Aquatic Life
Norwalk River Case study points to declines in sensitive aquatic insects as neonic levels rise.
The CT Department of Energy and Environmental Protection (CTDEEP) has tested the Norwalk River for insect life–for macroinvertebrate abundance and richness–periodically over the last 30 years. The US Geological Survey (USGS) has tested the Norwalk River monthly, with some gaps, for the last 30 years for pesticides, including the neonicotinoid, imidacloprid. Unfortunately, no studies have been designed with the specific goal of evaluating the effects of neonicotinoids on aquatic macroinvertebrate communities in Connecticut. Consequently, the data from the Norwalk River for aquatic macroinvertebrates and for imidacloprid concentration are not matched in time and space–they weren’t collected at the same times or from the same places–making evaluation of the potential effects of imidacloprid on macroinvertebrates particularly challenging. Nonetheless, these remain the best data in Connecticut to conduct a preliminary evaluation of potential effects of imidacloprid on aquatic fauna. (UConn, Presley, 2025)
The overall abundance of insect life has not changed significantly through time in the Norwalk River and the diversity of species has even increased as efforts to improve water quality in the river have been somewhat successful over the last 30 years. In contrast, the insects that are very sensitive to neonics, like the mayflies that trout and other fish rely on for food, have shown marked declines. Mayfly abundance in 2020 was only one quarter of that observed in 1989, while mayfly richness was only one third of that observed in 1989 (Figure 8). For macroinvertebrate abundance to remain relatively stable and for macroinvertebrate richness to increase through time, other species of macroinvertebrate must have become more pervasive or more abundant to offset the losses associated with mayflies.
Figure 8 Temporal trends in macroinvertbrate and mayfly abundance and richness in the Norwalk River. Macroinvertebrate abundance does not exhibit a significant temporal trend, whereas macoinvertebrate richness significantly increased through tine. In contrast, mayfly abundance and richness significantly decreased through time. See Uconn Report.