Acute fipronil toxicity induces high mortality rate for honeybees and stingless bees, with the latter facing heightened risk

Fipronil is an insecticide used against agricultural pests. It disrupts the central nervous system of insects, leading to hyperexcitation, paralysis, and eventual death. Despite being banned in many countries, Brazil extensively employs diverse commercial formulations of fipronil across various crops visited by bees. There is suspicion linked to honeybee colony losses and potential impact on the wild Brazilian bee population. This study aimed to assess the acute toxicity of fipronil on the Africanized honeybee species Apis mellifera and the stingless bee Scaptotrigona bipunctata. The latter represents native Brazilian stingless bees and is highly valued by stingless beekeepers. We conducted oral and topical exposure analyses to determine LC₅₀ and LD₅₀, respectively. We also evaluated food intake with syrup contaminated by varying fipronil doses using linear mixed models. The results demonstrated the high toxicity of fipronil to both bee species. In line with similar studies comparing exotic (A. mellifera) and native stingless bee species in Brazil, our findings indicate that S. bipunctata is less tolerant to pesticides (fipronil) than A. mellifera. As such, S. bipunctata was approximately 6.58 (LC₅₀) and 3.30 (LD₅₀) times more vulnerable than A. mellifera. We did not find significant differences for food intake. The extensive use of fipronil in Brazilian agriculture raises pressing concerns for honeybees and stingless bees. Great efforts are needed for conservation strategies and regulatory actions to safeguard both pollinator species (as well as wild bee population), as these managed bees provide significant income for most smallholders in rural areas.