peer reviewed sources.

http://www.webbee.org.br/bpi/pdfs/livro_02_richards.pdf

this paper was about the different types of bees and a certain type of bee can pollinate a specific flower that a different species can't. for example bumble bees are good for pollinating blueberry's, cranberry's, and tomato's. bees can be temperamental depending on the weather they chose to fly in. Honey bees prefer warmth vs cold. It hasn't been proven but a theory to why bees are dying while on farms is because of the pesticides sprayed. It mentions why bees disappear and conditions that affect them greatly, from there being less flowers and plants, to the human activity not giving enough free open land for bees to live naturally.

http://www.nal.usda.gov/ref/disappearing.pdf

this is a survey of the bee population in the united states I believe in the 1970's
and the disappearing disease.
the bees that would go pollinate fields would disappear and never return during the cold weather. a large majority of the bees would die during spring when it rains and is cold, luckily the summer helped revive the population, this is because of the warm weather and heat allowing them to grow. however some populations needed help recovering, and the hives that didn't need assistance weren't producing any more honey than the other hives.