Pathogens are one of the major factors attributed to bee species declines. Flowers can serve as hubs for bee pathogens and shared floral resources mediate the acquisition of pathogens from infected to healthy hosts. While previous research places emphasis on pathogen spillover from managed bees to wild bee species, there is a paucity of knowledge as to the role of non-bee floral visitors, such as ants, in shaping pathogen prevalence for bees. Here, we document the prevalence of two common bee pathogens, Nosema spp. and Crithidia spp., among honey bees, native bees, and ants at an ecological reserve in Southern California encompassing threatened coastal sage scrub habitat to understand how ecological factors, such as space and time, shape host-parasite interactions for these insect taxa. Nosema and Crithidia were detected in honey bees, native bees, and ants. Surprisingly, both pathogens were highly prevalent in ants. The interaction of sampling period x insect taxa had the most pronounced effects in influencing prevalence for both pathogens. Plant-host associations did not appear to be important in shaping pathogen prevalence for honey bees and native bees. This is the first study, to our knowledge, to detect Nosema and Crithidia in ants and several native bee species. We discuss two potential mechanisms, shared floral resources and consumption of infected prey, that may explain pathogen transmission between bees and ants. Ants may serve as a previously undescribed reservoir for Nosema and Crithidia and future research should define the potential for pathogen spillback from ants back into bee populations.

Bees are important in both agricultural and natural ecosystems for the pollination services that they provide. However, invasive ants that exploit floral nectar in these landscapes can act as resource competitors with bees. Competitive interactions between bees and ants can have repercussions for pollinator resource acquisition, plant or bee fitness, and, ultimately, ecosystem function. There are gaps in our knowledge about the behaviors and sensory mechanisms involved in interference competition between bees and nectivorous, non-pollinator antagonists. Here, we studied how invasive ants influence bees in the context of nectar foraging. First, we performed laboratory assays to assess the foraging behavior of bumble bees (Bombus impatiens) in response to live Argentine ants (Linepithema humile) or to a subset of ant chemical cues. We found that bees were deterred by the presence of live ants as well as the combination of olfactory and gustatory ant chemical cues. Next, we characterized specific ant-bee interactions and behavioral transitions at shared nectar resources. We found that two-thirds of bee behaviors did not involve feeding, and, of those, the majority of them were aggressive interactions with ants. Similarly, 70% of behavioral transitions did not involve feeding, of which 82.3% were transitions among aggressive behaviors. Consistent with predictions of interference competition, the number of ants at a shared resource decreased the probability of a bee feeding and increased the likelihood of a bee being bitten. Similarly, getting bitten by an ant decreased the probability that a bee would subsequently feed. Finally, we examined participation and foraging success of bees during experiments with live Argentine ants, as well as the influence of ant-bee interactions on bee foraging activity and movement. We found that the majority of bees never foraged during experimental trials, consistent with high levels of non-activity in other social insects. There was further evidence of foraging specialization among bees: foragers that came into contact with ants and were nonaggressive were less successful at resource acquisition, while the foragers exhibiting aggression toward ants were more successful.

California contains numerous horticultural nurseries: large lots encompassing patches of healthy containerized flowering plants year-round. While some ornamental plants attract pollinators, the ecological relationships between nurseries and wild bees remain uncharacterized. Management regimes such as systemic insecticide use may pose hazards to bees, while others, such as irrigation, may improve floral resources. My dissertation explored bee assemblages in nurseries, how individual bees forage on ornamental plants, and how plant management impacts bee reproduction.In Chapters I and II, I documented bee assemblages in 13 nurseries during spring, summer, and autumn over two years and detected over 150 species visiting over 90 plant varieties. Greater cultivation of native plants influenced the abundance, richness, and composition of wild bee assemblages. A wide variety of bee species use plants inside nurseries across seasons, and nursery characteristics can influence the abundance and types of bees present. In Chapter III, I used mark-recapture techniques to determine the fidelity of individual bees to nursery plants across days. I marked foraging sweat bees in nurseries and returned after 24 hours. I consistently rediscovered half of individuals across nurseries and seasons, with higher recapture rates on native plants. Nearly all recaptured individuals were found on the same plant species as the previous day, suggesting a high degree of plant fidelity. Wild bees thus regularly use nursery plants as resources over time. In Chapter IV, I explored how ornamental plant management—irrigation level and insecticide application—affect bee foraging and reproduction. In flight cages, irrigation level influenced floral resources and pesticide concentrations in nectar, but did not affect reproduction of solitary bees. Neonicotinoid treatment below the label rate almost entirely halted bee nest initiation and brood cell construction. These results underscore dangers posed by irresponsible use of nursery insecticides. This dissertation addressed the need for an understanding of bee community ecology in horticultural systems, foraging preferences between native and non-native ornamental plants, and impacts of nursery management practices on bee reproduction. This knowledge can be applied to inform practical management strategies that minimize exposure of wild bees to potential stressors, while simultaneously reflecting positively on environmentally minded nursery patrons.

Biodiversity can be maintained or disrupted by disturbance. With human activities drastically altering natural disturbance regimes in ecosystems, understanding the factors that most impact recovery is vital. California coastal sage scrub (CSS) is a fire-prone system with many endemic plant species, including fire-following forbs and shrubs. However, increased fire and plant invasions have impacted native recovery, which may have implications for plant-pollinator interactions. As biodiversity continues to be threatened, investigating post-disturbance recovery may help to predict outcomes of climate change and altered disturbance regimes across trophic levels.

In Chapter 1, I investigated how a wildfire influenced CSS plant composition at four- to six-years post-fire. I found cover and species richness metrics were influenced not only by fire, but also by site characteristics and seasonality. Overall, native annual forbs and invasives grasses contributed largely to composition patterns. I also found a positive association between increased precipitation and both fire-follower richness and floral resource richness. These results highlight the importance of employing multiple metrics and species-level investigations to assess post-fire plant composition more accurately.

In Chapter II, I assessed host plant suitability of CSS species by quantifying larval development and survival for commercially sourced painted lady butterflies (Vanessa cardui). Overall, larval survival was extremely low (0.1%), as was successful pupation and survival to adulthood. This contrasted with 63% survival for wild sourced larvae. These findings broaden our understanding of V. cardui ecology and host plant suitability for supporting complete development of larvae.

In Chapter III, I leveraged community science observations of V. cardui butterflies throughout California shrub/scrub ecosystems during and outside of irruptive migrations. I identified visitation to 329 plant species of various functional types and native status. 195 of the plants in my dataset had not previously been identified as V. cardui nectar plants. These findings broaden our understanding of the diverse floral resources supporting this butterfly.

The goals of this dissertation were to 1) explore the factors driving post-fire plant composition, 2) assess the suitability of various putative host plants for V. cardui larval survival, and (3) quantify migratory butterfly nectar plant use throughout a geographic range.

The Argentine ant, Linepithema humile (Mayr), is an extremely invasive ant species that has spread to urban, commercial and natural areas worldwide. This pervasive expansion has had detrimental ecological and economic effects, resulting in the allocation of vast amounts of resources to its control in urban and agricultural areas. New efforts are underway to control Argentine ants in ecologically sensitive habitats, such as California’s Channel Islands. I tested the treatment efficacy of application of thiamethoxam liquid bait in ecologically sensitive habitats using polyacrylamide beads as a delivery matrix to eradicate Argentine ants. This matrix is shown to be a promising eradication tool on San Clemente Island, CA with a 99.86% reduction in Argentine ant activity across five sites (176.95 hectares). As these eradication efforts are implemented, a standardized detection protocol becomes essential to gauge the success of such efforts and to ensure that remnant ant populations don’t go undetected during pre- or post-treatment stages. In aid of creating such protocols, I conducted field trials to assess 1) attractant efficacy for Argentine ant detection throughout the year and 2) pre- and post-treatment detection rates of Argentine ants. Traditional sucrose water bait traps can be enhanced with the addition of sugar-egg or a synthetic pheromone of the Argentine ant. The sugar-egg and pheromone bait traps performed equally well at detecting Argentine ants in areas with low levels of ant density. This information will be used to standardize detection protocols in a diversity of ecosystems and to refine Argentine ant eradication efforts on the Channel Islands.