Like most insecticides, imidacloprid is highly toxic to bees, with a contact acute LD50 = 0.078 μg a.i./bee and an acute oral LD50 = 0.0039 μg a.i./bee. Imidacloprid was first widely used in the United States in 1996 as it replaced 3 broad classes of insecticides. In 2006, U.S. commercial migratory beekeepers reported sharp declines in their honey bee colonies. This has happened in the past; however, unlike previous losses, adult bees were abandoning their hives. Scientists named this phenomenon colony collapse disorder (CCD). Reports show that beekeepers in most states have been affected by CCD. Although no single factor has been identified as causing CCD, the United States Department of Agriculture (USDA), in their progress report on CCD, stated that CCD may be "a syndrome caused by many different factors, working in combination or synergistically."