Deltamethrin insecticide harms aquatic caddisfly larvae, a pyrethroid pollution bioindicator

In a laboratory in southeastern Brazil, caddisfly larvae abandoned their protective silk-and-detritus cases, slowed their movements, and suffered severe damage to their guts after exposure to deltamethrin at concentrations far below levels routinely measured in the environment. A new study, published in the journal Ecotoxicology, provides the first detailed description of the midgut and fat body of the aquatic insect Grumichella boraceia and demonstrates that this little-known species may be among the most sensitive freshwater invertebrates yet tested against one of the world’s most widely used pyrethroid insecticides.

The research, conducted by Álvaro Domingues Ataide and colleagues at the Federal University of Viçosa, focused on a species of long-horned caddisfly described only in 2016. Trichoptera larvae, familiar to stream ecologists as the architects of portable cases, capture nets, and retreats built from silk and debris, occupy a central place in freshwater food webs. Because they respond measurably to environmental degradation, caddisflies are already widely used as bioindicators of water quality. What the new study adds is a cellular-level account of exactly how a commercial pyrethroid formulation attacks a sensitive non-target species, together with a toxicity benchmark that raises uncomfortable questions about the safety margins currently assumed for agricultural runoff.

The commercial product tested was Decis 25EC, a deltamethrin-based insecticide manufactured by Bayer CropScience. Deltamethrin belongs to the pyrethroid class, synthetic compounds derived from the insecticidal principle of chrysanthemum flowers. Pyrethroids act primarily on voltage-gated sodium channels in neurons, keeping them open and producing hyperexcitation, paralysis, and death. They also inhibit glutathione S-transferase, a key detoxifying enzyme in insects. Although the nerve and muscle are the classic targets, the new results show that the gut suffers profound structural damage as well, even when the dose is too low to kill most individuals quickly.

The team’s laboratory bioassay followed standard protocols from the US Environmental Protection Agency and the OECD. Larvae collected from a stream at Fazenda Remanso in Araponga, Minas Gerais, between January and October 2024, were acclimated and reared in glass aquariums fitted with gentle filters and natural stony substrate, at temperatures of 20 to 23 degrees Celsius. Groups of ten larvae, each between one and two centimeters long, were then exposed to four concentrations of the insecticide in dechlorinated water, with four replicates per treatment and a total of 200 individuals. Mortality was recorded every 24 hours for up to 72 hours, and lethal concentrations were estimated with a Probit model.

The results were striking. The 24-hour LC50, the concentration that kills half the exposed animals, came out at just 0.0037 micrograms of active ingredient per liter. That figure is 16.2 times lower than the field-relevant residual level of 0.06 micrograms per liter previously reported in a river of the same basin. In other words, concentrations already documented in the wild would be more than enough to kill a substantial fraction of the larvae within a day. At the pilot concentration of 0.06 micrograms per liter, mortality was a hundred percent.

The comparison with other aquatic arthropods sharpens the picture further. The mayfly Callibaetis radiatus, considered sensitive to pyrethroids, shows a 24-hour LC50 of 0.60 micrograms per liter for deltamethrin, more than 160 times higher than that of G. boraceia. The phantom midge Chaoborus obscuripes, a standard reference species for pyrethroid sensitivity, requires 0.027 to 0.075 micrograms per liter of lambda-cyhalothrin over longer exposure windows. Against these benchmarks, the Brazilian caddisfly stands out as exceptionally vulnerable, and the authors argue that this sensitivity makes it a strong candidate for inclusion in ecological risk assessments and freshwater biomonitoring programs.

Beyond lethality, the bioassay documented a disturbing behavioral signature. Larvae exposed to all four insecticide dilutions abandoned their cases within 24 hours, whereas control animals stayed inside their shelters and aggregated normally. Treated larvae showed reduced motility and a blunted response to mechanical stimulation. Case abandonment has been reported before, notably in the caddisfly Brachycentrus americanus exposed to the pyrethroid esfenvalerate, and it is usually interpreted as an escape response to contamination. The irony is grim: leaving the case increases exposure to predation, so even larvae that survive the chemical may pay an ecological price afterward. Similar reductions in movement and responsiveness have been described in lepidopteran larvae, including the fall armyworm Spodoptera frugiperda, treated with pyrethroids.

To understand the cellular basis of these effects, the team turned to histology. Surviving larvae exposed for 24 hours to the LC50 concentration were fixed in glutaraldehyde, embedded in historesin, sectioned at three micrometers, and stained with hematoxylin and eosin, with additional histochemical tests for total proteins using mercury-bromophenol blue and for neutral polysaccharides using the periodic acid-Schiff reaction. Every treated larva examined showed midgut damage compared with controls.

The normal midgut of G. boraceia proved to be an elegant structure. Its wall consists of a simple columnar epithelium wrapped in circular and longitudinal muscle layers, with a peritrophic matrix lining the lumen. The epithelium is dominated by columnar digestive cells bearing a well-developed apical brush border and a central ovoid nucleus packed with decondensed chromatin, the signature of active protein synthesis. Less numerous are goblet-like cells, identified by an apical invagination forming a cavity lined with its own brush border, and clusters of small regenerative cells nestle at the base of the epithelium, ready to replace worn cells. Histochemistry confirmed abundant proteins throughout the epithelium and carbohydrate-rich vesicles in the apical cytoplasm and brush border.

After deltamethrin exposure, this architecture fell apart. All five treated larvae displayed epithelial disorganization, with apical protrusions bulging from the digestive cells, some of them released as fragments into the gut lumen, and intense cytoplasmic vacuolization, some of the vacuoles corresponding to spherocrystals that failed to stain for protein. The pattern echoes damage reported in caterpillars exposed to deltamethrin and other insecticides, and in the mayfly C. radiatus treated with the same compound. The researchers interpret the increased apocrine secretion observed in treated larvae as a possible detoxification response, since similar secretions in other insects have been linked to enzymes involved in xenobiotic metabolism. Spherocrystals, meanwhile, have previously been associated with detoxification and osmoregulation in wasps and honey bees exposed to contaminants.

One organ remained untouched. The fat body of G. boraceia, organized into a peripheral parietal region and a visceral region hugging the midgut, is composed of trophocytes with irregular nuclei, basophilic cytoplasm, and lipid droplets. After exposure to the LC50 concentration, neither the structure of the fat body nor its protein and carbohydrate content differed from controls. The authors suggest that the fat body may have successfully detoxified the sublethal dose, a capacity documented in mosquito larvae exposed to ivermectin and in honey bees treated with imidacloprid. The contrasting picture, a wrecked midgut beside a pristine fat body, offers a rare anatomical snapshot of where a pyrethroid’s sublethal damage concentrates in an aquatic larva.

The study also carries an evolutionary footnote. Goblet cells in the insect midgut were long thought to be unique to caterpillars, where they pump potassium and support nutrient absorption and detoxification. Finding goblet-like cells in G. boraceia aligns with the only previous Trichoptera histological study, of Limnephilus stigma in 1980, and supports the view that goblet cells are a shared derived trait of the Amphiesmenoptera clade, which unites butterflies and moths with caddisflies as sister orders. The detailed anatomy reported here, the first for any species of the genus Grumichella, doubles as a contribution to comparative insect morphology in a group whose internal biology remains almost unstudied.

The practical message is sobering. Decis 25EC is toxic to G. boraceia at concentrations below those already measured in natural waters of the region, and it damages the gut and behavior of survivors. Because commercial formulations contain adjuvants that are themselves sometimes harmful to non-target insects, the results also underline the importance of testing products as they are actually applied, not just the purified active ingredient. For rivers draining agricultural land across the tropics, a recently described caddisfly may now serve as an early-warning sentinel, its abandoned cases and ravaged midguts signaling contamination long before fish or other vertebrates show effects.