For some background on how exactly bees use honey and nectar, the University of Florida Extension has a nice webpage here. Honeybees feed on nectar from flowers and convert that into honey. Nectar is the main source of carbohydrates for bees, and also provides them with some essential minerals. Honeybees feed on honey throughout the winter in their hive when they cannot collect nectar from flowers. Honeybees also collect pollen from flowers and bring it back to the hive in pollen baskets on their legs (you may have noticed the bright yellow or orange bumps on their back legs where these are located). Bees then store the honey long-term after adding enzymes from their glands to it to prevent bacterial growth. This modified pollen is often called "bee bread." Larval bees and newly emerged adults feed on the bee bread to acquire essential proteins to their development. Colonies require 15-55 kilograms of pollen a year! Different plants have different amounts of nutritional components in their pollen, so not all are nutritionally equivalent to bees.
Scientists were interested in studying the effects of honeybee diet on their resistance to pesticides, so they fed miticides (pesticides commonly used by beekeepers for harmful bee mites) to honeybees, and examined differences in their RNA after 7 days. They found the RNA differed in expression of genes linked to immunity, detoxification, and nutrition. The scientists then tried feeding some bees with a natural pollen diet, and others with artificial diets: soy protein or a no-protein diet. They then exposed the bees to a lethal dose of a common crop pesticide, and found honeybees that were fed the pollen diet had lower mortality and sensitivity to the pesticide.
Since humans have been changing our agricultural practices and increasing urbanization, it is likely that bees are not finding as many pollen sources and could be nutritionally-stressed more of the time now than in the past. This could make them more sensitive to the effects of the pesticides. Christina Grozinger, one of the scientists who worked on this study, said, "If we can figure out which diets and which flowering plants are nutritionally optimal for honey bees, we can help bees help themselves."