Effects of species-specific interactions with predation risk on the relative species sensitivities to a pesticide in water boatmen (Corixidae)

Pollution impacts on water bugs (Nepomorpha, Gerromorpha): state of the art and their biomonitoring potential

As water pollution poses an increasing risk worldwide, it is timely to assess the achievements of the aquatic macroinvertebrate ecotoxicology to provide a sound basis for the discipline's future and support the development of biomonitoring. Aquatic and semi-aquatic bugs (Hemiptera: Nepomorpha, Gerromorpha) are ubiquitous in almost all water types, sometimes in high densities, and play a significant role in organic material turnover and energy flow. Nevertheless, they are ignored in the water pollution biomonitoring schemes. Here, based on 300 papers, we review and evaluate the effects of chemical pesticides, microorganism-derived pesticides, insecticides of plant origin, heavy metals, eutrophication, salinisation and light pollution which are summarised for the first time. Our review encompasses the results of 100 laboratory and 39 semi-field/field experiments with 47 pesticides and 70 active ingredients. Pyrethroids were found to be more toxic than organochlorine, organophosphate and neonicotinoid insecticides to water bugs, like other macroinvertebrate groups. Additionally, in 10 out of 17 cases, the recommended field concentration of the pesticide was higher than the LC50 values, indicating potential hazards to water bugs. The recommended field concentrations of pesticides used in mosquito larvae control were found non-toxic to water bugs. As very few replicated studies are available, other findings on the effects of pesticides cannot be generalised. The microorganism-derived pesticide Bti appears to be safe when used at the recommended field concentration. Data indicates that plant-derived pesticides are safe with a high degree of certainty. We have identified three research areas where water bugs could be better involved in water biomonitoring. First, some Halobates spp. are excellent, and Gerris spp. are promising sentinels for Cd contamination. Second, Micronecta and, to a certain extent, Corixidae species composition is connected to and the indicator of eutrophication. Third, the species composition of the Corixidae is related to salinisation, and a preliminary method to quantify the relationship is already available. Our review highlights the potential of water bugs in water pollution monitoring.

Community composition modifies direct and indirect effects of pesticides in freshwater food webs

For environmental risk assessment, the effects of pesticides on aquatic ecosystems are often assessed based on single species tests, disregarding the potential influence of community composition. We, therefore, studied the influence of changing the horizontal (the number of species within trophic levels) and vertical composition (number of trophic levels) on the ecological effects of the herbicide linuron and the insecticide chlorpyrifos, targeting producers and herbivores, respectively. We tested how adding, to a single primary producer, 4 selected competing producer species, 0-1-4 selected herbivore species, and one selected predator species resulting in 1, 2 and 3 trophic levels, changes the effects of the two pesticides. Linuron decreased producer biovolume less (17%) when the 4 producers were added, because insensitive producers compensated for the loss of sensitive producers. However, linuron decreased producer biovolume 42% and 32% more as we increased the number of herbivore species from 0 to 4 and as we increased trophic levels from 1 to 3, respectively. The indirect negative effect of linuron on herbivore biovolume was 11% and 15% lower when more producer and herbivores were added, respectively. Adding a predator increased this indirect negative effect by 22%. Chlorpyrifos decreased herbivore biovolume about 10% less when adding multiple herbivore or producer species. However, adding a predator magnified the direct negative impact on herbivores (13%). Increasing the number of producer, herbivore species and adding trophic levels increased the indirect positive impact on producer biovolume (between 10% and 35%). Our study shows that changing horizontal composition can both increase and decrease the effects of the selected pesticides, while changing vertical composition by adding number of trophic levels always increased these effects. Therefore, single species sensitivity will not always represent a worst case estimate of ecological effects. Protecting the most sensitive species may not ensure protection of ecosystems.

Additive bioenergetic responses to a pesticide and predation risk in an aquatic insect

Ignoring natural stressors such as predation risk may contribute to the failure of ecological risk assessment of pesticides to protect freshwater biodiversity. To better understand combined effects of multiple stressors, bioenergetic responses are important as these inform about the balance between energy input and consumption, and provide a unifying mechanism to integrate the impact of multiple stressors with different modes of action. We studied in Enallagma cyathigerum damselfly larvae the single and combined effects of exposure to the pesticide chlorpyrifos and predation risk on life history (survival and growth rate) and bioenergetic response variables at the organismal level (assimilation and conversion efficiency) and the cellular level (cellular energy allocation CEA, energy storage Ea, and energy consumption Ec). Chlorpyrifos exposure almost halved the survival of the damselfly larvae, while predation risk had no effect on survival. Both exposure to the pesticide and to predation risk reduced larval growth rates. This was caused by a reduced conversion efficiency under chlorpyrifos exposure, and by a reduced assimilation efficiency under predation risk. Both chlorpyrifos and predation risk reduced the CEA because of a decreased Ea, and for chlorpyrifos also an increased Ec. The lower Ea was driven by reductions in the fat and glycogen contents. Effects of the pesticide and predation risk were consistently additive and for most variables the strongest response was detected when both stressors were present. The absence of any synergisms may be explained by the high mortality and hypometabolism caused by the pesticide. Our results indicate that CEA can be a sensitive biomarker to evaluate effects of not only contaminants but also natural stressors, such as predation risk, and their combined impact on organisms.

Development of acute and chronic sediment bioassays with the harpacticoid copepod Quinquelaophonte sp

Reliable environmentally realistic bioassay methodologies are increasingly needed to assess the effects of environmental pollution. This study describes two estuarine sediment bioassays, one acute (96 h) and one chronic (14 d), with the New Zealand harpacticoid copepod Quinquelaophonte sp. utilising behavioural and reproductive endpoints. Spiked sediments were used to expose Quinquelaophonte sp. to three reference compounds representing important categories of estuarine chemical stressors: zinc (a metal), atrazine (a pesticide), and phenanthrene (a polycyclic aromatic hydrocarbon). Acute-to-chronic ratios (ACR) were used to further characterise species responses. Acute sediment (sandy and low total organic content) 96 h EC50 values for the sublethal inhibition of mobility for zinc, atrazine and phenanthrene were 137, 5.4, and 2.6 µg/g, respectively. The chronic EC50 values for inhibition of reproduction (total offspring) were 54.5, 0.0083, and 0.067 µg/g for zinc, atrazine, and phenanthrene, respectively. For phenanthrene, a potentially novel mode of action was identified on reproduction. Quinquelaophonte sp. was found to be more sensitive than several other estuarine species indicating choice of test organism is important to characterising the effects of environmentally relevant levels of contamination. The bioassay sediment results demonstrate the sensitivity and suitability of Quinquelaophonte sp. as a tool for the assessment use of estuarine health.

Predator cues magnify effects of the pesticide endosulfan in water bugs in a multi-species test in outdoor containers

Pesticides have become major stressors in many aquatic communities. Laboratory studies suggest their impact may be further magnified in the presence of cues from predators. Despite their importance for ecological risk assessment, synergisms between pesticides and predator cues have not been confirmed under semi-natural outdoor conditions. We evaluated how the presence of predator cues and the presence of a non-corixid community affect the pesticide sensitivity of five water bug (Corixidae) species in an outdoor, multi-species container experiment. The experiment employed a full factorial design with two pesticide treatments, two predator cue treatments and two (non-corixid) community treatments (absence versus presence of Cloeon dipterum mayfly larvae, Ischnura elegans damselfly larvae and Physa acuta snails). The pesticide treatment negatively affected survival in Cymatia coleoptrata, and to a lesser extent, Sigara lateralis, but not in the other three Corixidae species (Hesperocorixa linnaei, Sigara iactans and Sigara striata). The addition of pesticides did not significantly affect body mass in the latter four species, unless combined with predator cues. To our knowledge this is the first report of this synergism under semi-natural, outdoor conditions. Neither lethal nor sublethal pesticide effects in the Corixidae depended on the community context, yet the presence of the non-corixid community when combined with predator cues reduced survival and body mass. Our results suggest that the here documented synergism between pesticides and predator cues may occur in nature.

Synergistic effects between pesticide stress and predator cues: conflicting results from life history and physiology in the damselfly Enallagma cyathigerum

There is increasing awareness that the negative effects of anthropogenic stressors may be magnified in the presence of natural stressors. Very few of these studies have included physiology, yet including physiological studies may help learning about the mechanistic base of such synergisms at the life history level and identify synergistic interactions not translated in life history traits. We studied in Enallagma cyathigerum damselfly larvae potential synergistic effects between exposure to the pesticide glyphosate and predator cues on a key life history trait, growth rate, its associated behavioural trait, food intake, and three types of physiological traits known to be affected by both stressors in isolation: the stress protein Hsp70, energy storage and variables related to oxidative stress and damage. The pesticide and predator cues reduced growth rate in an additive way. Food intake increased under pesticide exposure and was not affected by the predator cues, indicating physiological mediation of the growth reduction. One potential physiological mechanism was that both stressors additively increased Hsp70 levels, this may also have contributed to the reduced levels of total carbohydrates when exposed to predator cues. Chronic exposure to predator cues reduced oxygen consumption, possibly to avoid too high costs of an increased metabolic rate. This reduction did not occur in the presence of the pesticide, reflecting the need for energetically expensive defence mechanisms (such as Hsp70 upregulation). When both stressors were combined, there was a reduction of the antioxidant enzyme superoxide dismutase activity (SOD) and an associated increase of oxidative damage in lipids. While synergistic interactions were not present for growth rate and food intake, they were identified for antioxidant defence and oxidative damage. This novel type of "hidden" synergistic interaction may have profound fitness implications, and when ignored will lead to underestimations of the impact of pollutants in natural populations where predators are omnipresent.

Species-specific patterns of swimming escape performance and cholinesterase activity in a guild of aquatic insects exposed to endosulfan

Next to imposing direct lethal effects, pollutants may also indirectly impose mortality by making prey organisms more vulnerable to predation. We report that four water boatmen species differed strongly in direct endosulfan-imposed mortality, and only the species that suffered highest mortality, Sigara iactans, also showed a reduction in escape swimming speed. While head AChE activity was inhibited in all four species, body ChE was only inhibited in S. iactans where it covaried with escape swimming speed, indicating a mechanistic link between body ChE and swimming speed. Our study underscores the need for risk assessment to consider sublethal pollutant effects, which may considerably affect survival rates under natural conditions, also when testing concentrations of a pesticide that cause direct mortality. Such sublethal effects may generate discrepancies between laboratory and field studies and should be considered when designing safety factors for toxicants where the risk assessment is solely based on LC50 values.

Species-specific responsiveness of four enzymes to endosulfan and predation risk questions their usefulness as general biomarkers

General biochemical biomarkers are widely used in current ecotoxicology and may function as early warning signals. We have, however, poor knowledge on how ecologically similar species differ in their biomarker responsiveness and how predation risk may affect these biomarkers, potentially in an interactive way with pesticides. We evaluated this by exposing four corixid water bug species to combinations of endosulfan and predation risk and quantifying the activity of four general enzymatic biomarkers: acetylcholinesterase (AChE), phenoloxidase (PO), catalase (CAT) and superoxidedismutase (SOD). AChE activity was inhibited at an endosulfan concentration of 2 μg l(-1) and this did not differ significantly among species. Predation risk inhibited AChE activity with the same magnitude as endosulfan in one species, S. striata. Reduction in the investment of immune function following pesticide exposure, as measured by the activity of PO, was only observed in C. coleoptrata at 8 μg l(-1) while we observed an increase of PO levels in S. striata. Overall, PO was suppressed under predation risk at 8 μg l(-1) endosulfan. For SOD we observed a pesticide-induced increase across all species under predation risk, while for CAT the pesticide-induced increase was only present without predation risk. These results indicate that even within this group of ecologically similar and closely related species opposing biomarker responses may exist, as observed for PO. Effects of predation risk on all four enzymes, at a similar magnitude as the pesticide effects, further question their usefulness as general biomarkers.