So much of our agricultural productivity is dependent on the European honey bee (Apris mellifera) that it is no wonder that our attention is drawn to their plight. When the honey bee suffers, so does agriculture, and so, potentially do all who depend on the bounty that comes from animal pollinated angiosperms, the flowering plants from which we derive many of our most delicious and health-giving fruits and vegetables.
While honey bees are clearly not the only hard working pollinators that deliver a bounty to humans and other animals, their recent deaths from Colony Collapse Disorder (CCD) starting in 2006 have captured the world's attention. To date, CCD has been defined as a series of symptoms, but the cause and the cure have remained complex and elusive. CCD is not the only problem facing honey bees; in fact, in 2010 the overwintering losses were at the same unsustainable rates of over 30% but the cause seemed to be less from CCD than from other problems. Below is a list of the variety of issues facing honey bees.
The Varroa mite (Varroa destructor)
Is an external parasite that has spread from its original host, the Asian honey bee Apis cerana, to nearly all Western honey bees (Apis mellifera) worldwide. Virtually all European honey bees are highly vulnerable to Varroa mites, although some honey bee strains (VSH, Russians) show partial resistance to the mites. This mite weakens honey bees by sucking hemolymph (“blood”) from its host and by transmitting bee pathogens. A female mite reproduces by invading the cell of a bee larva just before capping. Once inside, the female lays eggs to produce offspring that feed together on the developing bee. The mother mite and her adult daughters emerge from the cell with the young adult bee host. Eventually, at high infestation rates, the mites overwhelm and kill the host colony. Beekeepers control Varroa mite populations by monitoring mite infestation rates and applying chemical treatments when mite populations become too large. Due to increased concerns over the effects of miticides on bees and mite resistance to commercial miticides, researchers are developing alternative approaches (“softer” chemical treatments, the genetics of mite resistance in honey bees, mite pheromones and hormones, and physical treatments) to control this mite.
This microscopic fungus can weaken or even kill colonies when the majority of workers become infected. Spores of the fungus survive on wax combs and stored food inside colonies. When workers eat these spores the fungus invades the lining of the intestine. Highly infected bees cannot digest efficiently and die earlier. Beekeepers use antibiotics and disinfection of hives to control this disease.
Thus far, more than 20 honey bee viruses have been identified. These viruses can impact bees in multiple ways, including killing developing larvae and pupae, decreasing the lifespan of adult bees, causing spasms and tremors, reducing cognitive skills, and impairing wing development so that bees cannot fly. Most honey bee colonies have multiple viruses, and the levels of these viruses can fluctuate throughout the year. Exposure to other stressors, particularly Varroa parasitization, can immunosuppress bees so that the effects of the viruses are more dramatic. The only treatment for viruses thus far is to feed the colony a solution of virus-specific RNA that enhances the bees' immune responses to these particular viruses, but these treatments only suppress the viral infections, and do not eradicate them. Other approaches that are being investigated include breeding bees with genetic resistance to the viruses.
American foulbrood (Paenibacillus larvae)
American foulbrood is an infection that kills young bees (brood) inside the wax cells in which they develop. This dead brood becomes a source of infection spread by workers nursing young brood. Some bees can detect and remove the diseased brood and this stops the disease from spreading. Beekeepers also use antibiotics to prevent the disease.
Pesticides are usually man made chemicals designed to kill pest organisms, that may injure plants or animals including humans. Pests cause economic damage by reducing crop yields directly or by producing crop, or ornamental plant diseases, or by competing with crops, or by reducing animal and human health, or by damaging buildings and structures. Pesticides are categorized according to their intended use as well as by their chemical composition. Pesticides are widely used and are divided into insecticides/acaricides, used to control insects and mites or ticks, fungicides used to control plant diseases; rodenticides, used to control rodents; and herbicides used to prevent weeds from competing with crops, grasses or ornamental plants. Pesticides usually contain an active ingredient, with a known mechanism for killing the target pests. Pesticides vary widely in their safety to humans and the environment and are sold as a formulation with added ingredients that augment the action of the active material when mixed in water for application. More than 1200 chemicals are registered for use in the United States and are used in some 18,000 separate products sold under a variety of trade names. People who apply the more toxic pesticides must have training and a state issued license to use these materials.
Some insecticides have warnings or bee hazards on their label because they are toxic to honey bees, causing honey bee deaths. If the insecticide has a sub-lethal affect on honey bees it may result in reduced larval survival, altered foraging behavior or shortened lifespan of adult bees. The extent of the sub-lethal affects is still unknown.
Honey bee colonies are healthier and stronger with access to pollen from diverse sources of flowering plants. However, floral diversity in landscapes has been reduced by intensive agriculture (single crops, few flowering weeds, limited hedgerows) and urbanization. In recent years, the pollination of early crops (such as almonds in California in February) has further increased the demand for strong colonies at times of year with few floral sources. Furthermore, changes in climate patterns may also affect seasonal availability of flowering plants. This requires beekeepers to use artificial sources (sugar syrup, corn syrup, and pollen substitutes) to try to meet the increased nutritional demands of their colonies.
Honey bee colonies are headed by a single queen who mates with an average of 12 males, and thus honey bee colonies are extremely genetically diverse. Several studies have demonstrated that genetic diversity improves the disease resistance and productivity of colonies, including their overwintering ability. Furthermore, strains of honey bees can have different traits - some forage for more pollen, while others are more adept at hygienic behavior, in which diseased or parasitized brood is removed. Several breeding programs are underway to develop stocks of bees that are more resistance to diseases and parasites, are better at overwintering in specific climates, and are productive and gentle.
The honey bee queen is responsible for producing all the workers in the colony, and she lays up to 1500 eggs a day. Poor quality queens can severely impact colony health. Queens with low egg-laying capacity can limit the numbers of health workers produced, while unhealthy queens can die or be killed by workers, causing a break in brood rearing that again limits colony growth and productivity. Poor quality queens are consistently cited by beekeepers as a major factor underlying colony failure, and a longitudinal study of colonies indicated that loss of a queen or lack of laying by a queen was one of the two factors linked to colony loss. Several factors seems to impact queen quality, including rearing conditions and mating number.
Proper management of honey bee colonies is a critical component of their health and productivity. Many of the stressors listed on this page can be mitigated by using the proper techniques. Beekeepers need to place their colonies is appropriate locations, which allow access to adequate foraging sites and are distant from areas where pesticides are being applied (honey bees can forage up to 5 km away from their colonies). Beekeepers can provide supplementary nutrition, in the form of sucrose solution or protein patties, during periods of low nectar flow. Beekeepers can monitor for pests, such as Varroa mites and Nosema microporidia, and use chemical or non-chemical methods to control these as needed. Beekeepers can minimize exposure to pathogens, such as viruses and bacteria, by systematically replacing used brood comb with fresh comb. Using genetic stocks of bees that more resistant to pests and pathogens is also an excellent way to reduce complications from these two stressors. Finally, rapid supercedure of poor quality queen honey bees can lead to colony losses, and thus purchases queens from excellent sources or rearing queens locally can improve colony productivity and health. For more information, please see the Honey Bee Best Management Practices guide that has been developed by the Managed Pollinator Coordinated Agriculture Program (CAP) and Project Apis m (PAm).