Morphological and toxicological effects of a deltamethrin-based insecticide on aquatic insect larvae: Grumichella boraceia (Trichoptera) as a bioindicator of pyrethroid contamination

Insect pest control is essential in agriculture to prevent losses and increase productivity. However, excessive insecticide use can negatively impact aquatic ecosystems. Deltamethrin, a widely used pyrethroid insecticide affects non-target insects by compromising the midgut and fat body—organs crucial for digestion, detoxification, and energy storage. Trichoptera larvae are sensitive to pesticides in aquatic environments, making them valuable bioindicators. This study describes the morphology and histopathological effects of a sublethal concentration of a deltamethrin-based insecticide on the fat body and midgut of Grumichella boraceia (Trichoptera) larvae. Larvae were collected and maintained in the laboratory, where the 24-hour LC₅₀ was estimated at 0.0037 µg L⁻¹, below the residual levels commonly found in aquatic habitats. The larval midgut epithelium consists of columnar digestive cells with an apical brush border and a central ovoid nucleus with decondensed chromatin, alongside goblet cells with a cavity, brush border, and basal nucleus. Regenerative cell nests are present at the base of the epithelium, with surrounding circular and longitudinal muscle layers. The fat body is well-developed, containing trophocytes with irregular nuclei rich in decondensed chromatin and basophilic cytoplasm with lipid droplets. While the fat body showed no histopathological alterations after deltamethrin exposure, the midgut presented epithelial disorganization, apical cell protrusions, cytoplasmic vacuolization with abundant spherocrystals, and apocrine secretion. This is the first histological description of the larval midgut and fat body in Grumichella , revealing similarities with Lepidoptera larvae. The findings demonstrate that the deltamethrin-based insecticide Decis 25EC ^® is toxic to G. boraceia , impairing its physiology and survival even at sub-residual concentrations.