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.

Acute sublethal exposure to toxic heavy metals alters honey bee (Apis mellifera) feeding behavior.

Heavy metal toxicity is an ecological concern in regions affected by processes like mining, industry, and agriculture. At sufficiently high concentrations, heavy metals are lethal to honey bees, but little is known about how sublethal doses affect honey bees or whether they will consume contaminated food. We investigated whether honey bees reject sucrose solutions contaminated with three heavy metals - cadmium, copper, and lead - as a measure of their ability to detect the metals, and whether ingesting these metals altered the bees' sucrose sensitivity. The metals elicited three different response profiles in honey bees. Cadmium was not rejected in any of the assays, and ingesting cadmium did not alter sucrose sensitivity. Copper was rejected following antennal stimulation, but was readily consumed following proboscis stimulation. Ingestion of copper did not alter sucrose sensitivity. Lead appeared to be palatable at some concentrations and altered the bees' sensitivity to and/or valuation of sucrose following antennal stimulation or ingestion of the metal. These differences likely represent unique mechanisms for detecting each metal and the pathology of toxicity. The bees' ability to detect and consume these toxic metals highlights the risk of exposure to these elements for bees living in or near contaminated environments.

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.

Laboratory bioassays on the impact of cadmium, copper and lead on the development and survival of honeybee (Apis mellifera L.) larvae and foragers.

Honeybees (Apis mellifera L.) have been widely distributed around the world to serve as pollinators for agriculture. They can encounter metal pollutants through various routes of exposure, including foraging on contaminated plant resources. Chronic and acute toxicity tests were conducted on larvae using artificial diets and on foragers using solutions of 50% sucrose, which contained cadmium (Cd), copper (Cu) and lead (Pb). We found that mortality increased in both larvae and foragers in a dose-dependent manner. Control larvae had higher relative growth indices (RGI) from day 6 to day 10 compared to all metal treatments, demonstrating substantial negative effects of metals on development. Copper was the least toxic to larvae with an LC50 of 6.97 mg L(-1). For foragers, Pb had the highest LC50, which was 345 mg L(-1). Foragers and larvae accumulated substantial quantities of all metals, and subsequent sucrose consumption decreased after dosing. Overall, honeybee larvae and foragers suffered detrimental effects when they were exposed to ecologically-relevant concentrations of Cd, Cu and Pb.

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.

Metal contaminant accumulation in the hive: Consequences for whole-colony health and brood production in the honey bee (Apis mellifera L.).

Metal pollution has been increasing rapidly over the past century, and at the same time, the human population has continued to rise and produce contaminants that may negatively impact pollinators. Honey bees (Apis mellifera L.) forage over large areas and can collect contaminants from the environment. The primary objective of the present study was to determine whether the metal contaminants cadmium (Cd), copper (Cu), lead (Pb), and selenium (Se) can have a detrimental effect on whole-colony health in the managed pollinator A. mellifera. The authors isolated small nucleus colonies under large cages and fed them an exclusive diet of sugar syrup and pollen patty spiked with Cd, Cu, Pb, and Se or a control (no additional metal). Treatment levels were based on concentrations in honey and pollen from contaminated hives around the world. They measured whole-colony health including wax, honey, and brood production; colony weight; brood survival; and metal accumulation in various life stages. Colonies treated with Cd or Cu contained more dead pupae within capped cells compared with control, and Se-treated colonies had lower total worker weights compared to control. Lead had a minimal effect on colony performance, although many members of the hive accumulated significant quantities of the metal. By examining the honey bee as a social organism through whole-colony assessments of toxicity, the authors found that the distribution of toxicants throughout the colony varied from metal to metal, some caste members were more susceptible to certain metals, and the colony's ability to grow over time may have been reduced in the presence of Se. Apiaries residing near metal-contaminated areas may be at risk and can suffer changes in colony dynamics and survival.

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.

Effects of selenium on development, survival, and accumulation in the honeybee (Apis mellifera L.).

Apis mellifera L. (Hymenoptera: Apidae) is an important agricultural pollinator in the United States and throughout the world. In areas of selenium (Se) contamination, honeybees may be at risk because of the biotransfer of Se from plant products such as nectar and pollen. Several forms of Se can occur in accumulating plants. In the present study, the toxicity of 4 compounds (selenate, selenite, methylselenocysteine, and selenocystine) to honeybee adult foragers and larvae was assessed using dose-response bioassays. Inorganic forms were more toxic than organic forms for both larvae (lethal concentration [LC50] selenate = 0.72 mg L(-1) , LC50 selenite = 1.0 mg L(-1) , LC50 methylselenocysteine = 4.7 mg L(-1) , LC50 selenocystine = 4.4 mg L(-1) ) and foragers (LC50 selenate = 58 mg L(-1) , LC50 selenite = 58 mg L(-1) , LC50 methylselenocysteine = 161 mg L(-1) , LC50 selenocystine = 148 mg L(-1) ). Inorganic forms of Se caused rapid mortality, and organic forms had sublethal effects on development. Larvae accumulated substantial amounts of Se only at the highest doses, whereas foragers accumulated large quantities at all doses. The present study documented very low larval LC50 values for Se; even modest transfer to brood will likely cause increased development times and mortality. The toxicities of the various forms of Se to honeybee larvae and foragers are discussed in comparison with other insect herbivores and detritivores.

Effects of selenium accumulation on phytotoxicity, herbivory, and pollination ecology in radish (Raphanus sativus L.).

Selenium (Se) has contaminated areas in the western USA where pollination is critical to the functioning of both agricultural and natural ecosystems, yet we know little about how Se can impact pollinators. In a two-year semi-field study, the weedy plant Raphanus sativus (radish) was exposed to three selenate treatments and two pollination treatments to evaluate the effects on pollinator-plant interactions. Honey bee (Apis mellifera L.) pollinators were observed to readily forage on R. sativus for both pollen and nectar despite high floral Se concentrations. Se treatment increased both seed abortion (14%) and decreased plant biomass (8-9%). Herbivory by birds and aphids was reduced on Se-treated plants, indicating a potential reproductive advantage for the plant. Our study sheds light on how pollutants such as Se can impact the pollination ecology of a plant that accumulates even moderate amounts of Se.

Selenium toxicity to honey bee (Apis mellifera L.) pollinators: effects on behaviors and survival.

We know very little about how soil-borne pollutants such as selenium (Se) can impact pollinators, even though Se has contaminated soils and plants in areas where insect pollination can be critical to the functioning of both agricultural and natural ecosystems. Se can be biotransferred throughout the food web, but few studies have examined its effects on the insects that feed on Se-accumulating plants, particularly pollinators. In laboratory bioassays, we used proboscis extension reflex (PER) and taste perception to determine if the presence of Se affected the gustatory response of honey bee (Apis mellifera L., Hymenoptera: Apidae) foragers. Antennae and proboscises were stimulated with both organic (selenomethionine) and inorganic (selenate) forms of Se that commonly occur in Se-accumulating plants. Methionine was also tested. Each compound was dissolved in 1 M sucrose at 5 concentrations, with sucrose alone as a control. Antennal stimulation with selenomethionine and methionine reduced PER at higher concentrations. Selenate did not reduce gustatory behaviors. Two hours after being fed the treatments, bees were tested for sucrose response threshold. Bees fed selenate responded less to sucrose stimulation. Mortality was higher in bees chronically dosed with selenate compared with a single dose. Selenomethionine did not increase mortality except at the highest concentration. Methionine did not significantly impact survival. Our study has shown that bees fed selenate were less responsive to sucrose, which may lead to a reduction in incoming floral resources needed to support coworkers and larvae in the field. If honey bees forage on nectar containing Se (particularly selenate), reductions in population numbers may occur due to direct toxicity. Given that honey bees are willing to consume food resources containing Se and may not avoid Se compounds in the plant tissues on which they are foraging, they may suffer similar adverse effects as seen in other insect guilds.