Toxicity and Morphological Alterations Caused By Azadirachtin in the Stingless Bee Nannotrigona testaceicornis Lepeletier, 1836 (Insecta: Hymenoptera: Apidae: Meliponini

Abstract

Nannotrigona testaceicornis are essential pollinators for biodiversity maintenance and agricultural production. However, during foraging in agroecosystems, they may be exposed to residues of bioinsecticides used for pest management. Thus, this study evaluated the toxicity of the commercial product AzaMax® (azadirachtin) to the stingless bee N. testaceicornis. Adult worker bees were exposed to this bioinsecticide by contact and ingestion at concentrations recommended for strawberry crops, and mortality was assessed after 24, 48, and 72 h. Morphological alterations of the midgut and changes in chromatin structure in brain cells were evaluated in the surviving stingless bees. Exposure resulted in low mortality, precluding the estimation of the LC₅₀, but revealed significant sublethal effects, such as disorganization of the midgut epithelium, loss of digestive cells, reduction of regenerative cells, and disappearance of the peritrophic membrane. An increase in chromatin condensation was detected at different exposure times and concentrations, suggesting disruption of gene regulatory processes. These findings indicate that although azadirachtin does not induce immediate lethality in N. testaceicornis, it may compromise digestive physiology and genomic integrity, potentially affecting individual performance and long-term colony maintenance. The results highlight the importance of considering sublethal parameters in bioinsecticide risk assessments, as they may reveal hidden and ecologically relevant impacts that are not detected by mortality alone. By incorporating both lethal and sublethal endpoints, this research contributes to the development of safer agricultural practices and supports the conservation of stingless bees, which are key pollinators in tropical ecosystems.