Abstract
Background:
Thiamethoxam, a neonicotinoid insecticide known for its harmful effects on bees and other pollinators, was banned in the European Union in 2013 because of its proven environmental risks. In Morocco, however, this compound is still used in agriculture and may contribute to the decline of honeybee populations.

Aim:
This study aimed to assess whether environmentally relevant doses of thiamethoxam affect the survival of honeybee (Apis mellifera) workers over a 10-day exposure period.

Methods:
Queenless worker microcolonies were established using 20-day-old bees and orally exposed to sucrose syrup containing increasing doses of thiamethoxam (0, 1, 10, 20, 50, and 70 ng/bee).

Results:
Bee mortality increased in a clear dose-dependent manner. At doses of 20 ng/bee or higher, 100% mortality was observed within 48 h, compared with 16.67% in the control group. The LD₅₀ values decreased over time (from 10.65 ng/bee at 24 h to 4.18 ng/bee at 72 h), indicating a time-dependent expression of thiamethoxam toxicity in honeybees.

Conclusion:
Overall, these results show that thiamethoxam is highly toxic to Apis mellifera and that high exposure levels can severely compromise colony survival in contaminated environments. The reduced reliability of LD₅₀ estimates at later time points is likely due to mortality saturation at higher doses, limiting the range of intermediate responses required for robust probit modelling.

Key words: Apis mellifera; Honey bee; LD₅₀; Neonicotinoid; Thiamethoxam.