Pollinator Declines

Domestic honey bees hives are down by 59% compared to 60 years ago with rapid declines over the last forty years. The populations of some native bee species may also be declining.
Pollinator Declines - Articles


Varroa mite parasitizing bee. Thought to be one of the major factors contributing to hive losses. Photo Alex Wild.

Researchers believe that long term honey bee declines are a result of a complex set of factors. The primary suspects are:

  • poor nutrition
  • pesticides
  • pathogens/ parasites
  • poor quality genetic stock

Pollinators became a hot topic when thousands of hives in the US were found strangely empty starting in 2006. Since then, nationwide surveys have found that beekeepers lose approximately one third of their colonies each winter. The losses in 2006 were attributed to "Colony Collapse Disorder," where collapsed colonies still had a queen and some young bees, but most of the adult bee population disappeared [17]. Since 2006, colonies losses have continued, but beekeepers have attributed these losses to factors other than Colony Collapse Disorder. However, the outbreak of Colony Collapse Disorder in 2006 served to bring international attention to long term, pervasive declines in honey bees and other pollinators.


Poor nutrition is thought to be a major factor leading to colony losses. Poor nutrition can be due to over-crowding honey bee colonies, using bees to pollinate crops that have low nutritional value, low nutritional value of food packets commercial beekeepers use inside hives during transport, and reduced abundance of flowering plant species (the bee's only natural source of food). Poor diet can also make bees susceptible to parasites and pathogens. Bees need high quality forage from diverse flowering plants.

Not surprisingly diet is important for bees, just like it is for us. Bees get their sugars from nectar, and their protein, fats and micronutrients from pollen. In order to balance their diets, most bee species need to gather pollen and nectar from a diverse range of flowers. When bees have a simple diet of just one type of pollen and nectar, they don't get all the vitamins, amino acids, fats, and antioxidants they need. Just like carotenoids, flavanoids, and alkaloids support healthy humans, researchers are finding that diverse diets support healthier bees [18-20].

When bees don't have sufficient forage, enough diversity, or high enough quality food sources, they are more susceptible to pathogens like Nosema (a gut parasite that causes dysentery), viruses and other parasites [18, 21]. When bee nutrition is low bees don't live as long. That can be a matter of life or death for the whole hive. A hive needs a sufficient number of bees to keep the hive warm all winter; when temperatures drop, the honey bees form a cluster that "shivers" to keep itself warm. If the population of winter bees is too low, they are unable to stay warm enough to survive the winter months.

Parasites and Pests

Imagine having a parasite the size of a dinner plate attached to your back sucking your blood. That is a little like what honey bees are dealing with when they are attacked by the varroa mite. The small hive beetle, the parasitic phorid fly, and tracheal mites are all also attacking honey bees. These parasites directly impact honey bee health, sapping their strength. They also interfere with honey bee immune systems and 'social immunity' behaviors. One study weighing the importance of 55 risk factors possibly involved in colony collapse disorder (CCD) found that aggressive mite control (indicated by presence of coumaphos miticide added by beekeepers in the hive) was the greatest indicator of healthy hives, though many other factors were involved [22]. Several other studies showed that high varroa mite levels are the number one factor linked to winter losses of honey bee colonies, and treatments with miticides can improve survival rates [23-26].

In order to understand how parasites are impacting pollinators it is important to understand how bee immune systems work. First consider your own immune system. Humans have white blood cells and antibodies which attack foreign invaders as well as defense mechanisms including gut environments hostile to pathogens. Bees have a somewhat parallel immune response [27]. They also have 'social immunity' as they work together to keep the hive healthy. For example, bees can show "hygienic behavior", where workers will locate and remove infested or diseased larvae and pupae. Honeybees secrete antimicrobial substances reducing the growth of pathogens in the colonies and raise their young in individual cells with strongly antimicrobial food sources (e.g. royal jelly). Another social immune response is an artificial 'fever' created in response to chalkbrood fungal infections, where bees will cluster and vibrate their muscles to raise the temperature high enough to kill the chalkbrood microbes. Honey bees also groom each other removing parasites.

Parasites often disrupt these hygienic behaviors. For example, when infected with mites, bees produce less of an enzyme, glucose oxidase, they usually secrete into food to sterilize it before feeding it to larvae. As a result young brood may be exposed to higher levels of microbes. Parts of the bee's individual immune systems also start to break down. Mite infected bees may produce less of the enzymes used to kill foreign invaders (think white blood cell like reactions) [28]. Mites can transmit viruses and immunosuppress bees, so mite-infected bees often have incredibly high levels of viruses.


Honey bees are battling a number of new and common pathogens. American foulbrood bacteria is notorious and easily recognized by the slimy goo that strings out of the cells when beekeepers disturb a cell while checking the hive. This bacterium spreads quickly through the hive, infecting larvae. European foulbrood bacteria, chalkbrood fungi and Nosema gut parasites all attack our bees, possibly including some new species or strains of Nosema. Finally, bees have more than 20 types of viruses that have been identified so far.

Many of the major problematic viruses that affect bees are the ssRNA viruses, distant relatives of polio and foot and mouth disease. After Varroa, viruses are the second factor that has been linked to colony losses in the winter [25, 29]. Viruses can cause muscle spasms, impair learning, and lead to early death. Also, these honey bee viruses can get transmitted to other bee species that forage on the same plants. We still don't fully know how these viruses affect other bee species, but there have been some studies with solitary bees. Solitary bees generally go into a hibernation-like state called diapause where their reduced metabolism helps them live through the winter. However, RNA virus infected bees don't go into diapause. Their metabolism stays normal and they don't live through the winter.

The interaction of viruses and other stressors including nutrition and immune compromise from pesticide toxin exposure may be multiplying the severity of the problem. Penn State's Dr. Cox-Foster explains how this happens with the analogous example of polio on the human population. When polio was widespread, only 1.5% of the infected population actually had the severe symptoms of the disease. Symptomatic individuals often had low nutrition. Similarly, bees might only show severe symptoms and death from ssRNA viruses when their immune systems are compromised due to low nutrition.

Poor quality genetic stock

Honey bee stocks can vary greatly. Some colonies are more aggressive than others, some are better at performing hygienic behavior, and some have lower levels of Varroa mites. There has long been an interest by beekeepers and researchers to try to "breed a better bee" that is more resistant to diseases and more productive. Several lines of bees that are more resistant to Varroa are commercially available, including the "Varroa Sensitive Hygiene" stock bred by the USDA, "Minnesota Hygienic" stock bred by Marla Spivak (University of Minnesota), and the "Russian" stock.

Honey bee queens typically mate with 10-20 males! This means that the offspring the queen produces is very genetically diverse. Genetically diverse colonies are more productive and healthier. Beekeepers are becoming increasingly concerned that their queens are low quality and not well-mated, which would lead to unhealthy colonies. Also, if a queen is low quality, she will die faster, leaving the colony without a queen. In surveys of commercial colonies, a "queen event" - where the queen was lost and/or replaced by a new queen - was linked with colony death [30]. There is a lot of interest in local queen breeding efforts, to improve the genetic stocks and the quality of the queens. Pennsylvania currently has a state-wide effort to breed better queens, led by Jeff Berta.


Honey bees across North America are exposed to multiple pesticides. The interactions between pesticides and long term effects can be difficult to predict. We will delve into recent information on pesticides and pollinators in our next issue.


15. (NRC), N.R.C., Status of Pollinators in North America, ed. C.o.t.S.o.P.i.N. America. 2007, Washington, DC: The National Academies Press.

16. United States Department of Agriculture, U., Report on the National Stakeholders Conference on Honey Bee Health, 2012: Alexandria, Virginia.

17. Cox-Foster, D.L., et al., A metagenomic survey of microbes in honey bee colony collapse disorder. Science, 2007. 318(5848): p. 283-287.

18. Alaux, C., et al., Diet effects on honeybee immunocompetence. Biology Letters, 2010. 6(4): p. 562-565.

19. Tasei, J.N. and P. Aupinel, Nutritive value of 15 single pollens and pollen mixes tested on larvae produced by bumblebee workers (Bombus terrestris, Hymenoptera : Apidae). Apidologie, 2008. 39(4): p. 397-409.

20. Morais, M., et al., Honeybee-collected pollen from five Portuguese Natural Parks: Palynological origin, phenolic content, antioxidant properties and antimicrobial activity. Food and Chemical Toxicology, 2011. 49(5): p. 1096-1101.

21. Schmidt, J.O., S.C. Thoenes, and M.D. Levin, Survival of Honey-Bees, Apis-Mellifer (Hymenoptera, Apidae), Fed Various Pollen Sources. Annals of the Entomological Society of America, 1987. 80(2): p. 176-183.

22. vanEngelsdorp, D., et al., Weighing Risk Factors Associated With Bee Colony Collapse Disorder by Classification and Regression Tree Analysis. Journal of Economic Entomology, 2010. 103(5): p. 1517-1523.

23. van Dooremalen, C., et al., Winter Survival of Individual Honey Bees and Honey Bee Colonies Depends on Level of Varroa destructor Infestation. Plos One 2012. 7.

24. Dainat, B., et al., Predictive markers of honey bee colony collapse. PLoS One, 2012. 7.

25. Genersch, E., et al., The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies. Apidologie, 2010. 41: p. 332-352.

26. Guzman-Novoa, E., et al., Varroa destructor is the main culprit for the death and reduced populations of overwintered honey bee (Apis mellifera) colonies in Ontario, Canada. Apidologie, 2010. 41: p. 443-450.

27. Evans, J.D., K. Aronstein, and Y.P. Chen, Immune pathways and defence mechanisms in honey bees Apis mellifera. Insect Molecular Biology, 2006. 15(5).

28. Yang, X.L. and D.L. Cox-Foster, Impact of an ectoparasite on the immunity and pathology of an invertebrate: Evidence for host immunosuppression and viral amplification. Proceedings of the National Academy of Sciences of the United States of America, 2005. 102(21): p. 7470-7475.

29. Chen, Y.P., et al., Israeli Acute Paralysis Virus: Epidemiology, Pathogenesis and Implications for Honey Bee Health. PLoS Pathog, 2014. 10(7).

30. vanEngelsdorp, D., et al., Idiopathic brood disease syndrome and queen events as precursors of colony mortality in migratory beekeeping operations in the eastern United States. Preventive Veterinary Medicine, 2013. 108(2-3): p. 225-233.

This article is part of a five part series describing pollinators, pollinator threats and on-farm conservation strategies as part of a collaboration between Penn State's Center for Pollinator Research and Penn State Extension Vegetable and Small Fruit Team.