmeijer, J.C. et al. 2006. Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands. Science 313:351-354.

Costanza, R. et al. 1997. The value of the world's ecosystem services and natural capital. Nature 387:253-260.

Cox-Foster et al. 2007. A metagenomic survey of microbes in honey bee colony collapse disorder. Science 318:283-287.

Cox-Foster et al. 2008. The latest buzz about colony collapse disorder. Science 319:725.
Davis, A.R., R.W. Shuel & R.L. Peterson. 1988. Distribution of carbofuran and dimethoate in flowers and their secretion in nectar as related to nectary vascular supply. Canadian Journal of Botany 66:1248-1255.

Edwards, M. & P. Williams. 2004. Where have all the bumblebees gone, and could they ever return? British Wildlife 15:305-312.

Fitzpatrick, U. et al. 2007. Rarity and decline in bumblebees: a test of causes and correlates in the Irish fauna. Biological Conservation 136:185-194.

Genersch, E. et al. 2006. Detection of deformed wing virus, a honey bee viral pathogen, in bumble bees (Bombus terrestris and Bombus pascuorum) with wing deformities. Journal of Invertebrate Pathology 91:61-63.

Greenleaf, S.S. & C. Kremen. 2006. Wild bees enhance honey bees' pollination of hybrid sunflower. PNAS 103:13890-13895.

Johnson, R. 2007. CRS Report for Congress: Recent Honey Bee Colony Declines. Congressional Research Service.

Kevan, P.G. 1975. Forest application of the insecticide fenitrothion and its effect on wild bee pollinators (Hymenoptera: Apoidea) of lowbush blueberries (Vaccinium spp.) in southern New Brunswick, Canada. Biological Conservation 7:301-309.

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McFrederick, Q.S. & G. LeBuhn. 2006. Are urban parks refuges for bumble bees Bombus spp. (Hymenoptera: Apidae)? Biological Conservation 1029:372-382.

Morandin, L.A. et al. 2007. Can pastureland increase wild bee abundance in agriculturally intense areas? Basic & Applied Ecology 8:117-124.

Morse, R.A. & N.W. Calderone. 2000. The value of honey bees as pollinators of U.S. crops in 2000. Cornell University. http://www.beeculture.com/beeculture/pollination2000

Otterstatter, M.C. & J.D. Thomson. 2008. Does pathogen spillover from commercially reared bumble bees threaten wild pollinators? PLoS ONE 3:1-9.

Thompson, H.M. 2001. Assessing the exposure and toxicity of pesticides to bumblebees (Bombus sp.). Apidology 32:305-321.

Thompson, H.M. & L.V. Hunt. 1999. Extrapolating from honeybees to bumblebees in pesticide risk assessment. Ecotoxicology 8:147-166.

Thomson, D. 2004. Competitive interactions between the invasive European honey bee and native bumble bees. Ecology 85:458-470.

Thomson, J.D. & K. Goodell. 2001. Pollen removal and deposition by honeybee and bumblebee visitors to apple and almond flowers. Journal of Applied Ecology 38:1032-1044.

Whittington, R. et al. 2004. Plant-species identity of pollen collected by bumblebees placed in greenhouses for tomato pollination. Canadian Journal of Plant Science 84:599-602.