Several species of American bumblebees are rapidly dying off, and scientists can't figure out the reason as per one new study published today in the Proceedings of the National Academy of Sciences.

The loads of four species have gone down by up to ninety six percent in the last ten to fifteen years according to the lead author of the study, Sydney A. Cameron, an entomologist working at the University of Illinois. She also reported that the bees' geographic ranges have contracted by anywhere from twenty three to eighty seven percent.

It is not merely an academic exercise: Bumblebees can help in pollination of about fifteen percent of all crops grown in the nation, having a value of three billion dollars. They are excellent pollinators, according to the study. This happens due to their long tongues, large body size, and high-frequency buzzing, by which pollen releases from flowers.

Bees also pollinate hothouse crops such as peppers, tomatoes and strawberries, and field crops such as squash, blueberries, watermelon, cranberries and raspberries.

Cameron stated that there can be two reasons for the decline number one can be the spread of a deadly fungus known as Nosema bombi and low genetic diversity, which makes the bees vulnerable to other pathogens and environmental pressures.

The scientists did a three-year country wide study of the changes in the distribution, genetic diversity, and health of as many as eight species of North American bumblebees by tallying data taken from surveys and museum records and found that population of four of the species were declining, and the other four remained steady.

This is totally different from the issue known as colony collapse disorder, a countrywide phenomenon that's the cause of the disappearance of honeybee populations.