A small study at Germany's Landau University found that bees are vulnerable to the electromagnetic radiation as a result of growing numbers of cell phones and wireless communication towers. Therefore, bees would not navigate and return to their hives when mobile phones were placed nearby. Some biologists even assume that bees disappeared as a result of global warming. Global warming may give the rise to pathogens such as the mites,viruses and fungi that are known to cause the death of bee colonies. The unusual hot-and-cold winter weather fluctuations in recent years may also have harmful effects on bee populations because they are accustiomed to more consistent seasonal weather patterns. A recent gathering of leading bee biologists yielded no consensus,but most agree that a combination of factors is likely to blame.