Effect of Comb Cell Width on the Activity of the Proteolytic System in the Hemolymph of Apis mellifera Workers

Simple Summary Honeybees are threatened by many biotic factors, e.g., microbes and parasites such as Nosema spp. and Varroa mites, or predators, as well as environmental factors such as weather conditions, pesticides, or food contaminants. Honeybee colonies have behavioral defense mechanisms against pathogens resulting from their social lifestyle. Moreover, these insects have innate immune barriers. One of the immune defense elements is the proteolytic system consisting of proteases and their inhibitors in the hemolymph (insect blood) and on the body surface. The proteolytic system is activated by both external and internal environmental factors. An important element of the nest environment is the comb. In feral bee colony nests, the bee comb cell size varies in width. In turn, bee colonies in Europe tend to be kept only on standard- (cell width approx. 5.50 mm) or small-cell (cell width approx. 4.90 mm) combs. We assessed the proteolytic system activity in the hemolymph of workers reared in a small-cell comb and a standard-cell comb in colonies kept simultaneously on standard- and small-cell combs. Simultaneous keeping of a colony on standard- and small-cell combs is a novel approach to the use of small-cell combs in beekeeping. The width of comb cells (small or standard) where workers were reared had a significant effect on the protein concentrations and the activities of proteases and protease inhibitors in hemolymph of workers. The protein concentrations in 1-day-old workers were higher in bees reared in small-cell combs than in those reared in standard-cell combs. The opposite was found in the groups of older bees (aged 7, 14 and 21 d). Moreover, the activities of proteases and their inhibitors in 1-day-old workers were always higher in bees reared in standard-cell combs, whereas opposite results were usually obtained in the group of the older workers. The differences between workers reared in the small-cell combs and those from the standard-cell combs may be associated with their different tasks. Workers reared in small-cell combs probably work outside the nest as foragers, whereas those reared in standard-cell combs work in the nest. However, this assumption requires confirmation. Abstract This study is a continuation of the innovative research of the impact of rearing of bee colonies simultaneously on standard- and small-cell combs on the traits of worker bees and bee colonies. Its aim was to compare the activities of proteases and their inhibitors in the hemolymph of workers reared in a small-cell comb (SMC) and a standard-cell comb (STC) in colonies kept simultaneously on standard- and small-cell combs. The width of comb cells in which workers are reared has a significant effect on the protein concentration and the proteolytic system in the hemolymph, which is reflected in the activities of proteases and their inhibitors. The protein concentrations in the 1-day-old workers were always higher (p ≤ 0.05) in the SMC than STC workers. The opposite was found in the older bee workers (aged 7, 14 and 21 d). The activities of proteases and their inhibitors in the 1-day-old workers were always higher (usually significantly at p ≤ 0.05) in STC than SMC workers, and opposite results were observed in the groups of the older workers (aged 7, 14 and 21 d). The differences between the workers from small-cell combs and those reared in standard-cell combs may be related to their different tasks. Workers reared in small-cell combs probably work as foragers outside the nest, whereas bees reared in standard-cell combs work in the nest. This hypothesis requires confirmation. To reduce the impact of accidental determinants on the results of single-season research on honeybees, it is advisable that such investigations should be conducted for several consecutive years.