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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2012)

Selenium (Se) has contaminated soils and plants in the western USA and other regions where pollination can be critical to the functioning of both agricultural and natural ecosystems, yet we know very little about how pollutants can impact insect pollinators. The overall goal of this dissertation was to investigate both the toxic effects of a plant-accumulated pollutant (Se) on a common pollinator, the honey bee (Apis mellifera L., Hymenoptera: Apidae).

I investigated the accumulation of Se in two phytoremediating species, Brassica juncea and Stanleya pinnata. The plants were irrigated in the greenhouse with selenate, and accumulated significantly quantities in the nectar (110 and 150 µg Se ml-1 ww) and pollen (1700 and 12900 µg Se g-1 dw, respectively). These concentrations are much higher than the Se LC50's for certain insects.

I used proboscis extension reflex bioassays to determine if the Se affected the gustatory response of honey bee foragers. Antennal stimulation with selenomethionine reduced PER at higher concentrations. Foragers dosed with selenate had higher mortality and reduced responses to sucrose, which may lead to decreases in incoming floral resources needed to support coworkers and larvae in the colony.

In a semi-field study, the weedy plant Raphanus sativus (radish) was exposed to selenate watering treatments to evaluate the effects on pollination ecology. Honey bee pollinators were observed to readily forage on R. sativus for both pollen and nectar despite high floral Se concentrations. Se treatments increased seed abortion and decreased plant biomass, but herbivory by birds and aphids was also reduced, indicating a potential tradeoff of phytotoxicity and protection from Se.

I used artificial diet bioassays to determine the toxicity of four Se compounds that occur in accumulating plants. Apis mellifera larvae were chronically fed Se, and the inorganic forms were more toxic (LC50 selenate = 0.72 µg g-1, LC50 selenite = 1.03 µg g-1) than the organic forms (LC50 methylselenocysteine = 4.09 µg g-1, LC50 selenomethionine = 6.04 µg g-1). All four forms decreased the percentage of larvae that pupated, and selenate and methylselenocysteine significantly decreased larval growth rates. Taken together, the dissertation research from the past five years represents a crucial first step towards understanding the impact environmental stressors can have on pollinator-plant interactions and specifically, the honey bee, Apis mellifera.

    Cover page: Selenium Accumulation in Brassicaceae Plant Species and its Biotransfer to Insect Pollinators
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2016)
    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.
      Cover page: Laboratory bioassays on the impact of cadmium, copper and lead on the development and survival of honeybee (Apis mellifera L.) larvae and foragers
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2017)
      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.
        Cover page: Arthropod communities in a selenium-contaminated habitat with a focus on ant species
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2012)
        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.
          Cover page: Selenium Toxicity to Honey Bee (Apis mellifera L.) Pollinators: Effects on Behaviors and Survival
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2020)
          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.
            Cover page: Joint effects of cadmium and copper on Apis mellifera forgers and larvae
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