Impact of sublethal doses of thiamethoxam and Nosema ceranae inoculation on the hepato-nephrocitic system in young Africanized Apis mellifera

We analyzed the morphological changes on the hepato-nephrocitic system (HNS) of Africanized honey bees exposed to thiamethoxam (TMX) and to Nosema ceranae (NOS) in isolation and co-exposure. We also analyzed the relative amount of hemocytes in TMX and/or Nosema-exposed bees. Newly emerged bees were exposed individually per os to TMX, at doses of 0.0856 ng/bee or 0.00856 ng/bee, or to TMX doses and 60,000 spores of N. ceranae per bee. Eight days after the single exposure, the results showed that TMX was extremely deleterious to the HNS at 0.00856 ng/bee, dramatically inducing morphological changes in HNS cells (trophocytes and oenocytes) and increasing hemocyte number. However, the treatment to TMX at 0.00856 ng/bee with additional NOS inoculation provoked the highest critical damage observed on HNS, collapsing the trophocytes, reducing the surface area of oenocytes, and inducing cell death in pericardial cells. Therefore, a sublethal dose of TMX, applied alone or simultaneously with NOS inoculation, triggered the disruption of the HNS and induced progressive damage on worker bees. The results demonstrated an interaction between biotic and abiotic agents in Africanized bees at individual level, whose impact at the colonial level remains to be assessed under field conditions.