Fumigilin B Every hive. Healthy bee are normally able to deal with Nosema. It is present to a degree in many hives. Only when the overall health of the bees declines does it become a problem. Along with treatment for Nosema, look for stressors that may have weakened the hives. Excessive moisture, Mite or SHB loads. ( I know a little early for the later at your latitude) But something in in the mix for the preponderance of the yard to be struggling with it.