Comparative toxicity of insecticides to the haemocytes of honeybee, Apis mellifera (Hymenoptera: Apidae) under laboratory conditions

The population density of the domesticated bee, Apis mellifera is affected on a large scale by pesticide application and climate changes. Furthermore, contamination of bee flora with insecticide residue is another cause of honeybee mortality. The current study assessed the toxicity of five insecticides, namely bifenthrin (Talstar (R) 10EC), endosulfan (Thiodon (R) 35EC), imidacloprid (Confidor (R) 200SL), ethofenprox (Trebon (R) 30EC) and diafenthiuron (Polo (R) 500SC) against haemocytes of the honeybee, A. mellifera under standard laboratory conditions. Analysis for counts of total haemocytes, differential haemocytes, and abnormalities was determined for larvae, pupae, and adult worker bees of A. mellifera. The haemolymph samples were assessed soon after, 30 and 60 min for selected insecticides applied at a concentration of recommended field doses. The total count of haemocytes was found to be significantly maximum in larval and pupal stages (35818 cells/mm(3), 3387 cells/mm(3)) while lowest (5493 cells/mm(3)) in adult workers, but differential counts of haemocytes remained somewhat insignificant for all insecticides applied. Variation of total haemocytes count of honeybee resulted for all tested insecticides. Results revealed an increase in haemocytes count (6662 cells/mm(3), 7237 cells/mm(3), 7081 cells/mm(3)) of adult honeybees exposed to diafenthiuron, ethofenprox, and imidacloprid soon after application; however, these counts dropped (4506 cells/mm(3), 4262 cells/mm(3), 3481 cells/mm(3), 4325 cells/mm(3)) after sixty minutes of application for all tested insecticides except for bifenthrin when compared with the untreated adult honeybee. While for differential count, granulocytes and plasmatocyte numbers increased in adult worker A. mellifera after the application of insecticides. Haemocytes immune response to insecticides exhibited abnormalities of various degrees, such as denucleation, cell shape distortion and agglutination. The findings indicate that insecticides influence the immunity of A. mellifera by altering of total and differential count of haemocytes, possibly leading to increased mortality and loss of the colony.