Populations of four of the eight common species of bumblebees in the United States are in steep decline, falling by up to 96 percent in recent decades, according to a study by researchers at the University of Illinois. The study, which compared present-day estimates of bee abundance with historical data collected by museums, also showed that the geographic ranges of the 
four species had contracted by 23 to 87 percent. The study, published in the Proceedings of the National Academy of Sciences, said the likely causes of the steep drop in bumblebees were disease, increasing use of pesticides, and low genetic diversity among bee populations as farmers rely on a small number of species to pollinate their crops. The sharp drop in bee numbers could have profound implications for the pollination of wild and farmed plants, as bees pollinate 90 percent of the world’s commercial plants, including most fruits, vegetables, and nuts. The study said that the four severely affected bee populations had higher infection levels of a pathogen called Nosemba bombi, which has taken a heavy toll on bee populations in Europe and has now reached the U.S.
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