The Roles of Diet and Habitat Use in Pesticide Bioaccumulation by Juvenile Chinook Salmon: Insights from Stable Isotopes and Fatty Acid Biomarkers.

Stable isotopes (SI) and fatty acid (FA) biomarkers can provide insights regarding trophic pathways and habitats associated with contaminant bioaccumulation. We assessed relationships between SI and FA biomarkers and published data on concentrations of two pesticides [dichlorodiphenyltrichloroethane and degradation products (DDX) and bifenthrin] in juvenile Chinook Salmon (Oncorhynchus tshawytscha) from the Sacramento River and Yolo Bypass floodplain in Northern California near Sacramento. We also conducted SI and FA analyses of zooplankton and macroinvertebrates to determine whether particular trophic pathways and habitats were associated with elevated pesticide concentrations in fish. Relationships between DDX and both sulfur (δ34S) and carbon (δ13C) SI ratios in salmon indicated that diet is a major exposure route for DDX, particularly for individuals with a benthic detrital energy base. Greater use of a benthic detrital energy base likely accounted for the higher frequency of salmon with DDX concentrations > 60 ng/g dw in the Yolo Bypass compared to the Sacramento River. Chironomid larvae and zooplankton were implicated as prey items likely responsible for trophic transfer of DDX to salmon. Sulfur SI ratios enabled identification of hatchery-origin fish that had likely spent insufficient time in the wild to substantially bioaccumulate DDX. Bifenthrin concentration was unrelated to SI or FA biomarkers in salmon, potentially due to aqueous uptake, biotransformation and elimination of the pesticide, or indistinct biomarker compositions among invertebrates with low and high bifenthrin concentrations. One FA [docosahexaenoic acid (DHA)] and DDX were negatively correlated in salmon, potentially due to a greater uptake of DDX from invertebrates with low DHA or effects of DDX on FA metabolism. Trophic biomarkers may be useful indicators of DDX accumulation and effects in juvenile Chinook Salmon in the Sacramento River Delta.

Using an internal body residue approach to assess acute pesticide toxicity in juvenile Chinook salmon (Oncorhynchus tshawytscha).

The use of internal body residues has the potential to improve toxicological assessments of hydrophobic pesticides. The acute toxicity of three classes of pesticides were assessed in juvenile Chinook salmon using internal body residues. Chinook salmon were exposed to two current-use pesticides bifenthrin and fipronil, and 4,4'- dichlorodiphenyldichloroethylene (DDE), which is a degradation product of the legacy pesticide dichlorodiphenyltrichloroethane (DDT). After 96-h of aqueous exposure to each pesticide, the pesticide content in whole-body Chinook salmon homogenates was measured using gas chromatography/mass spectrometry with methane negative chemical ionization. The wet-weight (ww) normalized lethal residue at 50% mortality (LR50) was lowest for bifenthrin (0.654 nmol/g ww), followed by fipronil (7.17 nmol/g ww) and the sum of fipronil and its sulfone, sulfide, and desulfinyl degradation products (8.72 nmol/g ww). No lethality was observed for DDE, even at the highest body residue (>116 nmol/g ww). LR50 estimates were also normalized to dry weight and lipid content and compared to field-caught fish to assess risk. The use of a risk quotient approach indicated that bifenthrin imparts the highest risk of acute toxicity in juvenile Chinook salmon among the three pesticides tested. In comparison to external dose metrics, the use of internal body residues has the potential to improve risk assessment by providing a more direct link between pesticide concentration at the receptor site and toxicological effects.

Assessing the Influence of Organic Carbon, Aging Time and Temperature on Bioaccessibility of Bifenthrin.

Tenax extraction, a measure of chemical desorption rates from sediments, was used to evaluate the bioaccessibility of bifenthrin in two different sediments exposed to three temperatures aged over a 56-d holding period. A 24-h single-point Tenax extraction was used and parent 14C-bifenthrin and polar metabolites were quantified in the sediment and Tenax. Bioaccessibility of bifenthrin was inversely related to the organic carbon (OC) content in the sediment, holding time, and temperature. Sequestration of the bifenthrin into slowly desorbing fractions within the sediment appears to have decreased degradation of the parent compound into metabolites and decreased the amount of parent compound bioaccessible for uptake by the Tenax. These results suggest that the environmental risk of bifenthrin to aquatic species is greatest immediately after the pesticide enters a waterbody after runoff, for low-OC content sediments, and in areas or seasons where water temperatures are colder.

Development of insecticide resistance in Hyalella azteca.

Hyalella azteca are epibenthic amphipods that have developed resistance to pyrethroid and organophosphate insecticides due to single amino acid substitutions in the voltage-gated sodium channel and the acetylcholinesterase-1 gene, respectively. Aquatic systems are often contaminated with several different types of insecticides, therefore there is a possibility that H. azteca have also developed resistance to other classes of insecticides. The aims of the current study were to verify that pyrethroid- and organophosphate-resistant H. azteca have retained their resistance after being cultured in the absence of selective pressure for 5 years (Escondido Creek population) and 9 years (Mosher Slough population), to determine if these populations have cross-resistance to carbaryl (carbamate) and 4,4'-dichlorodiphenyltrichloroethane (DDT; organochlorine), and determine whether previous field exposure to fipronil (phenylpyrazole) and imidacloprid (neonicotinoid) caused resistance in cultured pyrethroid- and organophosphate-resistant H. azteca populations. Escondido Creek and Mosher Slough H. azteca populations both maintained high tolerances for bifenthrin due to L925I and I936F amino acid substitutions. Resistance was also found for chlorpyrifos in the Escondido Creek and Mosher Slough populations with lower genotype frequencies of the G119S substitution, indicating that additional factors may be responsible for organophosphate resistance in this study. Mosher Slough H. azteca were moderately resistant to DDT, and Escondido Creek and Mosher Slough H. azteca were moderately resistant to carbaryl, suggesting cross-resistance. No differences were observed in acute toxicity values across the three populations of H. azteca for fipronil and imidacloprid, and this is possibly due to the lack of exposure to toxic concentrations of these insecticides in the field and lack of similar modes of action to pyrethroids and organophosphates. Resistance is known to be associated with fitness costs that can place insecticide-resistant populations at risk for decline through decreased survival and reduced fecundity.

Effect of temperature and dietary pesticide exposure on neuroendocrine and olfactory responses in juvenile Chinook salmon (Oncorhynchus tshawytscha).

Projected water temperature increases based on predicted climate change scenarios and concomitant pesticide exposure raises concern about the responses of aquatic organisms. To better understand the effect of pesticide mixtures and influence of water temperature to fish, juvenile Chinook salmon (Oncorhynchus tshawytscha) were dietarily exposed to a mixture of legacy and current use pesticides (p,p'-DDE, bifenthrin, chlorpyrifos, esfenvalerate, and fipronil) at concentrations detected from field-collected prey items in the Sacramento-San Joaquin Delta, California (Delta) and exposed under current and predicted future water temperature scenarios, 11, 14, or 17 °C, for 14 days. The expression of a subset of genes (deiodinase 2-dio2, gonadotropin releasing hormone 2-gnrh2, and catechol-o-methyltransferase-comt) involved in neuroendocrine, dopaminergic, and olfactory function previously shown to be altered by individual pesticide exposures germane to this study were determined and olfactory function assessed using a Y-maze behavioral assay. When total body burdens of pesticides were measured, a significant decrease in dio2 expression was observed in Chinook salmon exposed at 14 °C compared to fish kept at 11 °C. Increases in gnrh2 expression were also observed in fish exposed to 14 °C. Similarly, increases in comt expression was noted at 14 and 17 °C. Additionally, altered expression of all transcripts was observed, showing interactions between temperature and individual pesticide concentrations. Chinook salmon spent significantly more time actively avoiding the odorant arm at baseline conditions of 11 °C in the Y-maze. At higher temperatures, Chinook spent significantly more time not making a choice between the odorant or clean arm following exposure to the low pesticide mixture, relative to 11 °C. These results suggest that dietary exposure to pesticide mixtures can potentially induce neuroendocrine effects and behavior. Impaired olfactory responses exhibited by Chinook salmon could have implications for predator avoidance in the wild under increased temperature scenarios and impact populations in the future.

Predicting Resistance: Quantifying the Relationship between Urban Development, Agricultural Pesticide Use, and Pesticide Resistance in a Nontarget Amphipod.

Resistance alleles within the voltage-gated sodium channel (vgsc) have been correlated with pyrethroid resistance in wild populations of the nontarget amphipod, Hyalella azteca from California (CA), U.S.A. In the present study, we expand upon the relationship between land use and the evolution of pesticide resistance in H. azteca to develop a quantitative methodology to target and screen novel populations for resistance allele genotypes in a previously uninvestigated region of the U.S. (New England: NE). By incorporating urban land development and toxicity-normalized agricultural pesticide use indices into our site selection, we successfully identified three amino acid substitutions associated with pyrethroid resistance. One of the resistance mutations has been described in H. azteca from CA (L925I). We present the remaining two (vgsc I936F and I936V) as novel pyrethroid-resistance alleles in H. azteca based on previous work in insects and elevated cyfluthrin resistance in one NE population. Our results suggest that urban pesticide use is a strong driver in the evolution of resistance alleles in H. azteca. Furthermore, our method for resistance allele screening provides an applied framework for detecting ecosystem impairment on a nationwide scale that can be incorporated into ecological risk assessment decisions.

Dietary exposure to environmentally relevant pesticide mixtures impairs swimming performance and lipid homeostatic gene expression in Juvenile Chinook salmon at elevated water temperatures.

Aquatic organisms are exposed to complex mixtures of pesticides in the environment, but traditional risk assessment approaches typically only consider individual compounds. In conjunction with exposure to pesticide mixtures, global climate change is anticipated to alter thermal regimes of waterways, leading to potential co-exposure of biota to elevated temperatures and contaminants. Furthermore, most studies utilize aqueous exposures, whereas the dietary route of exposure may be more important for fish owing to the hydrophobicity of many pesticides. Consequently, the current study aimed to determine the effects of elevated temperatures and dietary pesticide mixtures on swimming performance and lipid metabolism of juvenile Chinook salmon, Oncorhynchus tshawytscha. Fish were fed pesticide-dosed pellets at three concentrations and three temperatures (11, 14 and 17 °C) for 14 days and swimming performance (Umax) and expression of genes involved in lipid metabolism and energetics were assessed (ATP citrate lyase, fatty acid synthase, farnesoid x receptor and liver x receptor). The low-pesticide pellet treatment contained five pesticides, p,p'-DDE, bifenthrin, esfenvalerate, chlorpyrifos and fipronil at concentrations based on prey items collected from the Sacramento River (CA, USA) watershed, with the high-pesticide pellet treatment containing a six times higher dose. Temperature exacerbated effects of pesticide exposure on swimming performance, with significant reductions in Umax of 31 and 23% in the low and high-pesticide pellet groups relative to controls at 17 °C, but no significant differences in Umax among pesticide concentrations at 11 or 14 °C. At 14 °C there was a significant positive relationship between juvenile Chinook salmon pesticide body residues and expression of ATP citrate lyase and fatty acid synthase, but an inverse relationship and significant downregulation at 17 °C. These findings suggest that temperature may modulate effects of environmentally relevant pesticide exposure on salmon, and that pesticide-induced impairment of swimming performance may be exacerbated under future climate scenarios.

The Use of Non-targeted Lipidomics and Histopathology to Characterize the Neurotoxicity of Bifenthrin to Juvenile Rainbow Trout (Oncorhynchus mykiss).

Due to the detection frequencies and measured concentrations in surface water, the type I pyrethroid insecticide, bifenthrin, has been of particular concern within the Sacramento-San Joaquin Delta in California. Concentrations have been detected above levels previously reported to impair neuroendocrine function and induce neurotoxicity to several species of salmonids. Metabolomic and transcriptomic studies indicated impairment of cellular signaling within the brain of exposed animals and potential alteration of lipid metabolism. To better understand the potential impacts of bifenthrin on brain lipids, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to mean bifenthrin concentrations of 28 or 48 ng/L for 14 days, and non-targeted lipidomic profiling in the brain was conducted. Brain tissue sections were also assessed for histopathological insult following bifenthrin treatment. Bifenthrin-exposed trout had a concentration-dependent decrease in the relative abundance of triglycerides (TGs) with levels of phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) significantly altered following 48 ng/L bifenthrin exposure. An increased incidence of histopathological lesions, such as focal hemorrhages and congestion of blood vessels, was noted in the brains of bifenthrin-treated animals, suggesting an association between altered lipid metabolism and neuronal cell structure and integrity.

Dietary Exposure to Bifenthrin and Fipronil Impacts Swimming Performance in Juvenile Chinook Salmon (Oncorhynchus tshawytscha).

Two commonly used insecticides, bifenthrin and fipronil, can accumulate in the prey of juvenile Chinook salmon, yet the effects of dietary exposure are not understood. Therefore, to better characterize the effect of a dietary exposure route, juvenile Chinook salmon were fed chironomids dosed with a concentration of 9 or 900 ng/g of bifenthrin, fipronil, or their mixture for 25 days at concentrations previously measured in field-collected samples. Chinook were assessed for maximum swimming performance (Umax) using a short-duration constant acceleration test and biochemical responses related to energetic processes (glucose levels) and liver health (aspartate aminotransferase (AST) activity). Chinook exposed to bifenthrin and bifenthrin and fipronil mixtures had a significantly reduced swimming performance, although not when exposed to fipronil alone. The AST activity was significantly increased in bifenthrin and mixture treatments and glucose levels were increased in Chinook following a mixture treatment, although not when exposed to fipronil alone. These findings suggest that there are different metabolic processes between bifenthrin and fipronil following dietary uptake that may influence toxicity. The significant reductions in swimming performance and increased levels of biochemical processes involved in energetics and fish heath could have implications for foraging activity and predator avoidance in wild fish at sensitive life stages.

Effects of temperature and salinity on bioconcentration and toxicokinetics of permethrin in pyrethroid-resistant Hyalella azteca.

Recent studies demonstrated pyrethroid resistance associated with voltage-gated sodium channel mutations in populations of the epibenthic amphipod, Hyalella azteca. Resistant populations were able to tolerate and bioconcentrate pyrethroids at concentrations significantly higher than toxic levels for non-resistant populations. In conjunction with elevated bioconcentration potential, environmental alteration particularly as a result of global climate change is anticipated to significantly alter abiotic parameters including temperature and salinity. These changes are expected to influence uptake and biotransformation of contaminants. Thus, the aims of the current study were a) to examine the bioconcentration potential of permethrin in two pyrethroid-resistant clades of H. azteca and b) assess the influence of temperature and salinity changes on toxicokinetic parameters. Two pyrethroid-resistant clades of H. azteca were exposed to 14C-permethrin at three salinities (0.2, 1.0 and 6.0 practical salinity units (PSU)) and temperatures (18, 23 and 28 °C). Tests were conducted for up to 36 h and uptake, elimination and biotransformation rates were calculated. Both populations demonstrated bioconcentration factors (BCFs) between five and seven times greater than published data for non-resistant H. azteca, with significant differences between clades. Calculated BCF values were comparable to field populations of resistant H. azteca, emphasizing the potential for elevated pyrethroid bioconcentration in the natural environment and increased exposure for predators consuming pyrethroid-resistant aquatic invertebrates. Alterations to temperature and salinity had no statistically significant effect on uptake or parent compound half-life in either population, though biotransformation was elevated at higher temperatures in both populations. Salinity had a variable effect between the two populations, with lower BCF values at 1.0 PSU in clade D H. azteca and greater BCFs at 6.0 PSU in clade C H. azteca. This is the first study to demonstrate the potential for future climate scenarios to influence toxicokinetics in pyrethroid-resistant aquatic organisms.

Bioavailability of legacy and current-use pesticides in juvenile Chinook salmon habitat of the Sacramento River watershed: Importance of sediment characteristics and extraction techniques.

The Sacramento River watershed, California, provides important rearing and migratory habitat for several species of conservation concern. Studies have suggested significant benefits for juvenile fish rearing in floodplain habitats of the watershed compared to the mainstem Sacramento River. However, the potential for contaminant exposure in each of these two habitats is poorly understood. Consequently, the present study aimed to determine the distribution and occurrence of bioavailable pesticides within two known salmon habitats using a suite of approaches including exhaustive chemical extraction, single-point Tenax extraction (SPTE) and ex situ passive sampling. Sediment samples were collected from sites within both habitats twice annually in 2019 and 2020, with inundation of the floodplain and high flows for both areas in 2019 and low flow conditions observed in 2020. Sediment characteristics including total organic carbon, black carbon and particle size distribution were determined to elucidate the influence of physical characteristics on pesticide distribution. Using exhaustive extractions, significantly greater sediment concentrations of organochlorines were observed in the floodplain compared to the Sacramento River in both years, with bioaccessible organochlorine concentrations also significantly greater in the floodplain (ANOVA, p < 0.05). Using both SPTEs and exhaustive extractions, significantly fewer pesticides were detected across both sites under low flow conditions as compared to high flow conditions (Poisson regression, p < 0.05). Sediment characteristics including percent fines and black carbon had significant positive relationships with total and bioaccessible pyrethroid and organochlorine concentrations. Fewer analytes were detected using low-density polyethylene (LDPE) passive samplers as compared to SPTEs, suggesting greater sensitivity of the Tenax technique for bioavailability assessments. These findings suggest that threatened juvenile fish populations rearing on the floodplain may have greater exposure to organochlorines than fish inhabiting adjacent riverine habitats, and that pesticide exposure of resident biota may be exacerbated during high-flow conditions.

Pesticide residues in juvenile Chinook salmon and prey items of the Sacramento River watershed, California - A comparison of riverine and floodplain habitats.

Juvenile Chinook salmon (Oncorhynchus tshawytscha) of the Sacramento River system encounter many anthropogenically-induced stressors while rearing and migrating to the Pacific Ocean. Located in a prominent agricultural region, the watershed serves as a source of notable contaminants including pesticides. Salmon rearing in riverine and floodplain areas are potentially exposed to these compounds via dietary exposure, which can vary based on selected food webs. Previous studies have suggested that juvenile Chinook salmon rearing in riverine and floodplain environments of the Sacramento River watershed are characterized by different dietary preferences, with potential for contrasting pesticide exposure between habitats. To examine the potential for pesticide exposure, juvenile Chinook salmon and known dietary items were collected in the mainstem Sacramento River and an adjacent floodplain, the Yolo Bypass, in 2019 and 2020, and analyzed for 33 pesticides, including degradates and isomers. Organochlorine pesticides including the DDX group (p,p'-DDT, p,p'-DDD and p,p'-DDE) were prevalent in all examined biota. There was a significantly greater number of total pesticide detections across all classes in zooplankton compared to macroinvertebrates, coupled with higher bifenthrin concentrations in zooplankton across regions and years, which may indicate different exposure potential depending on fish dietary preferences. Detection frequencies and concentrations of organochlorines were higher in prey items during flooding than in drought conditions, suggesting resuspension of legacy compounds. Significantly higher concentrations of organochlorines were recorded in floodplain rearing fish compared to the Sacramento River. These findings suggest that within these habitats, juvenile Chinook salmon feeding primarily on zooplankton within the water column may be exposed to a greater range of pesticides than those feeding on benthic macroinvertebrates, and that the benefits of floodplain rearing may come at a cost of increased organochlorine exposure.

Taking the Bite out of Mosquito Bites: The Role of Perceived Risk.

The present study examined how dimensions of perceived risk contributed to mosquito avoidance behaviors in a flood-prone area. Mosquito avoidance behaviors were classified into proactive (i.e. used repellant sources) and withdrawal/reactive (i.e. reduced outside activities) behaviors. After controlling for level of mosquito bites, increased scores for above normal perceived risk were associated with withdrawal/reactive avoidance behaviors; whereas, increased scores for normal perceived risk were associated with proactive avoidance behaviors. Efforts to improve mosquito avoidance behaviors should distinguish the type of perceived risk and the type of avoidance behavior. Greater congruence between perceived risk (i.e. normal risk of mosquito-borne illnesses) and avoidance behaviors (i.e. planning avoidance behaviors) will increase the effectiveness of education programs for disease prevention.

Enhanced trophic transfer of chlorpyrifos from resistant Hyalella azteca to inland silversides (Menidia beryllina) and effects on acetylcholinesterase activity and swimming performance at varying temperatures.

Chlorpyrifos, an organophosphate (OP) insecticide, is prevalent in aquatic systems globally and is often implicated in aquatic toxicity during storm events. Chlorpyrifos induces toxicity by inhibition of acetylcholinesterase (AChE) activity, which has been related to alterations to fish swimming performance. Resistance to organophosphate insecticides, including chlorpyrifos, is prevalent in populations of the epibenthic amphipod Hyalella azteca in areas with known OP exposure. Previous studies have demonstrated an elevated bioaccumulation potential of insecticide-resistant prey items, however the potential for trophic transfer of chlorpyrifos from OP-resistant prey items and associated neurotoxic effects in fish predators has not been studied. Consequently, the present study aimed to determine the potential for trophic transfer of chlorpyrifos from OP-resistant H. azteca to a known predator, the inland silverside, Menidia beryllina at two temperatures (18 and 23 °C) to simulate temperature changes associated with global climate change (GCC). Fish were fed either 14C-chlorpyrifos-dosed H. azteca or control animals for 7 d, after which total bioaccumulation, percent parent chlorpyrifos, brain AChE activity and swimming performance (ramp-Ucrit) were determined. Fish fed chlorpyrifos-dosed H. azteca bioaccumulated chlorpyrifos ranging from 29.9 to 1250 ng/g lipid, demonstrating the potential for trophic transfer. Lower bioaccumulation and greater biotransformation were observed in M. beryllina at 23 °C as compared to 18 °C, though this was not statistically significant. A significant 36.5% reduction in brain AChE activity was observed in fish fed chlorpyrifos-dosed H. azteca at 23 °C only, which may be attributed to increased biotransformation of parent chlorpyrifos to more potent AChE-inhibiting metabolites. Dietary chlorpyrifos exposure had no significant effect on swimming performance in M. beryllina, though ramp-Ucrit was significantly increased by 25% at 23 as compared to 18 °C. These findings confirm the potential for trophic transfer of chlorpyrifos from OP-resistant prey to fish predators and the potential for elevated temperatures to exacerbate the neurotoxic effects of chlorpyrifos.

Exposure to permethrin or chlorpyrifos causes differential dose- and time-dependent behavioral effects at early larval stages of an endangered teleost species.

Pyrethroid and organophosphate pesticides are two of the most commonly used classes of insecticide worldwide. At sublethal concentrations, permethrin (a pyrethroid) and chlorpyrifos (an organophosphate) impact behavior in model fish species. We investigated behavioral effects of environmentally relevant concentrations of permethrin or chlorpyrifos on early larval delta smelt Hypomesus transpacificus, a Critically Endangered teleost species endemic to the San Francisco Bay Delta, California, USA. Using a photomotor behavioral assay of oscillating light and dark periods, we measured distance moved, turn angle, meander, angular velocity, rotations, thigmotaxis (time spent in the border versus center), and swim speed duration and frequency. The lowest concentrations of permethrin used in the tests (0.05 and 0.5 µg l-1) caused significant increases in distance moved at 72 and 96 h, respectively. At 48, 72, and 96 h of exposure, 5 µg l-1 of permethrin caused a hyperactive state in which the larvae significantly decreased thigmotaxis, quickly turning in short bouts of activity, characterized by significant increases in rotations and freezing events. Larvae exposed to 0.05 µg l-1 chlorpyrifos significantly increased thigmotaxis at 72 and 96 h. In response to 5 µg l-1 chlorpyrifos, larvae significantly increased velocity at 72 h exposure, and significantly increased freezing events at 96 h. Behavioral data on larval delta smelt exposed to contaminants present in their limited habitat have the potential to aid evaluations of the suitability of spawning and rearing habitats for this endangered species, thus improving conservation management strategies focused on this sensitive life stage.

Bioaccumulation potential of chlorpyrifos in resistant Hyalella azteca: Implications for evolutionary toxicology.

Given extensive use of pesticides in agriculture, there is concern for unintended consequences to non-target species. The non-target freshwater amphipod, Hyalella azteca has been found to show resistance to the organophosphate (OP) pesticide, chlorpyrifos, resulting from an amino acid substitution in acetylcholinesterase (AChE), suggesting a selective pressure of unintended pesticide exposure. Since resistant organisms can survive in contaminated habitats, there is potential for them to accumulate higher concentrations of insecticides, increasing the risk for trophic transfer. In the present study, we estimated the uptake and elimination of chlorpyrifos in non-resistant US Lab, and resistant Ulatis Creek (ULC Resistant), H. azteca populations by conducting 24-h uptake and 48-h elimination toxicokinetic experiments with 14C-chlorpyrifos. Our results indicated that non-resistant H. azteca had a larger uptake clearance coefficient (1467 mL g-1 h-1) than resistant animals (557 mL g-1 h-1). The half-life derived from the toxicokinetic models also estimated that steady state conditions were reached at 13.5 and 32.5 h for US Lab and ULC, respectively. Bioaccumulation was compared between non-resistant and resistant H. azteca by exposing animals to six different environmentally relevant concentrations for 28 h. Detection of chlorpyrifos in animal tissues indicated that resistant animals exposed to high concentrations of chlorpyrifos were capable of accumulating the insecticide up to 10-fold higher compared to non-resistant animals. Metabolite analysis from the 28-h concentration experiments showed that between 20 and 50 % parent compound was detected in H. azteca. These results imply that bioaccumulation potential can be more significant in chlorpyrifos resistant H. azteca and may be an essential factor in assessing the full impacts of toxicants on critical food webs, especially in the face of increasing pesticide and chemical runoff.

The contribution of detoxification pathways to pyrethroid resistance in Hyalella azteca.

Chronic exposure to pyrethroid insecticides can result in strong selective pressures on non-target species in aquatic systems and drive the evolution of resistance and population-level changes. Characterizing the underlying mechanisms of resistance is essential to better understanding the potential consequences of contaminant-driven microevolution. The current study found that multiple mechanisms enhance the overall tolerance of Hyalella azteca to the pyrethroid permethrin. In H. azteca containing mutations in the voltage-gated sodium channel (VGSC), both adaptation and acclimation played a role in mitigating the adverse effects of pyrethroid exposures. Pyrethroid resistance is primarily attributed to the heritable mutation at a single locus of the VGSC, resulting in reduced target-site sensitivity. However, additional pyrethroid tolerance was conferred through enhanced enzyme-mediated detoxification. Cytochrome P450 monooxygenases (CYP450) and general esterases (GE) significantly contributed to the detoxification of permethrin in H. azteca. Over time, VGSC mutated H. azteca retained most of their pyrethroid resistance, though there was some increased sensitivity from parent to offspring when reared in the absence of pyrethroid exposure. Permethrin median lethal concentrations (LC50s) declined from 1809 ng/L in parent (P0) individuals to 1123 ng/L in the first filial (F1) generation, and this reduction in tolerance was likely related to alterations in acclimation mechanisms, rather than changes to target-site sensitivity. Enzyme bioassays indicated decreased CYP450 and GE activity from P0 to F1, whereas the VGSC mutation was retained. The permethrin LC50s in resistant H. azteca were still two orders-of-magnitude higher than non-resistant populations indicating that the largest proportion of resistance was maintained through the inherited VGSC mutation. Thus, the noted variation in tolerance in H. azteca is likely associated with inducible traits controlling enzyme pathways. A better understanding of the mechanistic and genomic basis of acclimation is necessary to more accurately predict the ecological and evolutionary consequences of contaminant-driven change in H. azteca.

Transcriptomic and Histopathological Effects of Bifenthrin to the Brain of Juvenile Rainbow Trout (Oncorhynchus mykiss).

The increased global use of pyrethroids raises concern for non-target aquatic species. Bifenthrin, among the most predominantly detected pyrethroids in the environment, is frequently measured in water samples above concentrations reported to induce neuroendocrine and neurotoxic effects to several threatened and endangered fish species, such as the Chinook salmon and steelhead trout. To better characterize the neurotoxic effect of bifenthrin to salmonids, rainbow trout were treated with environmentally relevant concentrations of bifenthrin (15 and 30 ng/L) for two weeks and assessed for changes in transcriptomic profiles and histopathological alterations. The top bioinformatic pathways predicted to be impaired in bifenthrin-exposed trout were involved in gonadotropin releasing hormone signaling, the dysregulation of iron homeostasis, reduced extracellular matrix stability and adhesion, and cell death. Subsequent histopathological analysis showed a significant increase in TUNEL positive cells in the cerebellum and optic tectum of bifenthrin-treated trout, relative to controls (p < 0.05). These findings suggest that low, ng/L concentrations of bifenthrin are capable of dysregulating proper neuroendocrine function, impair the structural integrity of the extracellular matrix and cell signaling pathways in the brain, and induce apoptosis in neurons of juvenile salmonids following bifenthrin treatment, which is consistent with metabolomic profiles demonstrating a common target and mechanism.

Pyrethroid bioaccumulation in field-collected insecticide-resistant Hyalella azteca.

Wild-type Hyalella azteca are highly sensitive to pyrethroid insecticides and typically do not survive exposure; however, pyrethroid bioaccumulation by insecticide-resistant H. azteca is an important potential risk factor for the transfer of pyrethroids to higher trophic species in aquatic systems. In the current study, four populations of pyrethroid-resistant H. azteca with corresponding sediment samples were sampled throughout the year, and nine-current use pyrethroids (tefluthrin, fenpropathrin, bifenthrin, cyhalothrin, permethrin, cyfluthrin, cypermethrin, esfenvalerate and deltamethrin) were measured. Bifenthrin was detected in every pyrethroid-resistant H. azteca tissue sample, up to 813 ng/g lipid, while cyhalothrin and permethrin were detected in fewer (18 and 28%, respectively) samples. Concurrent sampling of the sediment showed total pyrethroid concentrations exceeding toxic unit thresholds for non-resistant H. azteca survival, and confirmed the ubiquitous presence of bifenthrin at each site and sampling event. Bifenthrin concentrations in H. azteca tended to be higher in samples collected in winter months, and seasonal factors, such as temperature and rainfall, may have contributed to the noted differences in bioaccumulation. Finally, the bifenthrin and permethrin biota-sediment accumulation factors (BSAF) for pyrethroid-resistant H. azteca were similar to the BSAF values for less sensitive invertebrates, and therefore the development of resistance may enable an additional pathway for trophic transfer of pyrethroids in species that would otherwise be too sensitive to survive the exposure.

Effects of dietary cypermethrin exposure on swimming performance and expression of lipid homeostatic genes in livers of juvenile Chinook salmon, Oncorhynchus tshawytscha.

The increased use of pyrethroid insecticides raises concern for exposure to non-target aquatic species, such as Chinook salmon (Oncorhynchus tshawytscha). Cypermethrin, a type II pyrethroid, is frequently detected in surface waters and sediments at concentrations that exceed levels that induce toxicity to several invertebrate and salmonid species. To better understand the effects of cypermethrin to salmonids following dietary exposure, juvenile Chinook salmon were dietarily exposed to a 0, 200, or 2000 ng/g cypermethrin diet for a duration of 7, 14, or 21 days and assessed for body burden residues, swimming performance, lipid content, and lipid homeostatic gene expression. The average cypermethrin concentrations in fish dietarily exposed to cypermethrin for 21 days were 155.4 and 952.1 ng cypermethrin/g lipid for the 200 and 2000 ng/g pellet treatments, respectively. Increased trends of fatty acid synthase (fasn, r2 = 0.10, p < 0.05) and ATP citrate lyase (acly, r2 = 0.21, p < 0.001) mRNA expression were found in the fish livers relative to increasing cypermethrin body burden residues, though no significant changes in the mRNA expression of farnesoid X receptor or liver X receptor were observed. Furthermore, Chinook salmon dietarily exposed to cypermethrin did not have a significantly altered burst swimming performance (Umax). These results support studies that have suggested Umax may not be a sensitive endpoint when assessing the effects of certain pesticide classes, such as pyrethroids, but that dysregulation of fasn and acly expression may alter lipid homeostasis and energy metabolism in the liver of fish dietarily exposed to cypermethrin.