Modeling long-term effects of contaminants on non-biting midges (Chironomidae)

Dr. Alessandro Manfrin and his team developed a model to assess the long-term effects of contaminants on non-biting midge populations. Their study showed that exposure to the biological larvicide Bti reduces larval densities and adult emergence across multiple generations.

The Study

A team of scientists led by Dr Alessandro Manfrin (RPTU University of Kaiserslautern-Landau, iES Landau) and Dr Gregorio Alejandor López Moreira Mazacotte (Leibniz Institute of Freshwater Ecology and Inland Fisheries, Department of Ecohydrology and Biogeochemistry) (Fig. 1) has recently published a model they developed to assess the potential long-term impacts of contaminants on non-biting midge (Chironomidae) populations. The research was conducted as part of the DFG-funded RTG SystemLink under the supervision of Prof Ralf Schulz (RPTU University of Kaiserslautern-Landau, iES Landau).

Figure 1. Dr Alessandro Manfrin (panel a) is a postdoctoral researcher of the DFG-funded RTG SystemLink, at the iES of the RPTU University of Kaiserslautern-Landau in Landau. Dr Gregorio Alejandor López Moreira Mazacotte (panel b) is a postdoctoral researcher at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, Department of Ecohydrology and Biogeochemistry in Berlin

Contaminants and aquatic insects

Contaminants in freshwater ecosystems pose significant threats to biodiversity, particularly to merolimnic aquatic insects. These insects live as larvae in the water and then emerge as flying adults. They play a key role in both aquatic and terrestrial ecosystems. During their aquatic phase, they contribute to carbon cycling and are a major food source for predators such as fish. After their emergence, adult flying aquatic insects serve as important prey for riparian consumers such as spiders, birds, and bats.

Despite their ecological importance, the long-term effects of contaminants on aquatic insects remain poorly understood, as most research has focused on short-term impacts over only one or two generations. This leaves a large knowledge gap regarding how these stressors can influence aquatic insects across multiple generations, particularly across different geographic regions. Local environmental factors such as temperature and daylight, which regulate insect development and emergence, could, in fact, amplify or mitigate the effects of contaminants on aquatic insects. As a result, the response of aquatic insects to pollutants like insecticides may vary significantly depending on regional environmental conditions, highlighting the need for a long-term analytical approach in ecotoxicology studies.

Modelling contaminant effects on non-biting midges

The published study contributed to fill this gap. Using Chironomus riparius as an aquatic insect model organism and the biological larvicide Bacillus thuringiensis var. israelensis (Bti) as an example of a contaminant, the team developed an age-structured population model to simulate the long-term effects of Bti on C. riparius populations across two years of Bti applications in various geographic regions. Finally, to account for uncertainties associated with the strength of acute Bti exposure events, the researchers assumed two possible Bti application timing scenarios and three possible effect size scenarios based on contrasting empirical results of published studies showing mild (10% reduction), intermediate (50%), and strong (90%) effects of Bti on C. riparius larvae survival.

In all cases, Bti applications led to reduced larval densities and fewer emerging adults, with the effect becoming more pronounced over time, especially when Bti applications took place during periods with prevalence of younger and more vulnerable early-instar larvae (Fig. 2). In fact, the reduction of larval densities due to Bti (Fig. 2) translated into a reduction of emerging Chironomidae already during the first year of Bti application (Fig. 3) followed by a cross-generation reduction of the total number of laid eggs, larvae and emerging insects for the second year.

Figure 2. Difference in percentage of the cumulative emergence density compared to the control for the 1st and 2nd simulated year of Bti application, across the three latitudes. Each line represents a scenario with varying Bti toxicity on the emergence density: control (no reduction due to Bti), mild (10% increase in Bti-related loss rate), intermediate (50% increase in Bti-related loss rate), and strong (90% increase in Bti-related loss rate). We compared scenarios where Bti-applications occurred with prevalence of late-instar (panels a, c, e) or early-instar larvae (panels b, d, f).
Figure 3. Emergence density normalized to the control across is shown across the three latitudes over the 1st and 2nd simulated year of Bti application. Each line represents a scenario with varying Bti toxicity on the emergence density: control (no reduction due to Bti), mild (10% increase in Bti-related loss rate), intermediate (50% increase in Bti-related loss rate), and strong (90% increase in Bti-related loss rate). We compared a scenario where Bti applications occurred when there was a prevalence of late-instar larvae (panels a, c, and e), to a scenario where these applications happened during peaks of early-instar larvae (panels b, d, and f).

These findings highlight the importance of considering long-term ecological effects when using contaminants, as continuous exposure over years could significantly alter population dynamics. Understanding these long-term impacts is crucial for better managing freshwater ecosystems and preserving biodiversity. The published model is flexible and can be tuned and applied to different pollutants, scenarios, systems and species, and can support risk assessment for the regulation of pesticides to minimise negative impacts and potential long-term consequences that micropollutants can cause on natural freshwater systems.

Read the study

Alessandro Manfrin, Gregorio Alejandro L.M. Mazacotte, Jürg W. Spaak, Stephen E. Osakpolor, Carsten A. Brühl, Valeria Lencioni, Sara Kolbenschlag, Ralf B. Schäfer, Mirco Bundschuh, and Ralf Schulz, 2024. Modelling cumulative effects of acute exposure to toxicants on the life cycle of Chironomidae using Bti as a case study. Ecological Modelling, 494. The study can be found here

Acknowledgements

This study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—326210499/GRK2360.