Insecticide-induced changes in amphibian brains: How sublethal concentrations of chlorpyrifos directly affect neurodevelopment.

Widespread use of pesticides often contaminates natural habitats, exposing nontarget organisms to pesticides that were designed to control pest populations. Even low levels of pesticides can affect aquatic communities both directly and indirectly. Previous work has shown that trace amounts of the pesticide chlorpyrifos altered tadpole morphology and neurodevelopment in artificial ponds (mesocosms). To determine whether effects resulted from direct chlorpyrifos exposure or from disruption of the food web due to a pesticide-induced decline in zooplankton, we examined the impacts of chlorpyrifos on amphibian development in the presence of chlorpyrifos-resistant zooplankton, a key component of the aquatic trophic community. Northern leopard frog (Lithobates pipiens) tadpoles were reared through metamorphosis in mesocosms containing either 0 or 1 µg/L chlorpyrifos and either chlorpyrifos-resistant or chlorpyrifos-sensitive Daphnia pulex zooplankton. Developmental exposure to chlorpyrifos resulted in metamorphs with a relatively wider optic tectum, medulla, and diencephalon compared with controls, and this result was found regardless of the zooplankton population within the mesocosm. Thus, chlorpyrifos directly impacted brain development, independent of the effects on the trophic community. With respect to body shape, chlorpyrifos had no effect on body shape of metamorphs reared in mesocosms with chlorpyrifos-sensitive zooplankton, but body shape was sensitive to zooplankton population in the absence of chlorpyrifos. To conclude, low, ecologically relevant doses of organophosphorous pesticides can directly impact neurodevelopment in a vertebrate model. Environ Toxicol Chem 2018;37:2692-2698. © 2018 SETAC.

Effect of Simultaneous Amphibian Exposure to Pesticides and an Emerging Fungal Pathogen, Batrachochytrium dendrobatidis.

Amphibian declines have been linked to numerous factors, including pesticide use and the fungal pathogen Batrachochytrium dendrobatidis (Bd). Moreover, research has suggested a link between amphibian sensitivity to Bd and pesticide exposure. We simultaneously exposed postmetamorphic American toads (Anaxyrus americanus), western toads (A. boreas), spring peepers (Pseudacris crucifer), Pacific treefrogs (P. regilla), leopard frogs (Lithobates pipiens), and Cascades frogs (Rana cascadae) to a factorial combination of two pathogen treatments (Bd+, Bd-) and four pesticide treatments (control, ethanol vehicle, herbicide mixture, and insecticide mixture) for 14 d to quantify survival and infection load. We found no interactive effects of pesticides and Bd on anuran survival and no effects of pesticides on infection load. Mortality following Bd exposure increased in spring peepers and American toads and was dependent upon snout-vent length in western toads, American toads, and Pacific treefrogs. Previous studies reported effects of early sublethal pesticide exposure on amphibian Bd sensitivity and infection load at later life stages, but we found simultaneous exposure to sublethal pesticide concentrations and Bd had no such effect on postmetamorphic juvenile anurans. Future research investigating complex interactions between pesticides and Bd should employ a variety of pesticide formulations and Bd strains and follow the effects of exposure throughout ontogeny.

If you see one, have you seen them all?: Community-wide effects of insecticide cross-resistance in zooplankton populations near and far from agriculture.

The worldwide use of pesticides has led to increases in agricultural yields by reducing crop losses. However, increased pesticide use has resulted in pesticide-resistant pest species and recent studies have discovered pesticide-resistance in non-target species living close to farms. Such increased tolerance not only affects the species, but can alter the entire food web. Given that some species can evolve not only resistance to a single pesticide, but also cross-resistance to other pesticides that share the same mode of action, one would predict that cross-resistance to pesticides would also have effects on the entire community and affect community stability. To address this hypothesis, we conducted an outdoor mesocosm experiment comprised of 200 identical aquatic communities with phytoplankton, periphyton, and leopard frog (Lithobates pipiens) tadpoles. To these communities, we added one of four Daphnia pulex populations that we previously discovered were either resistant or sensitive to the insecticide of chlorpyrifos as a result of living close to or far from agriculture, respectively. We then exposed the communities to either no insecticide or three different concentrations of AChE-inhibiting insecticides (chlorpyrifos, malathion or carbaryl) or sodium channel-inhibiting insecticides (permethrin or cypermethrin). We discovered that communities containing sensitive Daphnia pulex experienced phytoplankton blooms and subsequent cascades through all trophic groups including amphibians at moderate to high concentrations of all five insecticides. However, communities containing resistant D. pulex were buffered from these effects at low to moderate concentrations of all AChE-inhibiting insecticides, but were not buffered against the pyrethroid insecticides. These data suggest that a simple change in the population-level resistance of zooplankton to a single insecticide can have widespread consequences for community stability and that the effects can be extrapolated to a wide variety of pesticides that share the same mode of action.

Wetland defense: naturally occurring pesticide resistance in zooplankton populations protects the stability of aquatic communities.

Anthropogenic stressors are ubiquitous and have been implicated in worldwide declines of terrestrial and aquatic species. Pesticides are one such stressor that can have profound effects on aquatic communities by directly affecting sensitive species and indirectly affecting other species via trophic cascades, which can alter ecosystem function. However, there is growing evidence that non-target species can evolve increased resistance. When such species are important drivers of the food web, then evolved resistance should help buffer communities from the effects of pesticides. To examine this possibility, we cultured four populations of the common zooplankton Daphnia pulex that we previously demonstrated were either sensitive or resistant to a common insecticide (i.e., chlorpyrifos) due to their proximity to agriculture. Using outdoor mesocosms that contained identical aquatic communities of phytoplankton, periphyton, and leopard frog tadpoles (Lithobates pipiens), we manipulated four D. pulex populations and four insecticide concentrations. As we monitored the communities for nearly 3 months, we found that the insecticide caused direct mortality of D. pulex in communities containing sensitive populations, and this led to a bloom of phytoplankton. In contrast, the insecticide caused much less direct mortality in communities containing resistant D. pulex populations, and the trophic cascade was prevented under low to moderate insecticide concentrations. Across all insecticide treatments, survivorship of leopard frogs was approximately 72 % in communities with resistant D. pulex but only 35 % in communities with sensitive D. pulex. To our knowledge, this is one of the first studies to use naturally occurring population variation in insecticide resistance to show that the evolution of pesticide resistance in zooplankton can mitigate the effects of insecticide-induced trophic cascades, and that this outcome can have far-reaching community effects.

Effects of Pesticide Mixtures on Host-Pathogen Dynamics of the Amphibian Chytrid Fungus.

Anthropogenic and natural stressors often interact to affect organisms. Amphibian populations are undergoing unprecedented declines and extinctions with pesticides and emerging infectious diseases implicated as causal factors. Although these factors often co-occur, their effects on amphibians are usually examined in isolation. We hypothesized that exposure of larval and metamorphic amphibians to ecologically relevant concentrations of pesticide mixtures would increase their post-metamorphic susceptibility to the fungus Batrachochytrium dendrobatidis (Bd), a pathogen that has contributed to amphibian population declines worldwide. We exposed five anuran species (Pacific treefrog, Pseudacris regilla; spring peeper, Pseudacris crucifer; Cascades frog, Rana cascadae; northern leopard frog, Lithobates pipiens; and western toad, Anaxyrus boreas) from three families to mixtures of four common insecticides (chlorpyrifos, carbaryl, permethrin, and endosulfan) or herbicides (glyphosate, acetochlor, atrazine, and 2,4-D) or a control treatment, either as tadpoles or as newly metamorphic individuals (metamorphs). Subsequently, we exposed animals to Bd or a control inoculate after metamorphosis and compared survival and Bd load. Bd exposure significantly increased mortality in Pacific treefrogs, spring peepers, and western toads, but not in Cascades frogs or northern leopard frogs. However, the effects of pesticide exposure on mortality were negligible, regardless of the timing of exposure. Bd load varied considerably across species; Pacific treefrogs, spring peepers, and western toads had the highest loads, whereas Cascades frogs and northern leopard frogs had the lowest loads. The influence of pesticide exposure on Bd load depended on the amphibian species, timing of pesticide exposure, and the particular pesticide treatment. Our results suggest that exposure to realistic pesticide concentrations has minimal effects on Bd-induced mortality, but can alter Bd load. This result could have broad implications for risk assessment of amphibians; the outcome of exposure to multiple stressors may be unpredictable and can differ between species and life stages.

Living on the edge: populations of two zooplankton species living closer to agricultural fields are more resistant to a common insecticide.

Ecological communities across the globe are exposed to diverse natural and anthropogenic stressors and disturbances that can lead to community-wide impacts. Contaminants are a group of anthropogenic disturbances that are ubiquitous in the environment and can trigger trophic cascades, increased susceptibility to pathogens, reduced biodiversity, and altered ecosystems. In these ecosystems, substantial attention has been given to evolved resistance in targeted pest species, but little attention has been given to the evolution of resistance in nontarget species in nature. For the present study, the authors used laboratory toxicity tests to determine if 2 common, co-occurring species of freshwater zooplankton (Simocephalus vetulus and Daphnia pulex) showed population-level variation in sensitivity to a common insecticide (chlorpyrifos). For both species, it was found that populations living near agricultural fields--a proxy for pesticide use--were more resistant to chlorpyrifos than populations collected from ponds far from agriculture. This finding is consistent with the evolution of resistance to pesticides. To the authors' knowledge, only 1 previous study (using Daphnia magna) has demonstrated this relationship. Collectively, these results suggest that evolved resistance may be common in zooplankton populations located near agriculture. Moreover, because zooplankton play a key role in aquatic food webs, it is expected that population variation in resistance would dramatically alter aquatic food webs, particularly with exposure to low concentrations of insecticides.