Bees around the world are dying off, a mysterious phenomenon that has been dubbed colony collapse disorder. Although a lot of potential causes have been theoretically linked to these mass die-offs, fewer direct links have been conclusively established. Two new studies, however, have shed some additional light on the subject.

The first study, published this past July in the journal PLoS One, linked a mix of pesticides and fungicides to bees' inability to fight off a common gut parasite called Nosema ceranae. The bees encountered these chemicals while pollinating agricultural crops, including blueberry, cranberry, cucumber, pumpkin and watermelon, and then linked the collapse of a colony to the pollen the bees carried back to their nests. Interestingly, the study found that bees often collected pollen not from the crops themselves but from other nearby flowers.

The study contrasted with the school of the thought that fungicides did not harm bees. "While fungicides are typically seen as fairly safe for honey bees," the authors wrote, "we found an increased probability of Nosema infection in bees that consumed pollen with a higher fungicide load. Our results highlight a need for research on sub-lethal effects of fungicides and other chemicals that bees placed in an agricultural setting are exposed to."

The second new study had less to do with colony collapse disorder than it did bee mortality in general. The study, published on Oct. 3 in the journal Scientific Reports, found that flowers exposed to diesel exhaust pollution had a smell that was different enough from normal that honeybees could not recognize the flowers. "Honeybees have a sensitive sense of smell and an exceptional ability to learn and memorize new odors," lead researchers Tracey Newman from the University of Southampton said in a press release. "Our results suggest that that diesel exhaust pollution alters the components of a synthetic floral odor blend, which affects the honeybee's recognition of the odor. This could have serious detrimental effects on the number of honeybee colonies and pollination activity."

The third study, funded by Bayer, disputes previous studies that have linked a group of pesticides called neonicotinoids or NNIs to colony collapse disorder. Bayer CropScience makes a NNI called clothianidin, which is used on corn, soy and other agricultural crops. NNIs were temporarily banned by the European Union earlier this year after studies linked them to mass bee deaths. Canada is considering taking the same action. Bayer, however, says NNIs are safe for bees and has submitted a study to Health Canada saying as much. Critics, however, told Canada's Global Post that the Bayer study was "outdated, simplistic and uninformative." Bayer has until 2015 to redo and resubmit the study.

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