Western honeybees are known to have more than 80 different parasites. Under typical conditions the honey bee's two immune systems are able to control infections. The first is the individual immune system, which is based inside the body of each bee and relies on immune molecules and cells in a similar manner to that of humans. The second immue system is called social immunity and is based on group behaviours. Bees will clean themselves or other sick bees and they may remove infected or dead brood from the hive.
As honey bees were moved around by humans they picked up new parasites which are not effectively recognised by their immune response. The lack of immune response allows these parasites to spread easily and cause so much damage that the colony eventually dies.
At the top of the list of harmful pests is the varroa mite, which mainly infects the brood of bees. The mites latch onto honey bees and suck their blood. This weakens the bee and also transfers other viruses and bacteria. Another more recently established pest of honeybees is the small hive beetle, whose larvae burrow and tunnel through combs consuming brood, pollen and honey. They also produce a repellent slime so bees do not remove them from the hive. As the combs are damaged honey starts to ferment and colonies collapse and die within weeks.
Industrial agricultural crops rely on chemicals to secure a successful harvest. To protect these monocultures from pests, toxic chemicals are applied to avoid crop losses. Modern pesticides are often systemic—they are not applied to a crop when it gets infected, but are continuously present within the plant, including in their pollen and nectar.
Modern agricultural pesticides such as neonicotinoids are highly toxic to honeybees. Unfortunately, they have been deemed harmless because they are systemic and bees are not directly exposed to them. Instead, honey bees pick up small doses of pesticides when they forage for food. Recent research shows that even small exposures of these substances found in nectar and pollen have sublethal effects on bees. The bees are not killed, but the pesticides negatively affect their brain, communication skills and the performance of colonies. Even worse, cocktails of different chemicals amplify such damaging effects on bees.
The honeybee colony is an incredibly complex social system that can adapt and cope with an amazing number of disturbances. However, continuous exposure to stress can result in serious health problems. Once a certain threshold has been reached, the colony is unable to recover and will collapse. Modern bees are exposed to a wide range of different stresses, such as novel parasites and pesticides as described above. However bee colonies can also experience nutritional stress caused by a lack of sufficient food from flowering plants or monocultures, droughts caused by global climate change, or inbreeding.
Furthermore, modern beekeeping requires honey bees be transported thousands or tens of thousands of kilometres each year to get them to pollinate flowering crops in different regions. However, the stress from traveling, loading and unloading can negatively impact a honey bee's immune system. Consequently, diseases that would be controllable under typical conditions can be extremely harmful to a stressed colony.