Closer to reality--the influence of toxicity test modifications on the sensitivity of Gammarus roeseli to the insecticide imidacloprid

Size- and sex-related sensitivity differences of aquatic crustaceans to imidacloprid

Field collected aquatic invertebrates are often used as test organisms in the refinement of the standard Tier 1 risk assessment of various pollutants. This approach can provide insights into the effects of pollutants on the natural environment. However, researchers often pragmatically select test organisms of a specific sex and/or size, which may not represent the sensitivity of the whole population. To investigate such intraspecies sensitivity differences, we performed standard acute toxicity and toxicokinetic tests with different size classes and sex of Gammarus pulex and Asellus aquaticus. Furthermore, toxicokinetics and toxicodynamics models were used to understand the mechanism of the intraspecies sensitivity differences. We used neonates, juveniles and male and female adults in separate dedicated experiments, in which we exposed the animals to imidacloprid and its bioactive metabolite, imidacloprid-olefin. For both species, we found that neonates were the most sensitive group. For G. pulex, the sensitivity decreased linearly with size, which can be explained by the size-related uptake rate constant in the toxicokinetic process and size-related threshold value in the toxicodynamic process. For A. aquaticus, female adults were least sensitive to imidacloprid, which could be explained by a low internal biotransformation of imidacloprid to imidacloprid-olefin. Besides, imidacloprid-olefin was more toxic than imidacloprid to A. aquaticus, with differences being 8.4 times for females and 2.7 times for males. In conclusion, we established size-related sensitivity differences for G. pulex and sex-related sensitivity for A. aquaticus, and intraspecies differences can be explained by both toxicokinetic and toxicodynamic processes. Our findings suggest that to protect populations in the field, we should consider the size and sex of focal organisms and that a pragmatic selection of test organisms of equal size and/or sex can underestimate the sensitivities of populations in the field.

Development of an Alternative Test System for Chronic Testing of Lotic Macroinvertebrate Species: A Case Study with the Insecticide Imidacloprid

There are currently few suitable test systems for the chronic toxicity testing of aquatic macroinvertebrates under stream conditions. Therefore, a new test system mimicking running water conditions was developed for testing with lotic insects. This system uses small test cages, with 10 of these suspended inside each 25-L container and rotating at 0.1 m/s, to create a water flow for the individual organism inside each cage. To test the performance of the new exposure system, chronic effects (21 d) of the neonicotinoid imidacloprid were investigated with field-collected larvae of the stonefly Protonemura sp. Endpoints were survival, growth, and/or emergence (depending on the developmental stage of the larvae at the start of the exposure). Two experiments conducted 1 yr apart showed good reproducibility: growth 10% effect concentration (EC10) values were 15.3 and 18.5 μg/L and no-observed-effect concentration (NOEC) values were 30.3 and 21.5 μg/L. A third experiment, performed with further-developed larval instars, showed a significant effect of imidacloprid on emergence (with EC10 of 5.97 μg/L and NOEC of 2.89 μg/L) and a significant effect on survival (with median lethal concentration of 44.7 µg/L). The results of the present study show that the newly developed test system provides a suitable approach for toxicity testing with stonefly larvae and potentially for other lotic macroinvertebrate species. Environ Toxicol Chem 2021;40:2229-2239. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Thermal plasticity and sensitivity to insecticides in populations of an invasive beetle: Cyfluthrin increases vulnerability to extreme temperature

Climate change increases average temperatures and the occurrence of extreme weather events, in turn accentuating the risk of organism exposure to temperature stress. When thermal conditions become stressful, the sensitivity of insects toward insecticides can be exacerbated. Likewise, exposure of insects to insecticides can subsequently influence their ability to handle stressful temperatures. Here, we investigated the effects of constant temperature and daily heat spikes, in presence/absence of insecticide treatment (cyfluthrin), on the condition (impairment of mobility) and thermal tolerance to cold (-6 °C) and heat (42.5 °C) of the terrestrial beetle Alphitobius diaperinus. The responses of insects from four populations (three farm-collected populations, one laboratory population) to different durations of extreme temperature exposure were compared. The results showed that the laboratory population was generally more sensitive to extreme cold and heat temperatures, with less than 50% of adults recovering after an exposure at -6 or +42.5 °C for 3h. Significant differences in the level of thermal tolerance were also found among insects from poultry farms. Cyfluthrin exposure incurred detrimental effects to insects' condition in all but one population. For two out of the four populations, mobility impairment was increased when adults were exposed to daily heat spikes (6 h per day at 38 °C) and cyfluthrin simultaneously, compared to cyfluthrin exposure at constant temperatures; yet, no significant interaction between the two stressors was found. Finally, using one farm collected population, effects of pre-exposure to cyfluthrin on extreme temperature tolerance provided another example of the toxicant-induced climate sensitivity in insects.

Single and combined effects of insecticides on multi-level biomarkers in the non-target amphipod Gammarus fossarum exposed to environmentally realistic levels

Aquatic media are ultimate recipients of various contaminants including pesticides pervasively applied in agrosystems. Characterizing the ecotoxicity of pesticides and their mixtures to aquatic wildlife at field-realistic levels is thus crucial for environmental risk assessment. This study aims at assessing the effects of two current-use insecticides, imidacloprid and chlorpyrifos, on Gammarus fossarum using multi-level biomarkers. In microcosms, gammarids were exposed for 72 h to insecticides tested individually or in mixture at 0.01, 0.1 and 1 μg/L of each chemical. Multi-metric responses were assessed at the individual level (behavioural traits: locomotion, respiration and amplexus formation) and the cellular level (enzymes involved in growth, moulting, digestion and cell stress). The results showed insecticide-elicited behavioural and biochemical responses from the lowest concentration of 0.01 μg/L. Overall, single exposures stimulated behavioural traits and inhibited enzymatic activities, highlighting subtle impacts at different organizational levels but these were not dose related. For binary mixtures, antagonistic effects (i.e. less-than-additive) on biomarkers were mainly observed when compared with single exposures. Multi-variable analyses indicated the complementarity of behavioural and biochemical biomarkers in identifying sublethal biological alterations and dose-dependent multiple action sites of insecticides. Besides, the mortality observed only for the mixture at 1 μg/L demonstrated a high lethal potential of insecticides in a simple binary combination. To conclude, this study demonstrates disturbances in individual performances and cellular impairments occurring at environmentally realistic exposure levels in a non-target wild species. Since the sublethal effects, such as those identified with this multi-biomarker approach, could lead to long-term alterations in population dynamics of agricultural areas, they constitute promising early endpoints for risk assessment of insecticides.

Recommendations for ecotoxicity testing with stygobiotic species in the framework of groundwater environmental risk assessment

As a consequence of the growing global dependence on groundwater resources, environmental risk assessments (ERA) for groundwater are increasingly required and, with that, ecotoxicological studies with groundwater fauna (stygofauna). However, the literature on the ecotoxicological studies with stygobiotic species (i.e. species that complete their life cycle exclusively in groundwater) has not expanded significantly since the first paper published in this field. The limitations regarding the use of stygobiotic species for ecotoxicological testing are clear and consistent across the globe; stygobiotic species are often 1) naturally present in low numbers, 2) difficult to collect, and 3) difficult to culture under laboratory conditions. This paper reviews the methods used in ecotoxicological studies performed with stygobiotic species, and provides ten recommendations for Good Laboratory Practice (GLP) for such tests. The recommendations focused on the following 10 points: 1) the taxonomic identification, the life stage/size and gender of the test organisms; 2) collection methodology of the organisms, including collection location, conditions and methods; 3) holding and acclimation conditions in the laboratory; 4) exposure conditions such as test set up and exposure time, number of replicates and densities of organisms in tests and in test vessels; 5) range-finding test set up and schedule; 6) final test design, including details of controls and treatments, and replication options; 7) incubation conditions, specifying temperature, pH and oxygenation levels throughout the test; 8) test duration; 9) observations and endpoints; 10) test validity criteria and compliance. The recommendations were developed for the purpose of supporting future short-term ecotoxicological testing with stygofauna through providing consistency in the protocols. A discussion of the potential implications for groundwater managers and decision-makers committed to ERA for groundwater is included.

An integrated study on Gammarus elvirae (Crustacea, Amphipoda): perspectives for toxicology of arsenic-contaminated freshwater

The Italian region Latium is characterized by extensive quaternary volcanic systems that contribute greatly to arsenic (As) contamination of freshwater, including drinking water supplies. However, knowledge of the possible toxic effects in these aquatic environments is, despite being highly relevant to public health, still limited. In this paper, we approach this issue using Gammarus elvirae, an amphipod species that inhabits rivers and streams in central Italy, including Latium. We explored the possibility of using G. elvirae in the toxicology of freshwater by addressing the most relevant issues. First, we tested the usefulness of hemocytes from G. elvirae in determining non-specific DNA damage by means of the Comet assay after exposure (24 h and 7 days) to different river water samples in Latium; second, we provided an interpretative overview of the usefulness of hepatopancreatic epithelial cells of G. elvirae as a means of assessing toxicity after long-term exposure to As and other pollutants; third, the LC (50-240 h) value for G. elvirae was estimated for arsenate, which is usually the dominant arsenic species in surface waters. Our study sheds light on G. elvirae at different levels, providing a background for future toxicological research of freshwater.

Aquatic degradation of Cry1Ab protein and decomposition dynamics of transgenic corn leaves under controlled conditions

The increasing cultivation of genetically modified corn plants (Zea mays) during the last decades is suggested as a potential risk to the environment. One of these genetically modified variety expressed the insecticidal Cry1Ab protein originating from Bacillus thuringiensis (Bt), resulting in resistance against Ostrinia nubilalis, the European corn borer. Transgenic litter material is extensively studied regarding the decomposition in soils. However, only a few field studies analyzed the fate of the Cry1Ab protein and the impact of green and senescent leaf litter from corn on the decomposition rate and related ecosystem functions in aquatic environments. Consequently, a microbial litter decomposition experiment was conducted under controlled semi-natural conditions in batch culture using two maize varieties: one variety with Cry1Ab and another one with the appertaining Iso-line as control treatment. The results showed no significant differences between the treatment with Cry1Ab and the Iso-line regarding loss of total mass in dry weight of 43% for Iso-line and 45% for Bt-corn litter, lignin content increased to 137.5% (Iso-line) and 115.7% (Bt-corn), and phenol loss decreased by 53.6% (Iso-line), 62.2% (Bt-corn) during three weeks of the experiment. At the end of the experiment Cry1Ab protein was still detected with 6% of the initial concentration. A slightly but significant lower cellulose content was found for the Cry1Ab treatment compared to the Iso-line litter at the end of the experiment. The significant higher total protein (25%) and nitrogen (25%) content in Bt corn, most likely due to the additionally expression of the transgenic protein, may increase the microbial cellulose degradation and decrease microbial lignin degradation. In conclusion a relevant year by year input of protein and therefore nitrogen rich Bt corn litter into aquatic environments may affect the balanced nutrient turnover in aquatic ecosystems.

Imidacloprid perturbs feeding of Gammarus pulex at environmentally relevant concentrations

Changes in food uptake by detritivorous macroinvertebrates could disrupt the ecosystem service of leaf litter breakdown, necessitating the study of shredding under anthropogenic influences. The impact of the neonicotinoid insecticide imidacloprid on the feeding rate of individual Gammarus pulex was measured at a daily resolution both during and after a 4-d exposure period. The authors found that imidacloprid inhibits feeding of G. pulex during exposure at concentrations ≥ 30 µg/L and that there was no recovery in feeding on transfer into clean media for 3 d. Exposure to imidacloprid at concentrations ≥ 0.81 µg/L and ≤ 9.0 µg/L resulted in increased feeding after exposure even though there was no significant effect on feeding during the exposure itself. Comparison with the literature shows that concentrations found to influence feeding lie within the range of estimated and measured environmental concentrations. Additionally, effects on feeding rate were observed at concentrations 2 orders of magnitude lower than those causing mortality. The lethal concentration for 50% of test organisms after 4 d of exposure (270 µg/L, literature data) and the effect concentration for a reduction in feeding by 50% (5.34 µg/L) were used for this comparison. The present study discusses the potential that effects on feeding may evoke effects at the population level or disturb leaf litter breakdown in the environment.

Effects of toxicants with different modes of action on Myriophyllum spicatum in test systems with varying complexity

At the international workshop Aquatic Macrophyte Risk Assessment for Pesticides (AMRAP), it was noted that the EU risk assessment under the directive 91/414/EEC for herbicides, based only on algae and the monocotyledonous duckweed species Lemna sp., offers no certain protection against some growth regulating auxins. Therefore, AMRAP members proposed the introduction of the dicotyledonous water milfoil Myriophyllum as additional test species. This study was aimed to compare toxicity results from three test systems (TS) with varying complexity, namely Water TS, Sediment TS and Microcosm TS using Myriophyllum spicatum as test organism. As test substances, the photosynthesis inhibiting herbicide isoproturon, the growth regulating auxins fluroxypyr and 2,4-dichlorophenoxyacetic acid (2,4-D), and the non-specific acting toxicant 3,5-dichlorophenol (3,5-DCP) were chosen. It was assessed if and why the sensitivity of M. spicatum towards the four toxicants varied in the different test systems and if the addition of sucrose to the medium used in the Water TS had an effect on the sensitivity of Myriophyllum. All TS were suitable for detecting negative effects of toxicants with different modes of action on M. spicatum. The lowest variability of endpoints was found in the Water TS with lowest experimental complexity. For auxins, the endpoint weight did not result in robust EC50 values in all TS, whereas root related endpoints, which are also ecologically relevant, turned out to be very sensitive with low variance. Sucrose in the medium of the Water TS did not seem to influence the sensitivity of M. spicatum towards isoproturon and 3,5-DCP but may have increased the sensitivity of M. spicatum roots when exposed to 2,4-D. However, the findings of all TS resulted in similar risk estimations if root endpoints were not considered.

Effects of low-dosed imidacloprid pulses on the functional role of the caged amphipod Gammarus roeseli in stream mesocosms

Effects of two series of imidacloprid pulses on caged amphipods (Gammarus roeseli) and their shredder efficiency for litter decomposition were studied for 70 days as part of a comprehensive stream mesocosm experiment. The duration of each imidacloprid pulse of 12µgL(-1) was 12h. About 250mL cages with an initial stock of 10 adult gammarids together with different conditioned litter substrates were used. Beside alder leaves (Alnus glutinosa), straw (× Triticosecale) was also used in different trials and tested for its suitability to serve as litter substrate. Results from tracer and microprobe measurements approved the suitability of the test system under low-flow condition of 10cms(-1) in the surrounding stream water. Population development followed a logistic growth function with a carrying capacity of 200 Ind cage(-1) for alder and 161 for straw. In the course of the study, the F1 generation reached sexual maturity and F2 offspring appeared. Increased nitrogen contents of gammarid-free trials compared to stocked ones after 70 days indicated that biofilm on both substrates was an important food source for G. roeseli. However, increased shredding activity by gammarids was only detected for alder during the second pulse series. During the remaining time and also for straw, losses of coarse particular organic matter were quite constant and slow indicating the dominance of transport limited decomposition processes on the litter surfaces. No effect of imidacloprid pulses on population levels and litter decomposition could be detected. However, the number of brood carrying females was reduced in the treatments compared to the control groups in the last 3 weeks of the study. In conclusion, repeated low-level and short-term exposition may have adverse long-term effects on G. roeseli in the field with regard to both the population size and the functional role as key shredder.

The insecticide imidacloprid causes mortality of the freshwater amphipod Gammarus pulex by interfering with feeding behavior

If an organism does not feed, it dies of starvation. Even though some insecticides which are used to control pests in agriculture can interfere with feeding behavior of insects and other invertebrates, the link from chemical exposure via affected feeding activity to impaired life history traits, such as survival, has not received much attention in ecotoxicology. One of these insecticides is the neonicotinoid imidacloprid, a neurotoxic substance acting specifically on the insect nervous system. We show that imidacloprid has the potential to indirectly cause lethality in aquatic invertebrate populations at low, sublethal concentrations by impairing movements and thus feeding. We investigated feeding activity, lipid content, immobility, and survival of the aquatic arthropod Gammarus pulex under exposure to imidacloprid. We performed experiments with 14 and 21 days duration, both including two treatments with two high, one day pulses of imidacloprid and one treatment with a low, constant concentration. Feeding of G. pulex as well as lipid content were significantly reduced under exposure to the low, constant imidacloprid concentration (15 µg/L). Organisms were not able to move and feed--and this caused high mortality after 14 days of constant exposure. In contrast, feeding and lipid content were not affected by repeated imidacloprid pulses. In these treatments, animals were mostly immobilized during the chemical pulses but did recover relatively fast after transfer to clean water. We also performed a starvation experiment without exposure to imidacloprid which showed that starvation alone does not explain the mortality in the constant imidacloprid exposure. Using a multiple stressor toxicokinetic-toxicodynamic modeling approach, we showed that both starvation and other toxic effects of imidacloprid play a role for determining mortality in constant exposure to the insecticide.

The neonicotinoid imidacloprid shows high chronic toxicity to mayfly nymphs

The present study evaluated the acute and chronic toxicity of imidacloprid to a range of freshwater arthropods. Mayfly and caddisfly species were most sensitive to short-term imidacloprid exposures (10 tests), whereas the mayflies showed by far the most sensitive response to long-term exposure of all seven arthropod species tested (28-d EC10 values of approximately 0.03 µg/L). The results indicated a high aquatic risk of chronic exposure of imidacloprid to mayflies.