After extensive research on the causes of pollinator decline, researchers have identified several contributing factors that can be grouped into four interacting categories: Parasites, Pesticides, Environmental Change, and Inferior Pollinator Management:
Parasites and Pathogens
Bees can be hosts and vectors to a large diversity of pathogens, and they are particularly vulnerable to disease. Both social and solitary bees can easily pick up parasites and pathogens at flowers that were recently visited by an infected individual, much like a human can pick viruses by touching a door handle recently used by someone with a cold. Furthermore, intensified agriculture requires the availability of large amounts of pollinators for relatively short periods of time when crops are flowering. As a consequence, managed pollinators are shipped across the country to follow the blooms. These migrations bring pollinators as well as pests to new landscapes, introducing novel parasites and pathogens that local populations have never before encountered. Local bees often lack an immune response to these introduced diseases and are particularly susceptible to them.
Industrial agricultural crops rely on chemicals to secure a successful harvest. To protect these monocultures from pests, chemicals are applied to the soil and the plants. 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 many bee species. Unfortunately, they have been deemed harmless because they are systemic and bees are not directly exposed to them. Instead, 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, performance, and immune system. Even worse, cocktails of different chemicals amplify such damaging effects on bees.
As the human population continues to increase, so does urban and agricultural land area. Natural pollinator habitat is decreasing every year, and pollinators are forced into fundamentally different environments. Many suburban, urban, and agricultural lands differ substantially in nesting habitat availability, floral composition, and predator communities. This drastic change in habitats often has negative effects on bee populations. Furthermore, climate change has a strong impact on the ecosystems used by pollinators. For example, early snow melt in some places has resulting in a mismatch of spring flowering times and pollinator emergence times. This can result in plants flowering in the absence of their pollinators, or pollinators emerging before floral resources are available.
Inferior Pollinator Management
As agriculture has intensified across the globe, so has pollinator management. Monocrop agriculture, where farmers plant a single type of crop over vast areas of land, results in massive, but short, flower booms. To ensure all of these flowers get pollinated, farmers typically import honeybees or other managed pollinators for the short duration of the bloom. For example, almond growers in California typically bring in thousands of honeybee colonies each year for only a few weeks while the almond trees flower. This deviation from traditional beekeeping has a wide range of negative effects on both the commodity bees and the local bee populations. The managed pollinators experience high levels of stress, inbreeding, and exposure to hostile environments as they are moved around the country. Native communities also feel the effects of these pollinator migrations in the form of introduced parasites and pests that the honeybees bring with them across the country, as well as heightened competition for floral resources when the honeybees are present.