Combined effects of Nosema ceranae and imidacloprid insecticide on africanized honeybees Apis mellifera

The Apis mellifera honeybees are importants pollinators both for native and cultivated plants. However, the contact with the agroecosystems may expose these insects to toxic substances, such as imidacloprid insecticide. Compound systemicity may lead to the contamination not only of forager honeybees, but also of other colony individuals, including larvae, through both pollen and nectar consumption containing imidacloprid residues. Thus, the first stage of this work evaluated the imidacloprid toxicity to both larvae and adult honeybees of hybrid Africanized A. mellifera. Results showed that larvae are more tolerant to imidacloprid than newly emerged honeybees, and alterations in larvae development were only observed at concentrations eight times higher than the average lethal concentration established for adult honeybees. Since not only insecticides, but also diseases such as nosemosis may weaken colonies and compromise its viability, the second stage of this work was the evaluation of the exposure effects of Africanized A. mellifera workers to imidacloprid insecticide and/or to honeybees infection by Nosema ceranae in order to understand how these factors act both isolated and together on organism of these insects. Honeybees survival decreased when they were exposed to insecticide and/or infected by parasite, and the combinations of these two stressors further compromised their longevity. Both N. cenarae as imidacloprid, either alone or in combination, led midgut cells of honeybees to stress, evidenced by HSP 70 chaperone immunostaining, and to cell death, probably by macroautophagy, as indicated by genic expression decrease of caspase-3. In the midgut, glutathione-S-transferase tissue activity was not altered, probably due to damage found on the epithelium that may affect both metabolization and biotransformation function. Once midgut was compromised, probably more quantity of insecticide reached hemolymph, leading cells from Malpighian tubules, organ responsible for toxic substances excretion from organism, to cell death. In the brain, the insecticide altered mushroom body synaptic activity, as verified by synapsin immunostaining, and it also caused cellular stress and increased of glial cells, which may indicate nerve tissue injury. Insecticide neurotoxic action also caused an increase in the activity of acetylcholinesterase and carboxylesterase enzymes in the heads of the honeybees. Altered synaptic activity and increased cellular stress in mushroom bodies were also observed for honeybees infected only by N. ceranae, indicating response to parasitism. Additionally, pathogen-insecticide combination and the stressors, when isolated, caused immunosuppression, reducing both the hemocytes circulating number in hemolymph and transcription of genes related to individual and social immune response. Based on results from this work, we may conclude that combination between N. ceranae and residual concentrations of Imidacloprid affected honeybees survival, due to insecticide action on their target organ (brain), impairment on digestory and excretory functions and honeybees imunity decreased. (AU)