Heavy metal exposure reduces larval gut microbiota diversity of the rice striped stem borer, Chilo suppressalis.

Cadmium (Cd), a widely distributed environmental pollutant in agroecosystems, causes negative effects on crops and herbivores through bottom-up processes. The gut microbial community of an insect can play a critical role in response to metal stress. To understand how microbiota affect the stress responses of organisms to heavy metals in agroecosystems, we initially used 16S rRNA sequencing to characterize the larval gut microbiota of Chilo suppressalis, an important agricultural pest, exposed to a diet containing Cd. The species richness, diversity, and composition of the gut microbial community was then analyzed. Results revealed that while the richness (Chao1 and ACE) of gut microbiota in larvae exposed to Cd was not significantly affected, diversity (Shannon and Simpson) was reduced due to changes in species distribution and relative abundance. Overall, the most abundant genus was Enterococcus, while the abundance of the genera Micrococcaceae and Faecalibaculum in the control significantly superior to that in Cd-exposed pests. Phylogenetic investigation of microbial communities by the reconstruction of unobserved states (PICRUSt) showed that the intestinal microorganisms appear to participate in 34 pathways, especially those used in environmental information processing and the metabolism of the organism. This study suggests that the gut microbiota of C. suppressalis are significantly impacted by Cd exposure and highlights the importance of the gut microbiome in host stress responses and negative effects of Cd pollution in agroecosystems.

Contaminants of emerging concern affect Trichoplusia ni growth and development on artificial diets and a key host plant.

Many countries are utilizing reclaimed wastewater for agriculture because drought, rising temperatures, and expanding human populations are increasing water demands. Unfortunately, wastewater often contains biologically active, pseudopersistent pharmaceuticals, even after treatment. Runoff from farms and output from wastewater treatment plants also contribute high concentrations of pharmaceuticals to the environment. This study assessed the effects of common pharmaceuticals on an agricultural pest, Trichoplusia ni (Lepidoptera: Noctuidae). Larvae were reared on artificial diets spiked with contaminants of emerging concern (CECs) at environmentally relevant concentrations. Trichoplusia ni showed increased developmental time and mortality when reared on artificial diets containing antibiotics, hormones, or a mixture of contaminants. Mortality was also increased when T. ni were reared on tomatoes grown hydroponically with the same concentrations of antibiotics. The antibiotic-treated plants translocated ciprofloxacin through their tissues to roots, shoots, and leaves. Microbial communities of T. ni changed substantially between developmental stages and when exposed to CECs in their diets. Our results suggest that use of reclaimed wastewater for irrigation of crops can affect the developmental biology and microbial communities of an insect of agricultural importance.

Effects of contaminants of emerging concern on Myzus persicae (Sulzer, Hemiptera: Aphididae) biology and on their host plant, Capsicum annuum.

Many countries are utilizing reclaimed wastewater for agriculture as water demands due to drought, rising temperatures, and expanding human populations. Unfortunately, wastewater often contains biologically active, pseudopersistant pharmaceuticals, even after treatment. Runoff from agriculture and effluent from wastewater treatment plants also contribute high concentrations of pharmaceuticals to the environment. This study assessed the effects of common pharmaceuticals on an agricultural pest, the aphid Myzus persicae (Sulzer, Hemiptera: Aphididae). Second instar nymphs were transferred to bell peppers (Capsicum annuum) that were grown hydroponically. Treatment plants were spiked with contaminants of emerging concern (CECs) at environmentally relevant concentrations found in reclaimed wastewater. M. persicae displayed no differences in population growth or microbial community differences due to chemical treatments. Plants, however, displayed significant growth reduction in antibiotic and mixture treatments, specifically in wet root masses. Antibiotic treatment masses were significantly reduced in the total and root wet masses. Mixture treatments displayed an overall reduction in plant root wet mass. Our results suggest that the use of reclaimed wastewater for crop irrigation would not affect aphid populations, but could hinder or delay crop production.

Acute exposure to selenium disrupts associative conditioning and long-term memory recall in honey bees (Apis mellifera).

A plethora of toxic compounds - including pesticides, heavy metals, and metalloids - have been detected in honey bees (Apis mellifera) and their colonies. One such compound is selenium, which bees are exposed to by consuming nectar and pollen from flowers grown in contaminated areas. Though selenium is lethal at high concentrations, sublethal exposure may also impair honey bees' ability to function normally. Examining the effect of selenium exposure on learning and memory provides a sensitive assay with which to identify sublethal effects on honey bee health and behavior. To determine whether sublethal selenium exposure causes learning and memory deficits, we used proboscis extension reflex conditioning coupled with recall tests 30min and 24h post-conditioning. We exposed forager honey bees to a single sublethal dose of selenium, and 3h later we used an olfactory conditioning assay to train the bees to discriminate between one odor associated with sucrose-reinforcement and a second unreinforced odor. Following conditioning we tested short- and long-term recall of the task. Acute exposure to as little as 1.8ng of an inorganic form of selenium (sodium selenate) before conditioning caused a reduction in behavioral performance during conditioning. And, exposure to 18ng of either an inorganic form (sodium selenate) or an organic form (methylseleno-l-cysteine) of selenium caused a reduction in the bees' performance during the long-term recall test. These concentrations of selenium are lower than those found in the nectar of plants grown in selenium-contaminated soil, indicating that even low-grade selenium toxicity produces significant learning and memory impairments. This may reduce foragers' ability to effectively gather resources for the colony or nurse bees' ability to care for and maintain a healthy colony.

Cadmium, copper, and lead accumulation and bioconcentration in the vegetative and reproductive organs of Raphanus sativus: implications for plant performance and pollination.

Several studies have found high levels of cadmium (Cd), copper (Cu), and lead (Pb) in honey bee hives located near urbanized or industrial areas. Insect herbivores and pollinators may come in contact with environmental contaminants in the leaves and flowers they forage upon in these areas. Our study quantified which of these metals are accumulated in the tissues of a common weedy plant that can serve as a route of exposure for insects. We grew Raphanus sativus (crop radish) in semi-hydroponic sand culture in the greenhouse. Plants were irrigated with nutrient solutions containing Cd, Cu, or Pb at four concentrations (control, low, medium, high). Plant performance, floral traits, and metal accumulation were measured in various vegetative and reproductive plant organs. Floral traits and flower number were unaffected by all metal treatments. Copper accumulated at the highest concentrations in flowers compared to the other two metals. Copper and Cd had the highest translocation indices, as well as higher bioconcentration factors compared to Pb, which was mostly immobile in the plant. Copper posed the highest risk due to its high mobility within the plant. In particular, accumulation of metals in leaves and flowers suggests that herbivores and pollinators visiting and foraging on these tissues may be exposed to these potentially toxic compounds.

A binomial sequential sampling plan for Bactericera cockerelli (Hemiptera: Triozidae) in Solanum lycopersicum (Solanales: Solanacea).

The tomato-potato psyllid Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) is a pest of many solanaceous plants, including tomato (Solanum lycopersicum L.) and potato (Solanum tuberosum L.). In tomato, feeding by nymphs is associated with "psyllid yellows." B. cockerelli also vectors "Candidatus Liberibacter psyllaurous," an infectious bacterium that causes "vein greening" disease. Decisions about management action are much more effective when guided by robust sampling. However, there are few previous studies of potato psyllid spatial distribution in tomato fields, and no published sequential sampling plans for the pest in tomato. We studied B. cockerelli in various tomato fields in California and used these data to generate a sequential sampling plan. We found that juvenile B. cockerelli in tomato fields exhibit an edge effect, an aggregated distribution, and individuals are primarily located on the bottom of leaves. Psyllids were concentrated in the upper segments of plants, but this changed over time. Finally, we present three binominal sequential sampling plans for managing tomato psyllids in tomato fields. These plans differed from both those for bell pepper (Capsicum annum L.) and potato, indicating that B. cockerelli needs to be sampled using crop-specific sampling plans.

The impact of selenium on insects.

Selenium, a naturally occurring metalloid, is an essential trace element for many higher organisms, including humans. Humans primarily become exposed to selenium by ingesting food products containing trace amounts of selenium compounds. Although essential in these small amounts, selenium exhibits toxic effects at higher doses. Previous studies investigating the effects on insects of order Blattodea, Coleoptera, Diptera, Ephemeroptera, Hemiptera, Hymenoptera, Lepidoptera, Odonata, and Orthoptera revealed impacts on mortality, growth, development, and behavior. Nearly every study examining selenium toxicity has shown that insects are negatively affected by exposure to selenium in their food. However, there were no clear patterns of toxicity between insect orders or similarities between insect species within families. At this time, the potential for control will need to be determined on a species-by-species basis. We suspect that the multiple modes of action, including mutation-inducing modification of important amino acids as well as impacts on microbiome composition, influence this variability. There are relatively few studies that have examined the potential effects of selenium on beneficial insects, and the results have ranged from increased predation (a strong positive effect) to toxicity resulting in reduced population growth or even the effective elimination of the natural enemies (more common negative effects). As a result, in those pest systems where selenium use is contemplated, additional research may be necessary to ascertain if selenium use is compatible with key biological control agents. This review explores selenium as a potential insecticide and possible future directions for research.

Cadmium contamination triggers negative bottom-up effects on the growth and reproduction of Frankliniella occidentalis (Thysanoptera: Thripidae) without disrupting the foraging behavior of its predator, Orius sauteri (Heteroptera: Anthocoridae).

Heavy metal contaminants may influence tri-trophic interactions among plants, herbivores, and their natural enemies and affect the results of pest management practices. We examined how the widely distributed heavy metal cadmium (Cd) could modify interactions between kidney bean, Phaseolus vulgaris L., western flower thrips, Frankliniella occidentalis Pergande, and a predator, Orius sauteri (Poppius) by examining Cd effects on the feeding damage on leaves, the growth and reproduction of the thrips, and the feeding and plant location selection behaviors of predators. Leaf feeding damage was significantly reduced only at the highest Cd treatment (625 mg L-1). Survival, reproduction, and population growth of thrips decreased with the increase of Cd treatment concentration (0, 25, and 625 mg L-1). The reproduction rate of thrips from the highest Cd treatment group was reduced to less than 30% of the controls. Predator choice of plants was not impacted at the lowest level of Cd treatment (25 mg L-1) when prey were excluded, but the predators were deterred from plants treated at the high level of Cd (625 mg L-1). However, the predators responded strongly to the presence of prey, and the Cd-based deterrence was effectively eliminated when prey were added. Thus, the presence of Cd can cause a bottom-up effect on the fitness of pests without disrupting the foraging behavior of its predator. Our results provide baseline data on the toxic impacts on the pest and predator, and indicate that the ecology of the system and the biological control efficiency would be potentially impacted by high levels of Cd (625 mg L-1).

Identification and impact of natural enemies of Bactericera cockerelli (Hemiptera: Triozidae) in Southern California.

Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) is a major pest of potato, (Solanum tuberosum L.), tomato (Solanum lycopersicum L.), and peppers (Capsicum spp.). The purpose of our research was to identify and determine the impact of natural enemies on B. cockerelli population dynamics. Through 2 yr of field studies (2009-2010) at four different sites and laboratory feeding tests, we identified minute pirate bug, Orius tristicolor (White) (Hemiptera: Anthocoridae); western bigeyed bug, Geocoris pallens Stål (Hemiptera:Geocoridae), and convergent lady beetle, Hippodamia convergens Guérin-Méneville (Coleoptera: Coccinellidae) as key natural enemies of B. cockerelli in southern California potatoes, tomatoes, and bell peppers. In natural enemy exclusion cage experiments in the potato crop and in American nightshade, Solanum americanum Miller, the number of B. cockerelli surviving was significantly greater in the closed cage treatments, thus confirming the affect natural enemies can have on B. cockerelli. We discuss how this information can be used in an integrated pest management program for B. cockerelli.

Risk assessment of selected insecticides on Tamarixia triozae (Hymenoptera: Eulophidae), a parasitoid of Bactericera cockerelli (Hemiptera: Trizoidae).

Tamarixia triozae (Burks) (Hymenoptera: Eulophidae) is an important parasitoid of the potato or tomato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Trizoidae), a serious pest of potato (Solanum tuberosum L.), tomato (Solanum lycopersicum L.), and other solanaceous vegetables in many countries. To produce a marketable crop, insecticides are required when B. cockerelli populations reach economically damaging levels. We evaluated 11 commonly used insecticides for their effects on T. triozae. Glass-surface residues of spinetoram, imidacloprid-cyfluthrin, abamectin, and tolfenpyrad caused 100% mortality of T. triozae in 72 h, and the leaf residue of spinetoram was extremely toxic to T. triozae adults; even 15-d-old residues caused 100% mortality. Cyantraniliprole, fenpyroximate, pymetrozine, spirotetramat, spiromesifen, and chenopodium oil did not cause significant mortality in either glass surface or leaf-residue bioassays. Ingestion of spinetoram, abamectin, and imidacloprid+cyfluthrin (Leverage) by the adults resulted in 100% mortality in 12 h, and tolfenpyrad, 75.0% mortality in 12 h; whereas chenopodium oil and pymetrozine showed moderate effects on adult survival. Ingestion of abamectin, imidacloprid-cyfluthrin, and spinetoram killed all adults in the first day of treatment, whereas female adults in the treatment of pymetrozine lived 80.8 d, which was similar to those in the control. Ingestion of abamectin, imidacloprid-cyfluthrin, chenopodium oil, and spinetoram killed all male adults in the first day, whereas ingestion of other insecticides did not cause significant mortality, but reduced percent parasitism. Abamectin, imidacloprid-cyfluthrin, and spinetoram had the most deleterious effects on T. triozae, and have the least potential for use in integrated control programs using this parasitoid.

Modeling and validation of oviposition by a polyphagous insect pest as a function of temperature and host plant species.

Modeling oviposition as a function of female insect age, temperature, and host plant suitability may provide valuable insight into insect population growth of polyphagous insect pests at a landscape level. In this study, we quantified oviposition by beet leafhoppers, Circulifer (= Neoaliturus) tenellus (Baker) (Hemiptera: Cicadellidae), on four common non-agricultural host plant species [Erodium cicutarium (L.) L'Hér. (Geraniaceae), Kochia scoparia (L.) Schrader (Amaranthaceae), Plantago ovata Forsskál (Plantaginaceae), and Salsola tragus L. (Amaranthaceae)] at two constant temperature conditions. Additionally, temperature-based oviposition models for each host plant species were validated, under semi-field and greenhouse conditions. We found that K. scoparia was the most suitable host plant, and optimal temperature for oviposition was estimated to be 30.6°C. Accordingly, beet leafhoppers appear to be well-adapted to high-temperature conditions, so increasing temperatures due to climate change may favor population growth in non-agricultural areas. Maximum total fecundity (Rm) was used as an indicator of relative suitability of host plants. S. tragus has been considered an important non-agricultural host plant, however, we found that S. tragus and E. cicutarium have lower Rm compared to K. scoparia and P. ovata. The combination of detailed experimental oviposition bioassays, modeling, and model validation is considered widely relevant and applicable to host plant assessments and modeling of population dynamics of other polyphagous insect pests.

Insect vector manipulation by a plant virus and simulation modeling of its potential impact on crop infection.

There is widespread evidence of plant viruses manipulating behavior of their insect vectors as a strategy to maximize infection of plants. Often, plant viruses and their insect vectors have multiple potential host plant species, and these may not overlap entirely. Moreover, insect vectors may not prefer plant species to which plant viruses are well-adapted. In such cases, can plant viruses manipulate their insect vectors to preferentially feed and oviposit on plant species, which are suitable for viral propagation but less suitable for themselves? To address this question, we conducted dual- and no-choice feeding studies (number and duration of probing events) and oviposition studies with non-viruliferous and viruliferous [carrying beet curly top virus (BCTV)] beet leafhoppers [Circulifer tenellus (Baker)] on three plant species: barley (Hordeum vulgare L.), ribwort plantain (Plantago lanceolata L.), and tomato (Solanum lycopersicum L.). Barley is not a host of BCTV, whereas ribwort plantain and tomato are susceptible to BCTV infection and develop a symptomless infection and severe curly top symptoms, respectively. Ribwort plantain plants can be used to maintain beet leafhopper colonies for multiple generations (suitable), whereas tomato plants cannot be used to maintain beet leafhopper colonies (unsuitable). Based on dual- and no-choice experiments, we demonstrated that BCTV appears to manipulate probing preference and behavior by beet leafhoppers, whereas there was no significant difference in oviposition preference. Simulation modeling predicted that BCTV infection rates would to be higher in tomato fields with barley compared with ribwort plantain as a trap crop. Simulation model results supported the hypothesis that manipulation of probing preference and behavior may increase BCTV infection in tomato fields. Results presented were based on the BCTV-beet leafhopper pathosystem, but the approach taken (combination of experimental studies with complementary simulation modeling) is widely applicable and relevant to other insect-vectored plant pathogen systems involving multiple plant species.

Effects of insecticides on behavior of adult Bactericera cockerelli (Hemiptera: Triozidae) and transmission of Candidatus Liberibacter psyllaurous.

The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is a serious pest of potatoes (Solanum tuberosum L.) that can cause yield loss by direct feeding on crop plants and by vectoring a bacterial pathogen, Candidatus Liberibacer psyllaurous. Current pest management practices rely on the use of insecticides to control the potato psyllid to lower disease incidences and increase yields. Although many studies have focused on the mortality that insecticides can cause on potato psyllid populations, little is known regarding the behavioral responses of the potato psyllid to insecticides or whether insecticides can decrease pathogen transmission. Thus, the objectives of this study were to determine the effects of insecticides on adult potato psyllid behaviors, the residual effects of insecticides on potato psyllid behaviors over time, and effects of these insecticides on Ca. L. psyllaurous transmission. Insecticides tested included imidacloprid, kaolin particle film, horticultural spray oil, abamectin, and pymetrozine. All insecticides significantly reduced probing durations and increased the amount of time adult psyllids spent off the leaflets, suggesting that these chemicals may be deterrents to feeding as well as repellents. Nonfeeding behaviors such as tasting, resting, and cleaning showed variable relationships with the different insecticide treatments over time. The insecticides imidacloprid and abamectin significantly lowered transmission of Ca. L. psyllaurous compared with untreated controls. The implications of our results for the selection of insecticides useful for an integrated pest management program for potato psyllid control are discussed.

Joint effects of cadmium and copper on Apis mellifera forgers and larvae.

Honey bees (Apis mellifera L.) are important ecological and agricultural resources. They are among the most widely available pollinators and provide products as well as services. Unfortunately, honey bee populations are susceptible to several environmental threats, including heavy metal exposure. Honey bees can be exposed to heavy metals when foraging on contaminated honey and pollen resources, and in some cases by airborne exposure. We studied the joint acute and chronic effects of cadmium (Cd) and copper (Cu) on A. mellifera. A 1:1 solution of the two heavy metals increased larval developmental duration and the mortality of both larvae and foragers in a dose-dependent way, decreased forager feeding, increased body metal burdens, and disrupted the sucrose response behavior of foragers. In combination, Cd and Cu demonstrated a weakly synergistic effect on foragers, but for larvae an initially antagonistic effect at low doses changed to strongly synergistic response at higher concentrations. The sucrose response threshold of foragers decreased significantly when they were dosed with increasing concentrations of the metal mixtures. Overall, the fitness of honey bee larvae and foragers is detrimentally affected when these metals co-occur.

Effects of terrestrial pollutants on insect parasitoids.

Parasitoids are important organisms in the regulation of insect herbivores in natural, urban, and agricultural ecosystems. The impact of pollutants acting on parasitoids has not been extensively reviewed. This prompted us to propose a falsifiable null hypothesis (pollutants have no effects on parasitoids) and two alternative hypotheses (pollution negatively or positively affects parasitoids) to assess in the available literature the effects of pollutants acting on parasitoids. We found 26 studies examining 39 biological systems that met our criteria for inclusion. Of these studies, 18 of the 39 biological systems (46.2%) supported the null hypothesis while 18 (46.2%) supported the first alternative hypothesis in which pollutants exhibited negative effects on parasitoids. Only a small percentage of the studies (7.6%, 3 of 39) supported the second alternative hypothesis suggesting that pollutants had positive effects on parasitoids. We provide a synthesis of the available data by pollution type, summarize trends for different pollutants, and suggest future areas of research.

Effects of High-Level Acylsugar-Producing Tomato Lines on the Development of Tomato Psyllids (Hemiptera: Triozidae).

Acylsugars have been shown to provide activity against numerous insect pests of tomatoes. Comparison of acylsugar levels in four tomato plant lines, FA7/AS, FA2/AS, CU071026, and 'Yellow Pear', found that the acylsugar contents in the elevated acylsugar lines were significantly higher than the commercial Yellow Pear (control) tomato plant line. Adult choice tests indicated that the tomato psyllid, Bactericera cockerelli, preferred to settle on the Yellow Pear and FA2/AS lines over the line with the highest content of acylsugars, FA7/AS, and the parental line, CU071026. The no-choice test demonstrated that adults laid fewer eggs on the high acylsugar tomato lines than on the control tomato line, Yellow Pear. For all high acylsugar lines, the relative growth index of the psyllid was significantly lower compared with the commercial line, indicating a reduced potential for population growth. Although some tomato psyllids completed their life cycle on the high acylsugar tomato plant lines, the percent survival of psyllids to the adult stage when developing on the high acylsugar lines was significantly less (range = 43.7-57.1%) than on the commercial tomato line (83.8%). All mortality occurred during the early stages of development (egg stage to third instar), which has implications for acquisition and transmission of Candidatus Liberibacter solanacearum, the causal agent of tomato vein greening disease. Therefore, with reduced attractiveness for tomato psyllids and significantly reduced survival, the high-acylsugar tomato plant lines have the potential to be part of an integrated pest management program for this pest.

Impact of inorganic pollutants perchlorate and hexavalent chromium on efficacy of Bacillus sphaericus and Bacillus thuringiensis subsp. israelensis against Culex quinquefasciatus (Diptera: Culicidae).

The effects of two widespread environmental pollutants, perchlorate and hexavalent chromium, were assessed on the efficacy of Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bsph) against fourth instars of Culex quinquefasciatus Say (Diptera: Culicidae) in 24-h laboratory bioassays. Although 250 mg/liter perchlorate, a level somewhat higher than would be considered ecologically relevant, did not affect the control provided by either larvicide, presence of 1.04 mg/liter hexavalent chromium, an ecologically relevant concentration, increased the efficacy of both Bti and Bsph by 21 and 80%, respectively. In the presence of hexavalent chromium, improved suppression could be expected from Bacillus applications at the current label rates. However, because hexavalent chromium has been shown to affect many taxa, we propose that the potential exists for increased susceptibility of nontarget organisms to Bacillus products in polluted habitats.

Effects of contaminants of emerging concern on Megaselia scalaris (Lowe, Diptera: Phoridae) and its microbial community.

Drought, rising temperatures, and expanding human populations are increasing water demands. Many countries are extending potable water supplies by irrigating crops with wastewater. Unfortunately, wastewater contains biologically active, long-lived pharmaceuticals, even after treatment. Run-off from farms and wastewater treatment plant overflows contribute high concentrations of pharmaceuticals to the environment. This study assessed the effects of common pharmaceuticals on a cosmopolitan saprophagous insect, Megaselia scalaris (Diptera: Phoridae). Larvae were reared on artificial diets spiked with contaminants of emerging concern (CECs) at environmentally relevant concentrations. Female flies showed no oviposition preference for treated or untreated diets. Larvae exposed to caffeine in diets showed increased mortality, and larvae fed antibiotics and hormones showed signs of slowed development, especially in females. The normal sex ratio observed in M. scalaris from control diets was affected by exposure to caffeine and pharmaceutical mixture treatments. There was an overall effect of treatment on the flies' microbial communities; notably, caffeine fed insects displayed higher microbial variability. Eight bacterial families accounted for approximately 95% of the total microbes in diet and insects. Our results suggest that CECs at environmentally relevant concentrations can affect the biology and microbial communities of an insect of ecological and medical importance.

Arthropod communities in a selenium-contaminated habitat with a focus on ant species.

The selenium contamination event that occurred at Kesterson Reservoir (Merced Co., CA) during the 1970-80s is a frequently cited example for the negative effects of contamination on wildlife. Despite the importance of arthropods for ecosystem services and functioning, relatively little information is available as to the impacts of pollution on arthropod community dynamics. We conducted surveys of the arthropod community present at Kesterson Reservoir to assess the impacts of selenium contamination on arthropod diversity, with a focus on ant species richness, composition and density. Trophic groups were compared to determine which arthropods were potentially receiving the greatest selenium exposure. Plant samples were analyzed to determine the selenium content by site and by location within plant. Soil concentrations varied across the study sites, but not across habitat types. Topsoil contained higher levels of selenium compared to core samples. Plants contained similar concentrations of selenium in their leaves, stems and flowers, but flowers contained the greatest range of concentrations. Individuals within the detritivores/decomposers and predators accumulated the greatest concentrations of selenium, whereas nectarivores contained the lowest concentrations. Species composition differed across the sites: Dorymyrmex bicolor was located only at the site containing the greatest soil selenium concentration, but Solenopsis xyloni was found at most sites and was predominant at six of the sites. Selenium concentrations in ants varied by species and collection sites. Nest density was also found to differ across sites, but was not related to soil selenium or any of the habitat variables measured in our study. Selenium was not found to impact species richness, but was a significant variable for the occurrence of two out of the eight native species identified.

Acute and chronic activity of perchlorate and hexavalent chromium contamination on the survival and development of Culex quinquefasciatus Say (Diptera: Culicidae).

Effects of water contamination with perchlorate and hexavalent chromium [Cr (VI)] on the mosquito Culex quinquefasciatus were assessed. The chronic (10-day) LC50s values for perchlorate and chromium were 74+/-8.0 mg/L and 0.41+/-0.15 mg/L, respectively. Relative Growth Index, a measure of growth and mortality rates in a population, was significantly reduced within 5 days for levels of perchlorate as low as 25 mg/L and for levels of chromium as low as 0.16 mg/L. Neither compound altered wing length of surviving adults. In combination, contaminants were synergistic, causing 14% more mortality than predicted. Acute (24-h) LC50 values for perchlorate and Cr (VI) were 17,000+/-3200 and 38+/-1.3 mg/L, respectively. Effects on mosquito larvae in contaminated environments are likely to be observed for Cr (VI) but not for perchlorate, which generally does not occur at levels as high as those shown here to affect larval mosquitoes.