Changing climate and reorganized species interactions modify community responses to climate variability.

While an array of ecological mechanisms has been shown to stabilize natural community dynamics, how the effectiveness of these mechanisms-including both their direction (stabilizing vs. destabilizing) and strength-shifts under a changing climate remains unknown. Using a 35-y dataset (1985 to 2019) from a desert stream in central Arizona (USA), we found that as annual mean air temperature rose 1°C and annual mean precipitation reduced by 40% over the last two decades, macroinvertebrate communities experienced dramatic changes, from relatively stable states during the first 15 y of this study to wildly fluctuating states highly sensitive to climate variability in the last 10 y. Asynchronous species responses to climatic variability, the primary mechanism historically undergirding community stability, greatly weakened. The emerging climate regime-specifically, concurrent warming and prolonged multiyear drought-resulted in community-wide synchronous responses and reduced taxa richness. Diversity loss and new establishment of competitors reorganized species interactions. Unlike manipulative experiments that often suggest stabilizing roles of species interactions, we found that reorganized species interactions switched from stabilizing to destabilizing influences, further amplifying community fluctuations. Our study provides evidence of climate change-induced modifications of mechanisms underpinning long-term community stability, resulting in an overall destabilizing effect.

Swimming Behavior of Daphnia magna Is Altered by Pesticides of Concern, as Components of Agricultural Surface Water and in Acute Exposures.

Pesticides with novel modes of action including neonicotinoids and anthranilic diamides are increasingly detected in global surface waters. Little is known about how these pesticides of concern interact in mixtures at environmentally relevant concentrations, a common exposure scenario in waterways impacted by pesticide pollution. We examined effects of chlorantraniliprole (CHL) and imidacloprid (IMI) on the sensitive invertebrate, Daphnia magna. Exposures were first performed using surface waters known to be contaminated by agricultural runoff. To evaluate the seasonal variation in chemical concentration and composition of surface waters, we tested surface water samples taken at two time points: during an extended dry period and after a first flush storm event. In surface waters, the concentrations of CHL, IMI, and other pesticides of concern increased after first flush, resulting in hypoactivity and dose-dependent photomotor responses. We then examined mortality and behavior following single and binary chemical mixtures of CHL and IMI. We detected inverse photomotor responses and some evidence of synergistic effects in binary mixture exposures. Taken together, this research demonstrates that CHL, IMI, and contaminated surface waters all cause abnormal swimming behavior in D. magna. Invertebrate swimming behavior is a sensitive endpoint for measuring the biological effects of environmental pesticides of concern.

Copper Exposure Affects Anti-Predatory Behaviour and Acetylcholinesterase Levels in Culex pipiens (Diptera, Culicidae).

Copper is an essential metal that occurs chronically in the environment and affects the development and physiology of aquatic insects. In excess amounts, it can impair their nervous system and behaviour. We tested the anti-predatory behaviour of Cx. pipiens larvae after seven days exposure with several concentrations of copper up to 500 mg L-1. We measured responses to non- consumptive (predation cues) and consumptive predation (dragonfly larvae) across two generations. We also tested the accumulated effect of copper on AChE enzyme activity. We exposed half of treated and control larvae to predation cues (water with predator odour and crushed conspecifics) and the other half to water without predation cues. We evaluated total distance moved and velocity. Copper reduced the distance moved and velocity, with stronger effects in the second generation. Copper had no significant effect on larvae eaten by dragonflies. Copper inhibited the AChE enzyme across both generations at 500 µg L-1. Copper can affect the nervous system directly by inhibiting AChE activity, and possibly also by impairing the olfaction sensors of the larvae, resulting in larval inability to detect predation cues.

Agricultural surface water, imidacloprid, and chlorantraniliprole result in altered gene expression and receptor activation in Pimephales promelas.

The toxicity of single pesticides is likely underestimated when considering complex pesticide mixtures found in agricultural runoff and this is especially true for newer pesticides with little toxicity data on non-target species. The goal of our study was to compare the toxicity of two newer pesticides, imidacloprid (IMI) and chlorantraniliprole (CHL), when an invertebrate and fish were exposed to single compounds, binary mixtures or surface water collected near agricultural fields. A secondary goal was to determine whether changes in select subcellular molecular pathways correspond to the insecticides' mechanisms of activity in aquatic organisms. We conducted acute (96 h) exposures using a dilution series of field water and environmentally relevant concentrations of single and binary mixtures of IMI and CHL. We then evaluated survival, gene expression and the activity of IMI toward the n-acetylcholine receptor (nAChR) and CHL activity toward the ryanodine receptor (RyR). Both IMI and CHL were detected at all sampling locations for May 2019 and September 2019 sampling dates and exposure to field water led to high invertebrate but not fish mortality. Fish exposed to field collected water had significant changes in the relative expression of genes involved with detoxification and neuromuscular function. Exposure of fish to single compounds or binary mixtures of IMI and CHL led to increased relative gene expression of RyR in fish. Furthermore, we found that IMI targets the nAChR in aquatic invertebrates and that CHL can cause overactivation of the RyR in invertebrates and fish. Overall, our finding suggests that IMI and CHL may impact neuromuscular health in fish. Expanding monitoring efforts to include sublethal and molecular assays would allow the detection of subcellular level effects due to complex mixtures present in surface water near agricultural areas.

Effect of long-term exposure to copper on survival and development of two successive generations of Culex pipiens (Diptera, Culicidae).

Aquatic invertebrates can be exposed to copper from various sources, including agricultural applications. For example, concentrations up to 1000 µg L-1 are found within rice fields, where copper-containing formulations are used as fungicides and algaecides. We conducted toxicity tests to study lethal and sublethal effects of copper sulfate pentahydrate on all immature stages across two generations of Culex pipiens mosquitoes as our model organism. Mortality was dose-dependent at concentrations of 500 µg L-1 and above in the first generation, and 125 µg L-1 and above in the second generation. The median lethal concentrations (LC50) of copper sulfate pentahydrate for larval Cx. pipiens were 476 ± 30.60 µg L-1 and 348.67 ± 23.20 µg L-1 for the first and second generations, respectively. Generation one pupation decreased from 96% in controls to 48% at 500 µg L-1, while the second-generation pupation decreased from 96% in controls to 17.5% at 500 µg L-1. Mortality during the pupal stage varied from 2 to 10% at 500 µg L-1 of first and second generations, respectively. Higher levels also delayed development to adulthood in both generations. The duration of the immature period was 14.8 days in controls in both generations, but when exposed at 500 µg L-1 it increased to 18.8 days in the first generation and to 20.5 days in the second generation. The chronic, multi-generation exposures in this study showed greater toxicity than reported for shorter exposures of Cx. pipiens, and confamilial taxa like Culex hortensis and Anopheles hispaniola.

Herbicide treatment alters the effects of water hyacinth on larval mosquito abundance.

Invasive aquatic weeds are managed with herbicides to reduce their negative impacts on waterways in many areas, including the California Delta Region. Herbicides create a dynamic environment of living and decomposing plant matter that could affect larval mosquitoes and other invertebrates, such as their predators and competitors. Our objective was to compare the number of larval mosquitoes in water or water hyacinth, before and after an herbicide treatment. We created replicated pond mesocosms with water hyacinth, water hyacinth treated with glyphosate and an oil adjuvant, open water, and water with glyphosate plus adjuvant. We sampled for larval mosquitoes and other aquatic invertebrates. Before herbicide addition, there was a trend for more larval mosquitoes in open water tanks than in tanks with water hyacinth. Herbicide application resulted in an immediate decrease of larval mosquitoes. As decay progressed, larval mosquitoes became most abundant in mesocosms with herbicide-treated hyacinth and very few larval mosquitoes were found in other habitat treatments. Although the numbers of predatory and competitor insects had some variation between treatments, no clear pattern emerged. This information on how invasive weed management with herbicides affects larval mosquitoes will allow control practices for larval mosquitoes and invasive weeds to be better integrated.

Environmentally relevant concentrations of herbicides impact non-target species at multiple sublethal endpoints.

There is concern over herbicide exposure effects on aquatic primary production and zooplankton as herbicides are found in aquatic ecosystems at concentrations potentially toxic to phytoplankton. We first aimed to determine the effect concentrations (growth inhibition) and mixture interactions of the herbicides diuron (0.5 to 50µg/L) and hexazinone (0.5 to 40µg/L) on the green algae Pseudokirchneriella subcapitata. Secondly, we evaluated chronic effects on Daphnia magna that were periodically fed on P. subcapitata that had been exposed to low, medium, and high concentrations. We hypothesized that based on the mode of action of the herbicides we would observe additive growth inhibition in algae, and sublethal effects on D. magna. Growth inhibition in P. subcapitata following mixture exposure was most consistent with the concentration addition (CA) concept; while the independent action (IA) model underestimated the combined effect. The lowest observed effect concentrations (LOEC) were 1.50µg/L hexazinone, 1.18µg/L diuron, and 0.125 TU (0.30µg/L diuron×0.12µg/L hexazinone) in the single and binary mixture exposures, respectively. High hexazinone exposure decreased D. magna survival (80% vs. 55.6%). Neonate number was reduced by 13.9% in high mixture and 23.5% in high hexazinone treatments. Gravid body length was reduced by 9.5% following exposure to the high mixture. Herbicide exposure decreased neonate size, especially in later broods. Herbicides decreased the phototaxic responses of neonates in most treatments. Herbicide exposure effects were detected at environmentally relevant concentrations, levels considered to be safe according to current USEPA aquatic life benchmarks, suggesting that these benchmarks need to be updated to improve ecological risk assessment. As herbicides are some of the most applied pesticides worldwide, sublethal endpoints can serve as sensitive early warning tools to indicate their presence and can support regulatory assessments and monitoring to protect aquatic life.

Environmental safety review of methoprene and bacterially-derived pesticides commonly used for sustained mosquito control.

Some pesticides are applied directly to aquatic systems to reduce numbers of mosquito larvae (larvicides) and thereby reduce transmission of pathogens that mosquitoes vector to humans and wildlife. Sustained, environmentally-safe control of larval mosquitoes is particularly needed for highly productive waters (e.g., catchment basins, water treatment facilities, septic systems), but also for other habitats to maintain control and reduce inspection costs. Common biorational pesticides include the insect juvenile hormone mimic methoprene and pesticides derived from the bacteria Bacillus thuringiensis israelensis, Lysinibacillus sphaericus and Saccharopolyspora spinosa (spinosad). Health agencies, the public and environmental groups have especially debated the use of methoprene because some studies have shown toxic effects on non-target organisms. However, many studies have demonstrated its apparent environmental safety. This review critically evaluates studies pertinent to the environmental safety of using methoprene to control mosquito larvae, and provides concise assessments of the bacterial larvicides that provide sustained control of mosquitoes. The review first outlines the ecological and health effects of mosquitoes, and distinguishes between laboratory toxicity and environmental effects. The article then interprets non-target toxicity findings in light of measured environmental concentrations of methoprene (as used in mosquito control) and field studies of its non-target effects. The final section evaluates information on newer formulations of bacterially-derived pesticides for sustained mosquito control. Results show that realized environmental concentrations of methoprene were usually 2-5µg/kg (range 2-45µg/kg) and that its motility is limited. These levels were not toxic to the vast majority of vertebrates and invertebrates tested in laboratories, except for a few species of zooplankton, larval stages of some other crustaceans, and small Diptera. Studies in natural habitats have not documented population reductions except in small Diptera. Bacterial larvicides showed good results for sustained control with similarly limited environmental effects, except for spinosad, which had broader effects on insects in mesocosms and temporary pools. These findings should be useful to a variety of stakeholders in informing decisions on larvicide use to protect public and environmental health in a 'One Health' framework.

Greater Species Richness of Bacterial Skin Symbionts Better Suppresses the Amphibian Fungal Pathogen Batrachochytrium Dendrobatidis.

The symbiotic microbes that grow in and on many organisms can play important roles in protecting their hosts from pathogen infection. While species diversity has been shown to influence community function in many other natural systems, the question of how species diversity of host-associated symbiotic microbes contributes to pathogen resistance is just beginning to be explored. Understanding diversity effects on pathogen resistance could be particularly helpful in combating the fungal pathogen Batrachochytrium dendrobatidis (Bd) which has caused dramatic population declines in many amphibian species and is a major concern for amphibian conservation. Our study investigates the ability of host-associated bacteria to inhibit the proliferation of Bd when grown in experimentally assembled biofilm communities that differ in species number and composition. Six bacterial species isolated from the skin of Cascades frogs (Rana cascadae) were used to assemble bacterial biofilm communities containing 1, 2, 3, or all 6 bacterial species. Biofilm communities were grown with Bd for 7 days following inoculation. More speciose bacterial communities reduced Bd abundance more effectively. This relationship between bacterial species richness and Bd suppression appeared to be driven by dominance effects-the bacterial species that were most effective at inhibiting Bd dominated multi-species communities-and complementarity: multi-species communities inhibited Bd growth more than monocultures of constituent species. These results underscore the notion that pathogen resistance is an emergent property of microbial communities, a consideration that should be taken into account when designing probiotic treatments to reduce the impacts of infectious disease.

An assessment of direct and indirect effects of two herbicides on aquatic communities.

Herbicides are often detected in watersheds at concentrations that are toxic to phytoplankton, potentially causing indirect effects on higher trophic organisms. The long-term effects of 5 applications over 30 d of binary mixtures of the herbicides diuron and hexazinone were assessed at "low" and "high" concentrations typically found in the environment, using mesocosms. Sixteen of 95 phytoplankton taxa, 3 of 18 zooplankton taxa, and 6 of 14 macroinvertebrate taxa responded negatively to contaminant exposures. Herbicide applications altered the phytoplankton community structure. Relative abundance of Cyanophyceae decreased following 5 applications from 52.1% in the control to 37.3% in the low treatment and to 25.9% in the high treatment, while Chlorophyceae increased to 50.6% in the low treatment and to 61.7% in the high treatment compared with the control (39.7%). Chlorophyceae had the greatest number of affected species (8), whereas 1 species within the Cyanophyceae was negatively affected on more than 1 sampling day. Further, chlorophyll a was reduced on 4 and 5 d out of the 8 total sampling days in the low and high treatments, respectively, compared with the control. These results highlight that integrating multiple taxa and contaminants with long-term exposures in ecological risk assessments of herbicides can facilitate the ability to make predictive and mechanistic generalizations about the role of herbicides in shaping patterns of species abundance in natural systems. Environ Toxicol Chem 2017;36:2234-2244. © 2017 SETAC.

A long-term assessment of pesticide mixture effects on aquatic invertebrate communities.

To understand the potential effects of pesticide mixtures on aquatic ecosystems, studies that incorporate increased ecological relevance are crucial. Using outdoor mesocosms, the authors examined long-term effects on aquatic invertebrate communities of tertiary mixtures of commonly used pesticides: 2 pyrethroids (permethrin, λ-cyhalothrin) and an organophosphate (chlorpyrifos). Application scenarios were based on environmentally relevant concentrations and stepwise increases of lethal concentrations from 10% (LC10) to 50% (LC50) based on laboratory tests on Hyalella azteca and Chironomus dilutus; repeated applications were meant to generally reflect runoff events in a multiple-grower or homeowner watershed. Pyrethroids rapidly dissipated from the water column, whereas chlorpyrifos was detectable even 6 wk after application. Twelve of 15 macroinvertebrate and 10 of 16 zooplankton taxa responded to contaminant exposures. The most sensitive taxa were the snail Radix sp., the amphipod H. azteca, the water flea Daphnia magna, and copepods. Environmentally relevant concentrations had acute effects on D. magna and H. azteca (occurring 24 h after application), whereas lag times were more pronounced in Radix sp. snails and copepods, indicating chronic sublethal responses. Greatest effects on zooplankton communities were observed in environmentally relevant concentration treatments. The results indicate that insecticide mixtures continue to impact natural systems over multiple weeks, even when no longer detectable in water and bound to particles. Combinations of indirect and direct effects caused consequences across multiple trophic levels.

A comparison of the sublethal and lethal toxicity of four pesticides in Hyalella azteca and Chironomus dilutus.

Laboratory toxicity testing is the primary tool used for surface water environmental risk assessment; however, there are critical information gaps regarding the sublethal effects of pesticides. In 10-day exposures, we assessed the lethal and sublethal (motility and growth) toxicities of four commonly used pesticides, bifenthrin, permethrin, cyfluthrin, and chlorpyrifos, on two freshwater invertebrates, Chironomus dilutus and Hyalella azteca. Pyrethroids were more toxic than the organophosphate chlorpyrifos in both species. Bifenthrin was most toxic to H. azteca survival and growth. Cyfluthrin was most toxic to C. dilutus. However, cyfluthrin had the greatest effect on motility on both H. azteca and C. dilutus. The evaluated concentrations of chlorpyrifos did not affect C. dilutus motility or growth, but significantly impacted H. azteca growth. Motility served as the most sensitive endpoint in assessing sublethal effects at low concentrations for both species, while growth was a good indicator of toxicity for all four pesticides for H. azteca. The integration of sublethal endpoints in ambient water monitoring and pesticide regulation efforts could improve identification of low-level pesticide concentrations that may eventually cause negative effects on food webs and community structure in aquatic environments.

The use of growth and behavioral endpoints to assess the effects of pesticide mixtures upon aquatic organisms.

Aquatic communities are often subject to complex contaminant mixtures, usually at sublethal concentrations, that can cause long-term detrimental effects. Chemicals within mixtures can effectively interact, resulting in synergism, antagonism or additivity. We investigated the tertiary mixture effects of two pyrethroids, lambda-cyhalothrin and permethrin, and the organophosphate chlorpyrifos, evaluating sublethal endpoints; immobility and growth, on Chironomus dilutus in 10-day exposures. We utilized a toxic units (TU) approach, based on median lethal concentrations (LC50) for each compound. The concepts of independent action and concentration addition were used to compare predicted mixture toxicity to observed mixture toxicity. Increased immobility resulted from mixture concentrations ≥1 TU (7.45 ng/L lambda-cyhalothrin × 24.90 ng/L permethrin × 129.70 ng/L chlorpyrifos), and single pesticides concentrations ≥0.25 TU (5.50 ng/L lambda-cyhalothrin, 24.23 ng/L permethrin, 90.92 ng/L chlorpyrifos, respectively). Growth was inhibited by pesticide mixtures ≥0.125 TU (1.04 ng/L lambda-cyhalothrin × 3.15 ng/L permethrin × 15.47 ng/L chlorpyrifos), and singly by lambda-cyhalothrin ≥0.25 TU (5.50 ng/L), and permethrin ≥0.167 TU (18.21 ng/L). The no observed effect concentrations (NOEC) for immobility and growth, for both mixture and single-pyrethroid exposure, were up to 8.0 and 12.0 times respectively lower than the corresponding NOEC for survival. The median effective concentrations (EC50) for growth (mixture and single-pyrethroid exposure) were up to 7.0 times lower than the respective LC50. This study reinforces that the integration of sublethal endpoints in monitoring efforts is powerful in discerning toxic effects that would otherwise be missed by solely utilizing traditional toxicity assessments.

Efficacy of spinosad in control of larval Culex tarsalis and chironomid midges, and its nontarget effects.

Formulations of spinosad for mosquito and midge control (e.g., Natular) are derived from the Saccharopolyspora spinosa bacterium. They offer a different mode of action from other larvicides. We tested the effectiveness of single-brood (2EC) and extended-release pellet (G30) formulations against larvae of Culex tarsalis and chironomid midges, at mid- to high label rates, in wetland mesocosms. We also monitored survival of mayfly nymphs and other nontarget insects. Both formulations were effective against mosquitoes for > 4 wk and yielded excellent midge control. Spinosad caused mortality of mayflies and other nontarget insects. Spinosad was less toxic to mayflies than to targets, and effects on mayflies were undetectable after day 21. The higher toxicity for mosquitoes indicates that future research could identify rates that reduce nontarget effects while maintaining high efficacy.

Control of mosquito larvae in seasonal wetlands on a wildlife refuge using VectoMax CG.

There is a great need for novel insecticides to control mosquitoes. VectoMax is a new mosquito larvicide that combines toxins from Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bs), and is designed to provide extended mosquito control. We tested the initial efficacy and longevity of control of mosquitoes using one of the formulations, VectoMax CG, in a full-scale study conducted in seasonal wetlands. VectoMax CG was applied by air at 8.9 kg/ha to 3 wetlands and 3 others were untreated controls. VectoMax CG controlled Culex tarsalis through day 28 and showed activity against Aedes melanimon. Use of this dual-material, extended-action formulation could minimize inspection visits and reduce application costs compared to Bti and Bs alone, and its combination of toxins may forestall resistance development.

Nonnative trout impact an alpine-nesting bird by altering aquatic-insect subsidies.

Adjacent food webs may be linked by cross-boundary subsidies: more-productive donor systems can subsidize consumers in less-productive neighboring recipient systems. Introduced species are known to have direct effects on organisms within invaded communities. However, few studies have addressed the indirect effects of nonnative species in donor systems on organisms in recipient systems. We studied the direct role of introduced trout in altering a lake-derived resource subsidy and their indirect effects in altering a passerine bird's response to that subsidy. We compared the abundance of aquatic insects and foraging Gray-crowned Rosy-Finches (Leucosticte tephrocotis dawsoni, "Rosy-Finch") at fish-containing vs. fishless lakes in the Sierra Nevada Mountains of California (USA). Introduced trout outcompeted Rosy-Finches for emerging aquatic insects (i.e., mayflies). Fish-containing lakes had 98% fewer mayflies than did fishless lakes. In lakes without fish, Rosy-Finches showed an aggregative response to emerging aquatic insects with 5.9 times more Rosy-Finches at fishless lakes than at fish-containing lakes. Therefore, the introduction of nonnative fish into the donor system reduced both the magnitude of the resource subsidy and the strength of cross-boundary trophic interactions. Importantly, the timing of the subsidy occurs when Rosy-Finches feed their young. If Rosy-Finches rely on aquatic-insect subsidies to fledge their young, reductions in the subsidy by introduced trout may have decreased Rosy-Finch abundances from historic levels. We recommend that terrestrial recipients of aquatic subsidies be included in conservation and restoration plans for ecosystems with alpine lakes.

Does synergized pyrethrin applied over wetlands for mosquito control affect Daphnia magna zooplankton or Callibaetis californicus mayflies?

BACKGROUND: Public health agencies may apply aerosolized synergized pyrethrin over wetlands repeatedly to control mosquitoes. This concerns wildlife managers because studies have shown the accumulation of pyrethroids, which are chemically similar to pyrethrin, in sediments in amounts that can be toxic to invertebrates. The authors tested whether repeated applications of synergized pyrethrin over wetlands caused mortality of two aquatic invertebrates: the zooplankton Daphnia magna Straus and a mayfly, Callibaetis californicus Banks. Fifteen wetland mesocosms were either exposed to repeated pyrethrin sprays or were protected by lids. Invertebrates in screened cages were placed in mesocosms before the fifth and eleventh spray, and directly into wetlands before spray 11. Six mesocosms were exposed to spray deposition. Caged adult mosquitoes were used to verify that sprays drifted over mesocosms. Sediments were analyzed for insecticide residues. RESULTS: There were no detectable effects of synergized pyrethrin on 36 h survival of Daphnia or mayflies, but most exposed adult mosquitoes died. Some exposed sediments yielded pyrethrin (< or =34.5 ng g(-1)); most showed piperonyl butoxide (PBO) (< or =14.9 ng g(-1)). CONCLUSIONS: Deposition of aerosolized 25% pyrethrin + 5% PBO may contaminate wetlands, but its application at rates used for mosquito control did not produce detectable effects on indicator species.

Nontarget effects of the mosquito adulticide pyrethrin applied aerially during a West Nile virus outbreak in an urban California environment.

In August 2006, a pyrethrin insecticide synergized with piperonyl butoxide (EverGreen Crop Protection EC 60-6, McLaughlin Gormley King Company, Golden Valley, MN) was sprayed in ultralow volumes over the city of Davis, CA, by the Sacramento-Yolo Mosquito and Vector Control District to control mosquitoes transmitting West Nile virus. Concurrently, we evaluated the impact of the insecticide on nontarget arthropods by 1) comparing mortality of treatment and control groups of sentinel arthropods, and 2) measuring the diversity and abundance of dead arthropods found on treatment and control tarps placed on the ground. We found no effect of spraying on nontarget sentinel species including dragonflies (Sympetrum corruptum), spiders (Argiope aurantia), butterflies (Colias eurytheme), and honeybees (Apis mellifera). In contrast, significantly higher diversity and numbers of nontarget arthropods were found on ground tarps placed in sprayed versus unsprayed areas. All of the dead nontarget species were small-bodied arthropods as opposed to the large-bodied sentinels that were not affected. The mortality of sentinel mosquitoes placed at the same sites as the nontarget sentinels and ground tarps ranged from 0% to 100%. Dead mosquitoes were not found on the ground tarps. We conclude that aerial spraying with pyrethrins had no impact on the large-bodied arthropods placed in the spray zone, but did have a measurable impact on a wide range of small-bodied organisms.

Effects of vegetation control on mosquitoes in seasonal freshwater wetlands.

Wetland restorations benefit wildlife, but wetland vegetation may promote mosquitoes. We studied the effects of removing joint grass (Paspalum distichum L.) on the abundance and distribution of immature mosquitoes in seasonal freshwater wetlands in California. In 4 >4-ha plots and one 29.8-ha field, joint grass was removed via herbicide and disking. Four untreated plots and one 4.1-ha field were controls. Fields were sampled every 7-14 days in early autumn by taking 75 dips along levees, plus a 25-dip circular transect 6 m from field edges. Untreated plots had 7 times more larvae and 20 times more pupae than did vegetation-removal plots. Similar densities of mosquitoes were found along edges and within fields.

Effects of lambda-cyhalothrin on mosquito larvae and predatory aquatic insects.

Agricultural insecticides can affect mosquito production in rice fields by controlling mosquitoes, disrupting biological control or contributing to selection of insecticide resistance. The duration of insecticidal activity of the pyrethroid lambda-cyhalothrin was quantified on predatory insects in rice fields and on three kinds of mosquito larva: a pyrethroid-susceptible strain of Culex tarsalis Coquillet, a pyrethroid-resistant strain of Cx pipiens L. (sensu lato) and non-resistant Cx pipiens s.l. Lambda-cyhalothrin killed most caged, susceptible mosquitoes for up to 21 days. It killed fewer resistant Cx pipiens s.l., but suppressed their survival for over a week. Lambda-cyhalothrin suppressed field populations of predatory insects through day 29. Agricultural use of lambda-cyhalothrin can provide incidental mosquito control. However, the pyrethroid persisted in sediment and gradually decreased in activity, which could contribute to selection of pyrethroid-resistant mosquitoes. Because caged mosquitoes showed good survival before predators recovered, disruption of biological control is possible. It is therefore advisable for growers and mosquito control agencies to communicate about pesticide use.