Effects of different pesticides and pathogen exposure on hemocytes in honeybee larvae (Apis mellifera)
Department of Zoology, Karl-Franzens-University Graz, Universitätsplatz 2, 8010 Graz; Austria
Honeybees (Apis mellifera) have developed an effective immune system, which differs significantly from the immune system of vertebrates, e.g. in the lack of antibody mediated defense. The innate immune system of insects is composed of humoral and cellular defense mechanisms. In the cellular defense mechanism, honeybees have 4 types of hemocytes within their hemolyph i.e., prohemocyte, plasmatocyte, granulocyte and oenocytoid. We evaluated whether pesticides and/or Paenibacillus larvae (Pl), a spore-forming bacterium causing American foulbrood, affect the cellular immune system. For this purpose we studied effects of different pesticides, such as dimethoate, clothianidin and fluvalinate on the total and differential hemocyte number in honeybee larvae. For dimethoate, the LD50 of adult bees, for clothianidin the 8-fold LD50 of adult bees was employed. Additionally, the influence of Pl spore infection, responsible for 50% mortality, on the above mentioned parameters was assessed as well as a combination Pl/pesticides. An in vitro bee larval rearing method was employed, where doses of pesticides were orally administered to the larvae within 4 days. In this treatment we observed a 50% increase in total hemocyte counts. The results point out that total hemocyte number increases highly significant under treatments with doses of dimethoate [120 ng/larva], clothianidin [32 ng/larva] and Pl. Our results further confirm a significant alteration of the number of differential hemocytes in pesticide-treated groups as compared to the control. This study shows that pesticide exposure has a significant effect on the recruitment of a cell-specific immune response in honeybee larvae.