New insights into unexplored potential pathway of chemical pollution: Do aquatic insect larvae accumulate pharmaceuticals from contaminated wastewater?

Protecting freshwater habitats is a priority within the EU, with many habitats at risk from pollution, over-abstraction (the excessive removal of water), degradation, and the impacts of climate change. Since the establishment and enforcement of the Urban Wastewater Treatment Directive (UWWTD) in the EU (initially adopted in 1991, with the revised directive entering into force in 2025), it has become clear that a substantial portion of antibiotic, psychoactive, and hormone drug pollution is resistant to wastewater treatment processes, and so eventually enters our waterways.

The build-up of these substances – known as pharmaceutically active compounds (PhACs) – in aquatic environments is known to pose a significant threat to many ecosystems and organisms. Some PhACs are considered hazardous or play a role in the development of microbial resistance to antibiotics, making them high priorities for removal from wastewater. 

Even if concentrations of individual PhACs fall below toxicity limits in our waterways, they can still accumulate in various species – an effect that has been observed in some fish and aquatic macroinvertebrates. The process of PhAC accumulation in aquatic organisms is likely to be dependent on interactions between organism biochemistry and ecology, the surrounding environment, and the physicochemical properties of the contaminants themselves, such as how they dissolve or become encapsulated within cells. However, laboratory and field studies regularly disagree in the amount of bioaccumulation they find, highlighting a lack of predictability in this process.

Research suggests that PhACs can be carried from aquatic to terrestrial environments by winged adults of aquatic insect species, such as mayflies and caddisflies. This is an understudied pathway for PhAC accumulation in terrestrial ecosystems as well as aquatic ones, given the huge numbers of winged insects produced annually in freshwater habitats.

A new study by Czech researchers explores the bioaccumulation of PhACs in aquatic insect larvae and airborne adults. The researchers assess bioaccumulation in three types of insect – the giant caseworker caddisfly Oligotricha striata, several species of Limnephilus caddisflies, and the minnow mayfly Siphlonurus aestivalis – at the larvae, pupae, and adult or subadult stages (referring to a four-day stage of incomplete metamorphosis prior to full maturity, as occurs in mayflies). The insects were reared in laboratory conditions and fed uncontaminated food, with one group of larvae also exposed to treated urban wastewater for up to three months. The larvae had been collected across four non-polluted pools in the Czech Republic, where water showed very low concentrations of a few PhACs.

Up to 49 PhACs were detected in the samples of exposure wastewater, and 15 different PhACs subsequently detected in exposed insects. Both larvae and adult insects took up significant quantities of PhACs from wastewater. The rate of accumulation was dependent on species, life stage, and type of PhAC; PhAC concentrations were higher in larvae than in adults and consistent in larvae throughout the exposure time. Adult insects had lower PhAC concentration at the start of the exposure time, with concentrations later increasing.

In aquatic species, PhACs can bioconcentrate (accumulate from water), bioaccumulate (accumulate from water and food), biomagnify (increase in concentration as contaminated individuals are consumed up the food chain), and bioamplify (become more concentrated as insect larvae either struggle to metabolise or excrete PhACs, consequently losing biomass as they undergo metamorphosis). Bioamplification of some substances, e.g. sertraline, differed significantly between individual insects, highlighting that some species may see a highly variable process from individual to individual. Notably, the study identifies bioamplification for the PhACs sertraline, norsertraline, venlafaxine, and theophylline for the first time in caddisfly species (but not in the mayfly). 

The best predictors of relative PhAC bioconcentration appeared to be the time taken for half of the substance to degrade (known as the PhAC’s biodegradation half-life – a measure of how the PhAC persists in the environment); and how the PhAC accumulates within the insect (known as the bioconcentration factor). 

Overall, the results show that certain PhACs can bioaccumulate and bioamplify across species in both aquatic larvae and aerial adults, albeit inconsistently. The finding supports the possibility of PhACs travelling beyond a contaminated water environment. The researchers call for further research to better understand the dynamics, effects, and accumulation of PhACs in such environments, and to reveal how these substances may spread across ecosystem boundaries. This need is especially pressing given the prevalence and potential impact of PhAC contamination on both aquatic and terrestrial ecosystems.

Reference: 

Let, M., Grabicová, K., Balzani, P., Musil, M., Roje, S., and Bláha, M. (2025) Bioaccumulation of Pharmaceutically Active Compounds from Treated Urban Wastewaters in Aquatic Insect Larvae and Aerial Adults. Environ. Sci. Technol. 2025, 59, 5293−5305.

To cite this article/service:

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.