Or more precisely, dead fruit flies or carrion on a tarweed plant can benefit the plant in more ways that most people would ever think about, say researchers in the UC Davis Department of Entomology.
Just as human tourists can be good for the economy, ‘insect tourists” can be good for a plant.
When the hairs of a “sticky plant” trap small insects or “insect tourists,” the “tourist trap” provides food for other predators, thus becoming a defensive mechanism that spares the plant from increased herbivore damage. Other beneficial results include greater plant fitness and increased fruit production.
“We conducted a large, simple field experiment to test the hypothesis that plant-trapped insects could enhance indirect defense by increasing predator densities,” said ecologist Billy Krimmel, a graduate student in the Jay Rosenheim lab, who worked with fellow ecologist Ian Pearse of the Richard Karban lab. Pearse is now a postdoctoral fellow in Walter Koenig’s laboratory at Cornell University, Ithaca, N.Y.
“Sticky plants-- those producing resinous, oily or hooked trichomes (hairs)--often entrap small insects that land on them as they pass by,” Krimmel said. “This insect carrion functions as a type of plant-provided food for defense.”
“This is the first example of such a plant-provided food being captured from the external environment,” Krimmel said. “We coined the term 'tourist trap', referring to the sticky hairs that catch insect passers-by.”
In their research, “Sticky Plant Traps Insects to Enhance Indirect Defence,” published in the journal Ecology Letters, the ecologists revealed that the trapped insect tourists “increased the abundance of a suite of predators, decreased herbivory and increased plant fitness.”
Later the journal Nature focused on the Krimmel-Pearse research in its ecology section: "When Plants Run the Food Chain."
"We have known for a long time that carnivorous plants entrap insects for their own benefit,” Pearse said. “In our current study, we found that the entrapment of insects by plants might be even more important and general than previously thought."
Krimmel and Pearse conducted their research in the Stebbens Cold Canyon Reserve, a UC Davis Nature Reserve located in Solano County, near the outlet of Lake Berryessa. Their sticky plant was tarweed (Madia elegans), an annual flowering California native plant in the family Asteraceae. It generally flowers in mid to late summer, from approximately June through September.
At our study site, tarweed's major herbivore is the specialist caterpillar Heliothodes diminutiva, which feeds largely on plant reproductive organs and can completely sterilise its host plants,” they wrote. The adult owlet moth, Heliothodes diminutive, lays its eggs on the developing buds. The emerging caterpillars can quickly devour all the flowers and buds.
“The suite of predators commonly found on tarweed,” they wrote, “includes the assassin bug Pselliopus spinicollis, two stilt bugs Hoplinus echinatus and Jalysus wickhami, the green lynx spider Peucetia sp. and the crab spider Mecaphesa schlingeri. All can navigate tarweed's sticky surface.”
Krimmel and Pearse chose 82 tarweed plants for their experiment. They placed dead Drosophila fruit flies to half of them, five flies per week through the growing season, and then monitored all the plants throughout the growing season.
“Because tarweed is a small, annual plant, we were able to do full counts of arthropods on all plants each week, and measure lifetime fruit production by the plants, allowing us to relate our experimental treatment to plant lifetime fitness,” the authors wrote.
“The addition of 5 dead fruit flies (carrion) to plants each week over the growing season increased the abundance of all surveyed predatory arthropods associated with M. elegans plants by 76 percent to 450 percent,. For P. spinoicollis, the most abundant predator, this effect was strongest during the early growth season in June and July.”
Specifically, “the addition of carrion (fruit flies) to M. elegans plants produced a 60 percent decrease in bud damage caused by H. diminutiva, the dominant lepidopteran herbivore in this system and increased lifetime fruit production by 10 percent,” the researchers said.
Jay Rosenheim's USDA research grant helped fund the project. Krimmel received two other grants: a National Science Foundation/Graduate Research Fellowship and a Jastro-Shields Research Scholarship.
Those oak trees (Quercus lobata) in California’s Central Valley have a lot of gall.
Ian Pearse, Maxwell Joseph and Melanie Gentles of the UC Davis Department of Entomology recently surveyed 1234 oak apple galls in Davis and Woodland (Yolo County) and Vacaville (Solano County) and got a better understanding of the gall-making wasps and the organisms that prey upon them or live with them.
“Oak apple galls are themselves a complex ecosystem, with over 20 species of insects, that are in many people’s backyards,” said Pearse, who is studying for his doctoral degree in entomology with major professor Rick Karban. “The galls and their wasps are not a major problem for oaks but are themselves food for other organisms such as birds and other insects.”
The wasp (Andricus quercuscalifornicus), a member of the Cynipidae family, lays her eggs on the leaves or twigs of a valley oak, which then forms a gall or a structure that resembles an apple hanging from the tree. “The gall is actually very beneficial, and necessary, for the insect,” Pearse said. In reality, the insect “’coerces’ the plant to make it a great home.”
“This community of insects has been poorly described for most cynipid-induced galls on oaks in North America, despite the diversity of these galls,” Pearse said. Cynipids are small solitary wasps that produce galls on oaks and other plants.
Their research, published in a recent edition of the international journal Biodiversity and Conservation, shed light on the natural history of the common oak apple gall and its parasitoid and inquilines community. They found that the composition of the insect community varies with galls of different size, phenology and location.
The researchers discovered that the gall maker “most often reached maturity in larger galls that developed later in the season. “The parasitoid Torymus californicus (family Torymidae) was associated with smaller galls, and galls that developed late in the summer,” they wrote. “The most common parasitoid, Baryscapus gigas (family Eulophidae), was more abundant in galls that developed late in the summer, though the percentage of galls attacked remained constant throughout the season.”
“Parasitoids and inquilines, in general, had a longer emergence period and diapauses within the gall than the gall-inducer,” they wrote. “The association of different parasite species with galls of different size and phenology suggests that different parasite species utilize galls with slight differences in traits.”
Galls provide their inducer with a consistent food source, a predictable abiotic environment, and a refuge from potential enemies, Pearse said.
Galls, which hang on the valley oaks like apples, are especially visible this time of year.
Researcher Maxwell Joseph, who received his bachelor of science degree last year from UC Davis, is now studying for his doctorate in ecology and evolutionary biology at the University of Colorado. Melanie Gentles, who holds a master's degree from UC Davis, is now the UC Davis campus arborist.
When does personality matter?
Jonathan Pruitt, a postdoctoral fellow with the Center for Population Biology, UC Davis Department of Evolution and Ecology, will speak on “From Individuals to Populations to Communities: When Does Personality Matter?” at the next UC Davis Department of Entomology seminar, set for Wednesday, Jan. 19.
His talk, open to the pubic, is from 12:10 to 1 p.m. in 1022 Life Sciences Addition (LSA), corner of Hutchison Drive and Kleiber Hall Drive. It will be webcast live and then archived on the UC Davis Department of Entomology website. Graduate student Meredith Cenzer of the Louie Yang lab will host the talk.
This is the third in a series of seminars for the winter quarter planned by graduate student Ian Pearse of the Rick Karban lab. Graduate students host the lectures, which take place every Wednesday noon in 1022 LSA through March 9.
If you missed the first two lectures, not to worry. They're archived here.
Rob Dunn, assistant professor, Department of Biology North Carolina State University, launched the series with a talk on “Using Collaborative Approaches to See the Geography and Future of Life: Lessons From Ants.”
Amanda Hodson, postdoctoral scholar in the UC Davis Department of Land, Air and Water Resources, spoke this week on "Ecological Influence of the Entomopathogenic Nematode, Steinernema carpocapsae, on Soil Arthropods in Pistachio Orchards."
When you visit the Peter J. Shields Oak Grove in the UC Davis Arboretum, you'll see one of the most diverse mature oak collections in the United States. More than 80 kinds of oaks, including scientifically documented trees native to the United States,Central America, Europe and Asia are planted there.
The dominant native oak is the Valley oak, Quercus lobata Née.
What Ian Pearse, a UC Davis researcher in the Department of Entomology, wanted to know was this: "Why do insects interact with some non-native plant species but not others?"
In a study encompassing three summers and 57 species of introduced (non-native) oaks in the grove, he found that many insects that target California’s native oak trees will also feed on non-native oaks planted near them, but with one distinct difference: the insects tend to do more damage to the non-native oaks that are closely related to the natives, than they do to the distant relatives.
“This is a confirmation of the ideas dating back to Darwin,” said Pearse, a doctoral candidate in the Department of Entomology who studies with major professor and noted insect ecologist Rick Karban.
Pearse and co-author Andrew Hipp of the Illinois-based Morton Arboretum and Field Museum of Natural History, Chicago, published their results in a recent edition of the journal, Proceedings of the National Academy of Sciences (PNAS).
“The insects were mostly small moths, fairly inconspicuous,” Pearse said. “They don’t cause a lot of damage. This was more of a theoretical study, of how insects on native oaks also tend to interact with non-native oaks that are similar.”
“Ian's study is important for several reasons,” said Karban. “Our collective intuition about what makes some introduced plants, including crop species, more susceptible to herbivores than others is poorly developed. By using a large number of oak species planted in a common environment, and accounting for the relatedness of the species, Ian can answer that question with a great deal of elegance and power. His finding that relatedness of the various oaks to the native species explains a lot of the picture and provides considerable insight.”
The journal cover features an image of the mural “Oak Family Tree,” from the UC Davis Arboretum oak collection. The mural, created through the UC Davis Art/Science Fusion Program, taught by entomology professor and artist Diane Ullman and artist Donna Billick, depicts the evolutionary relationships of 29 oak species and the animals associated with each species.
“The project was a collaboration with the arboretum,” Ullman said, noting that Emily Griswold, a national leader in oak conservation and the Arboretum’s assistant director of horticulture, “provided the leadership and knowledge base from the arboretum.”
The PNAS article is drawing widespread interest from ecologists, taxonomists and oak enthusiasts. Pearse is the first person to create a phylogeny of the oaks in Shields Oak Grove. Internationally recognized oak expert John Tucker (1916-2008), former UC Davis botany professor and a former director of the Arboretum, helped plant the trees nearly half a century ago.
And now we know more about the insects that interact with those oaks.