WASHINGTON — There were 2,631,220 honey bee colonies at operations with five or more colonies on Jan. 1, 2018, down slightly from 2,641,090 a year earlier, the U.S. Department of Agriculture said in its annual Honey Bee Colonies report released Aug. 1.

Colonies at operations with five or more colonies on April 1 totaled 2,692,660, also down slightly from 2,694,150 on April 1, 2017, the U.S.D.A. said.

Honey bee colonies lost during the January-March quarter of 2018 were 425,220, or 16%, of total colonies, while 512,940 colonies were added during the quarter. There were 270,000 colonies, or 10%, lost during the April-June period of 2018, while 725,650 colonies were added.

In 2017, January-March colony losses were 398,650, or 15% of the total, while 478,240 colonies were added. In the April-June period of last year, 285,590 colonies, or 11%, were lost, and 613,360 were added. In July-September, 394,810 colonies, or 13%, were lost, and 284,370 were added. The October-December period saw the largest quarterly loss at 424,860 colonies, or 15% of the total, while 204,510 were added. The April-June period saw the smallest number of colony losses in 2017 and the highest number added.

Varroa mites were the top stressor for operations with five or more colonies last year, the U.S.D.A. said, with 55% affected in the October-December period, the highest of any quarter in 2017. There were 40.8% of colonies affected during the January-March period of 2018 and 53.4% during April-June of this year.

There were 77,770 colonies lost to colony collapse disorder symptoms from January through March 2018, up 15% from the same quarter last year, and 48,190 colonies lost in April-June, up 46% from a year earlier.

Disease stressors affected 4.3% of colonies during the January-March period of 2018 compared with 7% in the same period last year. April-June disease stressors were 11.4% this year and 4.6% last year. Pesticide stressors affected 10.3% of colonies in January-March 2018 compared with 8.9% in 2017, and 13% in April-June 2018 compared with 12.3% in the same period last year.

The number of colonies in each state changes by quarter as bees are moved seasonally to pollinate various crops, although California always has the largest number of colonies due in part to the large number of nut and fruit trees. California is the nation’s largest producer of almonds, pistachios and English walnuts.

During the January-March period of 2018, California had 44% of all honey bee colonies (operations with five or more colonies), followed by Florida with 9%, Texas with 8%, Idaho with 6% and Georgia with 5%. During the April-June period, the top five states in order were California, Texas, Florida, Georgia and North Dakota. During July-September of 2017 the top five states were California, North Dakota, Florida, Montana and South Dakota, and in October-December the top five were California, North Dakota, Florida, Texas and Georgia.

The U.S.D.A., as well as public institutions and some private entities, continue work to stabilize honey bee numbers.

“Bee populations in North America have been in decline since the 1940s,” the U.S.D.A.’s National Institute of Food and Agriculture (N.I.F.A.) said in a July 17 report. “This is of great concern to the agriculture industry because about 75% of specialty crops depend on the services of pollinators, of which bees are the most economically important.”

Honey bees contribute about $15 billion in crop value, the N.I.F.A. said. The agency has invested nearly $50 million since 2008 in research to investigate the reasons for declining bee populations, promote pollinator health, reduce colony losses and restore pollinator habitats. Honey bee projects and studies are underway at Michigan State University, the University of Minnesota, Washington State University, the University of Maryland and others.