The Bohart Museum of Entomology is showcasing the innovative research of UC Davis evolutionary ecologist Leslie Saul-Gershenz, who studies how blister beetle nest parasites mimic the sex pheromone of digger bees.
Bohart associate Emma Cluff created the wall display, “Digger Bees and Their Nest Parasites,” which examines the life cycle, research process, results, research challenges and implications.
The display runs through May 16. The Bohart Museum is located in Room 1124 of the Academic Surge Building on Crocker Lane, UC Davis campus. It is open to the public, Monday through Thursday, from 9 a.m. to noon and from 1 to 5 p.m., except on holidays.
Gershenz, associate director of research, Wild Energy Initiative, at the John Muir Institute of the Environment, UC Davis, researches the chemical ecology and parasite-host interactions of these solitary native bees and their nest parasites across the western U. S., including the coastal sand dunes of Oregon and the Mojave Desert in south-central California.
Gershenz, who holds a doctorate in entomology from UC Davis, did much of her work at the Mojave National Preserve, where she tracked the solitary bee Habropoda pallida and its nest parasite, a blister beetle, Meloe franciscanus.
The larvae of the parasitic blister beetle produce a chemical signal or allomone, similar to that of a female bee's pheromone, to lure males to the larval aggregation. The larvae attach to the male bee on contact and then transfer to the female during mating. The end result: the larvae wind up in the nest of a female bee, where they eat the nest provisions and likely the host egg.
Saul-Gershenz' experiments found the allomones “released by each population of M. franciscanus triungulin (larvae) mimic the pheromones released by a specific species of Habropoda bees native to their local habitat,” Cluff wrote in the display. “Leslie found that these differences had a genetic basis. She also found that local bee species were more attracted to the allomones released by their local triungulin population.”
“The M. franciscanus triungulin hatching is synchronized with the emergence of adult female Habropoda bees,” the display reads. “The triungulins aggregate on plant stems and release an allomone blend which attracts male bees. The aggregation of triungulins hop on to male bees who have chosen to investigate the allomone. Once the male bees find a real female bee, they mate in a ‘mating ball' at which time the triungulins transfer to the female. All this effort is so that the triungulins can get a free ride to the nest that the female bee lays her eggs in. Once inside the nest burrow, the triungulins will feed on the net provisions and likely the egg itself and will remain there until they emerge as adults the following winter.”
Results? “Leslie's experiments found that the allomones released by each population of M. franciscanus triungulin mimic the pheromones released by a the specific species of Habropoda bees native to their local habitat,” Cluff wrote. “Leslie found that these differences had a genetic basis. She also found that local bee species were more attracted to the allomones released by their local triungulin population.”
The research contributes to the understanding of the communication signals of bees in the genus Habropoda. Saul-Gershenz is currently finishing two research papers: the basic biology of digger bee Habropoda pallida, and the biology of the silver digger bee Habropoda miserabilis.
She and her husband, Norman, are the co-founders of the Bay Area-based SaveNature.Org. The international conservation consortium works with partners to protect ecosystems around the world.
No large aggregations of the species had been seen in that area for nearly a century.
Saul-Gershenz confirmed that the digger bees are Habropoda miserabilis and were probably common in the sandy dunes of that area as late as the 1920s. When non-native ivy, eucalyptus and ice plants took over their habitat, the bees disappeared.
“The discovery of a thriving native bee colony on the western side of the Presidio is the latest example of how the removal of invasive plants and the restoration of dunes and grasses at the former military base have helped bring back coastal habitat that thrived in San Francisco for tens of thousands of years before the city was built,” said Saul-Gershenz, who received her doctorate in entomology from UC Davis and is now associate director of research for the Wild Energy Initiative, John Muir Institute of the Environment, on campus.
“I am very happy to see this nest site at the Presidio,” said Saul-Gerzhenz, who has worked on the biology, chemical ecology and parasite interactions of this group of bees in the genus Habropoda for many years. She has researched Habropoda at other sites, including the Oregon coast and the Mojave desert.
“This nest parasite M. franciscanus was originally described from the dunes in San Francisco near Lake Merced by Van Dyke in 1928,” Saul-Gershenz said. “It is presumed to be locally extirpated in San Francisco due to habitat alteration. However its host bee, H. miserabilis appears to have finally found a suitable nest location in a sand dune area being restored by the Presidio Trust in the Presidio National Park. The resiliency of nature provides hope for the future.”
In a news story published in the San Francisco Chronicle on March 29, reporter Peter Fimrite quoted the UC Davis entomologist as saying that the silver digger bees were “all but gone” by the mid-20th century. Since 1960, only a couple of specimens have been found at the Presidio. Saul-Gershenz collected a digger bee specimen near Baker Beach in 1998, but no large aggregations have been found--until now.
"Biologists have reported a more than tenfold increase in the number of native plants in the Presidio, including at least four that are federally listed endangered or threatened, among them the Presidio clarkia," wrote Fimrite. "The Franciscan manzanita, which was believed to be extinct in the wild, was discovered in the Presidio in 2009. It was the first of its kind seen in its native San Francisco since the old Laurel Hill Cemetery was bulldozed in 1947 and paved over for homes."
Silver digger bees "have made a comeback, after being nearly wiped out, in several other areas in California, including Bodega Marine Reserve in Bodega Bay and Lanphere Dunes in the Humboldt Bay National Wildlife Refuge after both places were restored," Fimrite related.
Brian Hildebidle, stewardship coordinator for the Presidio Trust, discovered the colonies while surveying a dune restoration project.
Gershenz and collaborator Jocelyn Millar of UC Riverside and others deciphered the sex attraction of Habropoda miserabilis and the deceptive mimicking blend used by its nest parasite Meloe franciscanus working with a population on the coast of Oregon (Saul-Gershenz et al. 2018). They documented a new parasite-host location system while conducting research on related species in the same genus Habropoda pallida found in the Mojave and Sonoran Desert (Saul-Gershenz and Millar 2006).
The scenario begins when aggregations of beetle larvae of Meloe franciscanus emit chemical signals that mimic the sex pheromones of female bees luring male digger bees to make contact. The Meloe larvae then attach to males bees on contact, Habropoda pallida, from California's Mojave Desert and H. miserabilis from the coastal dunes of Oregon.
During subsequent copulations, the larvae transfer from males to females, hitching a ride on female bees to their nests, where the larvae feed on the provisions and the bee egg, and emerge as adults the following winter, said Saul-Gershenz. The research paper, “Deceptive Signals and Behaviors of a Cleptoparasitic Beetle Show Local Adaptation to Different Host bee Species,” appears in the current edition of Proceedings of the National Academy of Sciences (PNAS).
In solving the puzzle, the scientists tested whether geographically isolated populations of M. franciscanus larvae--from Oregon's coast and California's Mojave Desert--use local adaptations to exploit their respective hosts, H. miserabilis and H. pallida.
“Interestingly, male H. miserabilis were attracted to conspecific females and to aggregations of local Meloe larvae, but not to male bees,” Saul-Gershenz said. “Importantly, male bees of both bee species were more attracted to local parasite larvae than larvae from the distant locale because the larvae tailored their pheromone-mimicking blends to the pheromones of their local hosts. Additionally, the larval aggregation adapted their perching height at each location to the patrolling height of local male bees.”
In their abstract, the scientists wrote: "Chemosensory signals play a key role in species recognition and mate location in both invertebrate and vertebrate species. Closely related species often produce similar but distinct signals by varying the ratios or components in pheromone blends to avoid interference in their communication channels and minimize cross- attraction among congeners. However, exploitation of reproductive signals by predators and parasites also may provide strong selective pressure on signal phenotypes. For example, bolas spiders mimic the pheromones of several moth species to attract their prey, and parasitic blister beetle larvae, known as triungulins, cooperatively produce an olfactory signal that mimics the sex pheromone of their female host bees to attract male bees, as the first step in being transported by their hosts to their nests.”
“In both cases, there is strong selection pressure on the host to discriminate real mates from aggressive mimics and, conversely, on the predator, parasite, or parasitoid to track and locally adapt to the evolving signals of its hosts,” the co-authors pointed out. “Here we show local adaptation of a beetle, Meloe franciscanus (Coleoptera: Meloidae), to the pheromone chemistry and mate location behavior of its hosts, two species of solitary bees in the genus Habropoda. We report that M. franciscanus' deceptive signal is locally host-adapted in its chemical composition and ratio of components, with host bees from each allopatric population preferring the deceptive signals of their sympatric parasite population. Furthermore, in different locales, the triungulin aggregations have adapted their perching height to the height at which local male bees typically patrol for females. "
Saul-Gershenz said that the study “provides strong evidence for two different but complementary types of local adaptation in geographically isolated populations of a parasitic insect.” Specifically, the beetles locally adapt their deceptive chemical signals to the differing pheromone blends of their local host species and “the local nest parasites are significantly more attractive to male bees than nonlocal parasites, using transplant experiments.” The scientists identified the attractant blends for the two host species and the compounds that the beetle larvae produce to attract their hosts. They also showed that the two parasite populations have evolved divergent host-matching behaviors.
“The blister beetle Meloe franciscanus has turned out to be an engaging research subject, commented Saul-Gershenz, who received her doctorate in entomology from UC Davis, studying with Neal Williams and Steve Nadler, professor and chair of the UC Davis Department of Entomology and Nematology. She is now an associate director of research, Wild Energy Initiative, John Muir Institute of the Environment, UC Davis. “The larvae cooperate with their siblings for a brief period; they mimic the pheromone of their hosts; they are locally adapted to different hosts both chemically and behaviorally; and their emergence times are plastic across their geographic range. It has been fantastic to unravel this species' puzzle.”
She credited the counsel of the “true native bee icons in my field"--Robbin Thorp, UC Davis distinguished emeritus professor of entomology; research entomologist Jim Cane, Agricultural Research Service of U.S. Department of Agriculture; research entomologist Tom Zavortink, Bohart Museum of Entomology and former professor and chair of the University of Francisco Department of Biology; blister beetle (Meloidae) expert John Pinto, UC Riverside emeritus professor; and emeritus entomologist Rick Westcott, Oregon Department of Agriculture.
Future plans? Saul-Gershenz and Millar will continue exploring chemical communication signals as reproductive isolating mechanisms and the effect of eavesdropping parasites, parasitoids and predators on these signals. “I also plan to continue collaborating with Dr. Rebecca Hernandez and her lab members (UC Davis Department of Land, Air and Water Resources, and the Wild Energy Initiative of the John Muir Institute of the Environment) on the intersection of utility-scale solar energy development and our wildlife resources,” Saul-Gershenz said. In addition, she will continue her research on the impact on native bee diversity and pollination services from utility-scale solar development in the western deserts.
The research drew funding from Sean and Anne Duffey and Hugh and Geraldine Dingle Research Fellowship, the Community Foundation's Desert Legacy Fund, California Desert Research, Disney Wildlife Conservation Fund, and UC Davis Department of Entomology and Nematology fellowships.
Leslie Saul-Gershenz, a postdoctoral scientist in the UC Davis Department of Entomology and Nematology beginning January 2016, received the $220,000 grant from Cooperative Ecosystem Studies Unit of the Bureau of Land Management for the first year of the study.
“The grant will fund research to determine the type and extent of impacts that utility-scale solar installations on public lands may have on pollinator-plant webs in desert ecosystems,” Saul-Gershenz said. “Pollinators play a vital role in maintaining functional ecosystems. This project addresses the need for documenting instances of impacts from fragmentation of pollinator trap lines, loss of vegetation habitat for different life stages of pollinators, disruption of dependencies between endemic plants or endemic invertebrates and their respective companion pollinators or host plants, and potential demographic population declines from pollinator mortalities induced by specific types of renewable energy technology.”
Her co-principal investigators are pollination ecologist Neal Williams, associate professor in the department, and Lynn Kimsey, director of the Bohart Museum of Entomology and UC Davis professor of entomology. They will collaborate with native pollinator specialist Robbin Thorp, distinguished emeritus professor of entomology at UC Davis and a Bohart Museum associate; research associate Thomas Zavortink of the Bohart Museum; Terry Griswold of the U.S. Department of Agriculture's Agricultural Research Service Bee Biology Lab; and John Ascher of the National University of Singapore.
Saul-Gershenz is known for her bee-parasite research on solitary ground-nesting bees in the genus Habropoda and its nest parasite, a blister beetle, Meloe franciscanus. The larvae of the parasitic blister beetle produce a chemical signal that mimics the sex pheromone of female solitary bee to lure males to the larval aggregation. The larvae attach to the male bee and then transfer to the female during mating. The end result: a larva winds up in the nest of a female bee, where it eats the nest provisions and likely the host egg.
The Mojave and Sonoran Deserts are biological hot spots of biodiversity supporting more than 689 species of bees and 1512 species of plants in the Mojave Desert alone, Saul-Gershenz said.
The grant cites several publications:
Baldwin, B. 2015. Personal Communication. U. C. Berkeley, Jepson Herbarium. Number of species of plants in the Mojave Desert.
Griswold, T., Higbee, S. and Messinger. O. (2006). Pollination Ecology Final Report for Biennium 2003, Clark County, Nevada (2004-2005). Logan, Utah, USDA-ARS Bee Biology
Zavortink, T. and Kimsey, L. “Bees (Hymenoptera, Apoidea) of the Imperial Sand Dunes Recreation Area, Imperial County, California.” In preparation.
Another book, pending publication in December, also will contain their work: the second edition of Pheromones and Animal Behaviour (Cambridge University) by Tristram Wyatt.
Saul-Gershenz, a PhD candidate in the Neal Williams lab, UC Davis Department of Entomology and Nematology, and UC Riverside professor Jocelyn Millar and staff research associate Steven McElfresh study a group of solitary ground-nesting bees, in the genus Habropoda and its nest parasite, a blister beetle, Meloe franciscanus.
They are examining the host range of Meloe blister beetles and how these nest parasites cooperate to mimic the sex pheromone bees. The larvae of the parasitic blister beetle produce a chemical signal or a pheromone similar to that of the female solitary bee to lure males to the larval aggregation, said Saul-Gershenz. The larvae attach to the male bee and then transfer to the female during mating. The end result: the larvae wind up in the nest of a female bee, where they eat the nest provisions and likely the host egg.
- Keeping the Bees: Why All Bees Are at Risk and What We Can Do to Save Them by Laurence Packer and published in 2011 by HarperCollins Publishers, Ltd.
- Cuticular Hydrocarbons: Biology, Biochemistry and Chemical Ecology by editors A. Bagnères-Urbany and G. Bloomquist and published in 2010 by Cambridge University Press.
- The Other Insect Societies by James T. Costa, and published in 2006 by the Belknap Press of Harvard University Press.
Saul-Gershenz is the lead author of “Blister Beetle Nest Parasites Cooperate to Mimic the Sex Pheromone of the Solitary Bee Habropoda pallida (Hymenoptera: Apidae)," peer-reviewed research co-authored by Millar and McElfresh and published in the April 2012 edition of the Mojave National Preserve Science News.
The solitary bee is the first native bee to emerge in the spring on the Kelso Dunes in the Mojave National Preserve, she said. The adult beetles emerge on the dunes in the winter months at Kelso Dunes and feed exclusively on the leaves of Astragalus lentiginosus, which leafs out in January.
The bee's emergence is synchronized with the onset of blooms of the Borrego milkvetch, which is the sole host plant of adults of the blister beetle at Kelso Dunes.
“The Mojave Desert ecosystem supports 689 species of bees, which is the highest bee diversity in North America,” the UC Davis scientist said. The wide variety of insects endemic, or known only to that area, include a fly, scarabs, crickets, weevils, a bee, aphid wasp and scores yet to be described.
Saul-Gershenz, Williams and Millar received several grants including one from the Desert Legacy Fund, California Desert Research Program at The Community Foundation to study digger bee ecology and conservation. They're working with SaveNature.Org, which Saul-Gershenz co-founded. The relationship between the bee and the blister beetle is part of the research.