A curious case: caddisfly cases built from brick and sewage overflow microplastics

Caddisflies larvae have been constructing cases made from vegetation or sediment grains for millions of years, but in the Anthropocene many alternative building materials have become available for caddisfly larvae. We collected caddisfly cases in three distinctive locations in The Netherlands, differing in degree of urbanization and pollution from sewage overflows. This resulted in a total of 1121 caddisfly cases, of which 311 presumably contained artificial material. Our results show that microplastics (MPs) from sewage overflows, along with littered bricks -whether from masonry waste in urban areas or historic dumping for streambank protection- affect how caddisfly larvae construct their cases. In the most polluted stream, more than half of all cases were found to include artificial material. The inclusion of masonry waste and MPs by the larvae as building and ballast material might alter the specific weight of the cases, which may influence the caddisfly larvae's functioning and behaviour. This is the first time a sewage overflow is directly linked to the inclusion of MPs in cases by caddisfly larvae and actions should be taken to minimize pollution from these overflows. Especially the MP load of sewage overflows may impact both the aquatic and the terrestrial ecosystem, as caddisfly larvae have a key role in the food chain and may transport these plastics into a broad and diverse food web. Sewage overflow events are expected to increase due to climate change, and it is expected that there will be three times as much human-made mass than dry biomass on Earth in 2040. This will, besides many other effects, impact caddisfly case construction in an even more dramatic way than is the case today.