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Fipronil is an insecticide widely used to control a great number of pests, thus the aim of this study was to determine the lethal dose and lethal concentration (LD(50) and LC(50)) of this insecticide to the stingless bees Scaptotrigona postica Latreille, 1807. The LD(50) and LC(50) values obtained after 24 h of exposition were of 0.54 ng a.i./bee and 0.24 ng a.i./μL diet, respectively. These values were considered highly toxic to stingless bees.

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The stingless bee Scaptotrigona postica is an important pollinator of native and cultivated plants in Brazil. Among the factors affecting the survival of these insects is the indiscriminate use of insecticides, including the neonicotinoid imidacloprid. This work determined the toxicity of imidacloprid as the topical median lethal dose (LD50) and the oral median lethal concentration (LC50) as tools for assessing the effects of this insecticide. The 24 and 48 h LD50 values were 25.2 and 24.5 ng of active ingredient (a.i.)/bee, respectively. The 24 and 48 h LC50 values were 42.5 and 14.3 ng a.i./µL of diet, respectively. Ours results show the hazard of imidacloprid and the vulnerability of stingless bees to it, providing relevant toxicological data that can used in mitigation programs to ensure the conservation of this species.

This study demonstrates the toxic effects of fipronil. An increase in the number of pyknotic profiles of Kenyon cells of mushroom bodies was observed even at the sublethal doses of 0.27 ng AI bee(-1) and 0.24 ng AI µL(-1) in the topical and oral treatments respectively. Also, differences in the number of pyknotic profiles were dose and time dependent.