The article, “Vehicle Pollution Is Associated with Elevated Insect Damage to Street Trees,” is published in the Journal of Applied Ecology. It received the Editor's Choice Award.
“This research reveals strong effects of vehicle pollution on insect damage to trees,” said Meineke, who conceived the idea for the project, funded by the department. “Trees next to highways are exposed to multiple stressors, including urban heat, pollution, and insects, all of which affect one another and tree health. Our research strongly suggests planting trees that are less susceptible to herbivory near highways.”
Her team included her colleague, UC Davis distinguished professor Richard “Rick” Karban, who co-wrote the manuscript, and junior specialist David Eng, then of the Meineke lab. The study targeted vehicle pollution in the Sacramento Valley “and adds to a now growing chorus of studies demonstrating the scientific value of intra-urban gradients of particular variables (heat, pollution, surrounding vegetation),” they wrote.
They suspect that “vehicle pollution depresses defensive pathways within trees and reduces the concentrations of key compounds that protect against herbivore damage.”
The researchers demonstrated that leaf damage to a native oak species (Quercus lobata), known as the valley oak,” is “substantially elevated on trees exposed to vehicle emissions.”
“Together, our studies demonstrate that the heterogeneity in vehicle emissions across cities may explain highly variable patterns of insect herbivory on street trees,” they wrote. “Our results also indicate that trees next to highways are particularly vulnerable to multiple stressors, including insect damage. To combat these effects, urban foresters may consider installing trees that are less susceptible to insect herbivory along heavily traveled roadways.”
The valley oak is a deciduous, long-lived tree that can reach up to 98 feet in height and live up to 600 years. It is known to tolerate wildfires.
“Past studies hint at the potential role of vehicle pollution as a driver of leaf nutritional quality for chewing herbivores. At one site in the United Kingdom, trees within 100 meters of motorways were much more likely to be severely defoliated than trees at further distances. Elevated herbivory was attributed to elevated nitrogen dioxide (NO2) along highways.” Another study in the Los Angeles Basin, showed that “herbivore communities on oak trees at more polluted natural areas tended to be more dominated by chewing herbivores compared to less polluted natural areas.”
The UC Davis researchers wrote that their results “demonstrate that highly polluted, highway-adjacent habitats are associated with shifts in plant-insect interactions and that this topic may be ripe for future research into how roadside environments may affect insect conservation and plant performance in cities.”
Their study highlights the importance of planting decisions along major roadways. “The concept of ‘right tree, right place' has long stated that tree selection should be aimed at maximizing the performance in urban areas,” they wrote. “Quercus lobata and other species that are highly susceptible to herbivores may provide ecosystem services sub-optimally along highways, and may have shorter lifespans due to chronic damage promoted by on-road pollution trees.”
“Identifying tree species that are robust to pollution, and resistant to insects that may benefit from pollution, could be a novel consideration in planting decisions. This consideration may become even more important as many cities become drier and hotter, and insect herbivores have disproportionate impacts on tree growth. Because city-owned trees are planted and cannot themselves evolve in response to climate change, we may be required to develop new cultivars to promote robust trees along roadways.”
In a newly published article titled, “Flower Plantings Support Wild Bee Reproduction and May Also Mitigate Pesticide Exposure Effects,” in the Journal of Applied Ecology, the eight-member team related that "to address the potential of flower plantings to mitigate bee pesticide exposure and effects and support bee reproduction, we established replicated sites where half contained flower plantings. We used sentinel bees to assess nesting and reproduction, quantified local and landscape flower availability, and used bee-collected pollen to quantify bee pesticide exposure and forage resource use. We asked the following questions:
- To what extent do bees use flower plantings?
- Do flower plantings promote bee reproduction?
- Do flower plantings modify bee pesticide exposure and effects on reproduction?"
The paper is the work of co-lead authors Maj Rundlöf and Clara Stuligross, and co-authors Arvid Lindh, Rosemary Malfi, Katherine Burns, John Mola, Staci Cibotti and Professor Williams.
- Sustainable agriculture relies on pollinators, and wild bees benefit yield of multiple crops. However, the combined exposure to pesticides and loss of flower resources, driven by agricultural intensification, contribute to declining diversity and abundance of many bee taxa. Flower plantings along the margins of agricultural fields offer diverse food resources not directly treated with pesticides.
- To investigate the potential of flower plantings to mitigate bee pesticide exposure effects and support bee reproduction, we selected replicated sites in intensively farmed landscapes where half contained flower plantings. We assessed solitary bee Osmia lignaria and bumble bee Bombus vosnesenskii nesting and reproduction throughout the season in these landscapes. We also quantified local and landscape flower resources and used bee-collected pollen to determine forage resource use and pesticide exposure and risk.
- Flower plantings, and their local flower resources, increased O. lignaria nesting probability. Bombus vosnesenskii reproduction was more strongly related to landscape than local flower resources.
- Bees at sites with and without flower plantings experienced similar pesticide risk, and the local flowers, alongside flowers in the landscape, were sources of pesticide exposure particularly for O. lignaria. However, local flower resources mitigated negative pesticide effects on B. vosnesenskii reproduction.
- Synthesis and applications. Bees in agricultural landscapes are threatened by pesticide exposure and loss of flower resources through agricultural intensification. Therefore, finding solutions to mitigate negative effects of pesticide use and flower deficiency is urgent. Our findings point towards flower plantings as a potential solution to support bee populations by mitigating pesticide exposure effects and providing key forage. Further investigation of the balance between forage benefits and added pesticide risk is needed to reveal contexts where net benefits occur.
Journal of Applied Ecology, a publication of the British Ecological Society, publishes novel, high-impact papers on the interface between ecological science and the management of biological resources.
The two-year research, led by Ola Lundin, a former postdoctoral fellow in the Neal Williams lab, UC Davis Department of Entomology and Nematology and published in the Journal of Applied Ecology, details what plants proved most attractive to honey bees, wild bees and other pollinators, as well as what drew such natural enemies as predators and parasitic wasps.
The research, “Identifying Native Plants for Coordinated Habitat Management of Arthropod Pollinators, Herbivores and Natural Enemies,” is co-authored by Williams, professor of entomology and a Chancellor's Fellow at UC Davis; and project specialist Kimiora Ward of the Williams lab.
“I hope this study can inform selection of plants that support pollinators and natural enemies without enhancing potential pests,” said Lundin, first-author of the paper and now a postdoctoral fellow in the Department of Ecology, Swedish University of Agricultural Sciences, Uppsala.
“Planting wildflowers is a key strategy promoted nationally to support wild and managed bees,” said Williams. “Successful adoption of these plantings in agricultural landscapes will require that they not only support pollinators but that they also avoid supporting too many pests. Plant selection going forward will need to balance multiple goals of pollinators pest management and other functions. This research is a first step on the path to identifying plants that will meet these goals."
The three scientists, who conceived the ideas and developed the methodology for the research project, established 43 plant species in a garden experiment on the grounds of the Harry H. Laidlaw Jr. Honey Bee Research Facility at UC Davis. They selected plant species that were drought-tolerant; native to California (except for buckwheat, Fagopyrum esculentum, known to attract natural enemies and widely used in conservation biological control); and, as a group, covered a range of flowering periods throughout the season.
“For early season bloom, Great Valley phacelia (Phacelia ciliata) was a real winner in terms of being attractive for both wild bees and honey bees,” Lundin said. “Elegant Clarkia (Clarkia unguiculata) flowers in late spring and was the clearly most attractive plant for honey bees across the dataset. The related Fort Miller Clarkia (C. williamsonii) was also quite attractive for honey bees and had the added benefit that a lot of minute pirate bugs visited the flowers.”
Lundin said that common yarrow (Achillea millefolium) “attracted “attracted the highest numbers of parasitic wasps but also many herbivores, including Lygus bugs.”
“In general a lot of parasitic wasps were found on Asteraceae species (the daisy family) and this was a somewhat surprising result considering that they have narrow corollas, and for parasitic wasps relatively deep corollas that can restrict their direct access to nectar. Under the very dry conditions in late summer, Great Valley gumplant (Grindelia camporum) and Vinegarweed (Trichostema lanceolatum) both performed well and attracted high numbers of wild bees.”
The team found that across plant species, herbivore, predator and parasitic wasp abundances were “positively correlated,” and “honey bee abundance correlated negatively to herbivore abundance.”
The take-home message is that “if you're a gardener or other type of land manager, what you'd likely prefer would be a mix of some of the most promising plant species taking into account their individual attractiveness for these arthropod groups, plus several more factors including costs for seed when planting larger areas,” Lundin said.
“Plant choice can also depend on how you weigh the importance of each arthropod group and whether you are interested in spring, summer or season-long bloom,” Lundin added. Those are some of the questions that the Williams lab plans to explore in future projects.
Williams praised the “uniquely capable team that came together.”
“Ola is an emerging leader in considering integrated management of pests and pollinators and Kimiora is a known expert in developing regionally-relevant plant materials to support pollinators,” Williams said. “They and some talented UC Davis undergraduates--notably Katherine Borchardt and Anna Britzman--compiled a tremendously useful study.”
The overall aim of the study “was to identify California native plants, and more generally plant traits, suitable for coordinated habitat management of arthropod pollinators, herbivores and natural enemies and promote integrated ecosystem services in agricultural landscapes,” the researchers wrote.
More specifically, they asked:
- Which native plants among our candidate set attract the highest abundances of wild bees, honeybees, herbivores, predators, and parasitic wasps,
- If the total abundances of arthropods within these functional groups across plant speacies are related to the peak flowering week, floral area, or flower type of the focal plant species, and
- If the total abundances of arthropods within these functional groups are correlated to each other across plant species.
“A first critical step for design and implementation of multifunctional plantings that promote beneficial arthropods while controlling insect pests is to identify suitable plant species to use,” the authors wrote in their abstract. “We aimed to identify California native plants and, more generally, plant traits suitable for the coordinated management of pollinators (wild bees and honey bees), insect herbivores and arthropod natural enemies (predators and parasitic wasps).”
At the time, the Laidlaw grounds included nearly 50 bee colonies: some 20 to 40 honey bee colonies, and eight managed research colonies of the yellow-faced bumble bee, Bombus vosnesenkii.
The project received funding from the USDA Resources Conservation Service, USDA Agricultural Marketing Service, USDA National Institute of Food and Agriculture and a Swedish foundation for scientific research, the Carl Tryggers Stiftelse for Vetenskaplig Forskning.