Over the past decade, beekeepers in the U.S. and other countries have had problems keeping their bees healthy. Some of the potential causes of their problems include the Varroa destructor mite, pesticides, pathogens and the reduction of floral resources due to land development or conversion.

To make matters worse, there has been a lack of useful, long-term data sets on the size of honeybee populations, which makes it difficult to quantify actual changes in honeybee abundance and to determine what causes population declines.

To help remedy this problem, Dr. Steven Highland at Utah State University and Dr. Rosalind James with the U.S. Department of Agriculture's Agricultural Research Service studied the few long-term data sets available — three in total — to see if they contained useful information. Their findings were recently published in Environmental Entomology.

"We cannot tell if colony numbers are declining if we don't have prior knowledge of their previous numbers," Highland said. "Our article identifies both the utility and limitations for each data set and describes, for the first time, how these data sets can be used together to elucidate how the colonies are being used in each state, where the population declines are happening and where there are actual colony number increases."

The three data sets they studied included: (1) a yearly survey of honey producers in each state by the USDA's National Agricultural Statistics Service (NASS); (2) a quinquennial "Census of Agriculture," which is a census of U.S. agricultural commodities also performed by NASS, and (3) data sets from states that require honeybee keepers to register their colonies annually.

"We found the USDA 'Census of Agriculture' provides the most accurate long-term data set, even though the data are taken only once every five years," Highland said. "As this data set has rarely been used for quantifying changes in honeybee abundance, we hope our assessment will help researchers better identify and quantify the most important causes for change in bee abundance and availability."

While Highland and James found that the data sets can indeed be useful for measuring honeybee abundance changes over the years, they acknowledged some limitations of the data.

"Which of these long-term data sets, or which combination of data sets, are best used for evaluating changes in honeybee colony numbers depends on the location, nature and purpose of the study," they wrote. "For all three, it should be acknowledged that none identifies the number of healthy colonies that were split into two or more. Also, while some degree of colony loss can be determined using these data sets, none actually counts the numbers of lost colonies. Furthermore, the data do not include information concerning how the colonies were used in any given year," such as for pollination, honey production, queen rearing, etc.

"More detailed data on colony gains and losses would improve our ability, in the future, to assess the impact that different human and environmental factors have on honeybee health and productivity," Highland and James said.

The full article, "The Similarity & Appropriate Usage of Three Honey Bee (Hymenoptera: Apidae) Datasets for Longitudinal Studies," is available at http://dx.doi.org/10.1093/ee/nvv227.

Environmental Entomology is published by the Entomological Society of America, the largest organization in the world serving the professional and scientific needs of entomologists and people in related disciplines. Founded in 1889, the society has nearly 7,000 members affiliated with educational institutions, health agencies, private industry and government.