The decline of honeybee colonies has slowed slightly since last fall, but the Agriculture Department says a mysterious combination of ailments is still affecting honeybee populations. Research at USDA’s Agricultural Research Service Bee Research Laboratory in Beltsville, Md., found that honeybee colonies declined by 29 percent between September 2008 and early April. That’s an improvement over the last two years, when researchers found that 32 percent and 36 percent, respectively, of all beekeepers surveyed lost hives. Domestic honeybee stocks have been waning since 2004 due to colony collapse disorder, which causes adult bees to inexplicably leave their broods. Bees now appear also to be suffering from other ailments. Jeff Pettis, USDA’s top bee scientist, cautions that bees are still dying at rates that could put some keepers out of business. “While the drop in losses is encouraging, losses of this magnitude are economically unsustainable for commercial beekeeping,” Pettis said.