Evaluation of the distribution and impacts of parasites, pathogens, and pesticides on honey bee (Apis mellifera) populations in East Africa

Abstract

In East Africa, honey bees (Apis mellifera) provide critical pollination services and income for small-holder farmers and rural families. While honey bee populations in North America and Europe are in decline, little is known about the status of honey bee populations in Africa. We initiated a nationwide survey encompassing 24 locations across Kenya in 2010 to evaluate the numbers and sizes of honey bee colonies, assess the presence of parasites (Varroa mites and Nosema microsporidia) and viruses, identify and quantify pesticide contaminants in hives, and assay for levels of hygienic behavior. Varroa mites were present throughout Kenya, except in the remote north. Levels of Varroa were positively correlated with elevation, suggesting that environmental factors may play a role in honey bee host-parasite interactions. Levels of Varroa were negatively correlated with levels of hygienic behavior: however, while Varroa infestation dramatically reduces honey bee colony survival in the US and Europe, in Kenya Varroa presence alone does not appear to impact colony size. Nosema apis was found at three sites along the coast and one interior site. Only a small number of pesticides at low concentrations were found. Of the seven common US/European honey bee viruses, only three were identified but, like Varroa, were absent from northern Kenya. The number of viruses present was positively correlated with Varroa levels, but was not correlated with colony size or hygienic behavior. Our results suggest that Varroa, the three viruses, and Nosema have been relatively recently introduced into Kenya, but these factors do not yet appear to be impacting Kenyan bee populations. Thus chemical control for Varroa and Nosema are not necessary for Kenyan bees at this time. This study provides baseline data for future analyses of the possible mechanisms underlying resistance to and the long-term impacts of these factors on African bee populations.

Figure 1. Geographic location of surveyed apiaries.

Twenty-four apiaries were surveyed throughout Kenya with an additional three apiaries (25–27), see supplemenatry material, surveyed for ecological effects on colony health. The location and numerical designation of the apiaries is indicated on the map.

Figure 2. Association of Varroa infestation with elevation and colony size.

A. Levels of Varroa mites were positively correlated with elevation, with colonies at higher elevations having significantly higher average numbers of Varroa (r(53) = 0.44, p = 0.001). B. Levels were also positively correlated with colony size ((48) = 0.35, p = 0.013). Varroa counts were converted to logarithmic scale.

Figure 3. Association of viral diversity with colony size and Varroa.

A. Colony size (the number of frames of bees) was not affected by viral diversity (the number of viruses in a colony), H(2) = 2.74, p = 0.254. B. However, colonies with different number of viruses had significantly different numbers of Varroa (H(2) = 13.10; p = 0.0014). Colonies with 1 or 2 viruses had significantly higher Varroa loads than colonies that had no viruses (p<0.05, Wilcoxon pairwise tests, different letters denote significant differences). The number of colonies in each group is indicated at the bottom of each bar. Varroa counts were converted to logarithmic scale.

Figure 4. Association of hygienic behavior with colony location, size, parasite and pathogen loads.

A. There were no significant correlations between hygienic behavior and elevation (r(35) = −0.18, p = 0.289). B. Hygienic behavior was not correlated with colony size (r(35) = −0.22, p = 0.196). C. There was, however, a significant negative correlation between hygienic behavior and the numbers of Varroa in these colonies (r(35) = −0.42, p = 0.011; Varroa counts were converted to logarithmic scale). D. Hygienic behavior was also significantly associated with viral diversity (H(2, 34) = 6.43, p = 0.040); levels were lower in colonies with 1 type of virus versus colonies with 0 or 2 viruses (p<0.05, Wilcoxon pairwise tests, different letter denote significant differences). The number of colonies in each group is indicated at the bottom of each bar.