UC Riverside

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.