In this blogpost, Alex Roodt talks about how pesticides which are accumulated by emerging aquatic insects during their development can be retained after metamorphosis and may increase the dietary exposure of terrestrial insectivores.
Midges as pesticide vectors
Insects, which emerge from freshwaters, are an important source of food for terrestrial insectivores hunting at the boundary between the aquatic and terrestrial habitats. During their development in freshwater ecosystems impacted by agriculture, industry or waste water, the aquatic life stages of emerging insects are regularly exposed to a broad range of chemical pollutants. This includes exposure to a wide range of industrial chemicals, pharmaceuticals and pesticides. There is a growing body of work indicating that many of these chemicals can be accumulated by the developing insect larvae or nymphs, and some may even be retained after metamorphosis into a flying adult insect. Emerging aquatic insects can therefore act as vectors of contamination which transport pollutants from the aquatic into the terrestrial habitat. These contaminated insects can thus increase the dietary exposure of terrestrial insectivores to a wide variety of pollutants. In the case of pesticides, this phenomenon has been researched for many chemicals which are no longer permitted to be used (e.g. organochlorine insecticides) but information is missing for most currently used chemicals. This subject was investigated in an experiment conducted within the SystemLink research training group at the Institute for Environmental Sciences of the University of Koblenz-Landau.
Our experimental setup
In the study, the larvae of the non-biting midge, Chironomus riparius, were exposed to a mixture of nine fungicides and herbicides at sublethal concentrations during their development as aquatic larvae. The pesticides were chosen to cover a range of lipophilicities because this property is associated with accumulation of chemicals by aquatic organisms. The concentrations of the pesticides were measured in the larvae shortly before pupation and twice more during the adult life stage of both sexes of the insects. The concentrations were measured in the adults directly after emerging because it was hypothesised that males lose more body mass during metamorphosis and thus could concentrate the retained pesticides to a greater extent than the females. The pesticide concentrations were measured in the adults for a second time after they had completed the terrestrial stage of their life cycle (i.e. they had mated and used up their stored energy reserves). The second measurement was performed in order to test the hypothesis that pesticide concentrations would increase during the terrestrial life stage because of body weight loss as energy reserves are consumed. Additionally, the effect of egg laying was hypothesised to reduce pesticide concentrations in the females.
Bioaccumulation of pesticides in larvae and adults
The results of the experiments revealed that all nine pesticides were accumulated during the larval development and eight of them were still present in the adults after metamorphosis. The average concentrations in the adults were less than or equal to the concentrations in the larvae. A strong effect of pesticide lipophilicity was not found, possibly because of differing rates of metabolism of the individual pesticides. Much to the researchers’ surprise, higher concentrations of four of the pesticides were found in the female compared to the male adults directly after emerging, with the difference ranging from a factor of 2 to 270 (Figure 1). Only two fungicides were found to have up to five times greater concentrations in the male insects. The concentrations of the pesticides in the adults changed in a more predictable manner over the course of the terrestrial life stage. Concentrations in the females were strongly reduced, implying potential loss of pesticides though egg laying while concentrations in males tended to remain similar or increase.
Expect the unexpected
The unexpected results from this study provide new insights into the complexity of the process of pesticide retention through insect metamorphosis and provide researchers with new information in order to refine their hypotheses. Overall, the study provides new information on the potential for dietary exposure of terrestrial insectivores to currently used pesticides after consuming emerging aquatic insects from polluted waterbodies. This information is especially relevant considering the declines in insectivorous bird and bat populations in recent years.
The paper titled “Emerging midges transport pesticides from aquatic to terrestrial ecosystems : importance of compound- and organism-specific parameters´´ was authored by Alexis P. Roodt, Nina Röder, Sebastian Pietz, Sara Kolbenschlag, Alessandro Manfrin, Klaus Schwenk, Mirco Bundschuh and Ralf Schulz and published as open access in Environmental Science and Technology.