Posts Tagged: Borers
After shothole borer invasion, UC Irvine becomes ‘perfect testing ground’
Results help inform best practices for managing the disease-causing beetle
The University of California, Irvine campus is home to a vast urban forest consisting of approximately 30,000 trees located in a mix of landscape, riparian and open space settings. In the mid-2010s, that forest came under threat from an invasive species of beetle that arborists and pest researchers were just learning about – the polyphagous shothole borer.
The tiny beetles, which may have arrived in California from their native Southeast Asia via infested shipping materials, tunnel into trees and introduce a fungus that serves as food for adult beetles and their larva.
As the fungus grows, it colonizes the tree's vascular system, blocking transport of water and nutrients. This causes a disease called Fusarium dieback that can kill branches or entire trees.
One reason the beetles were such a threat at UCI was the high number of sycamores on campus, especially in Aldrich Park at the campus center. Hundreds of cottonwoods, native willows, golden rain and coral trees also were affected. In total, the beetles attacked more than 2,000 trees, including 75 different tree species.
A variety of approaches to controlling beetle
To better understand and tackle this problem, UCI's Facilities Management department and Office of Environmental Planning and Sustainability collaborated with researchers affiliated with UC Agriculture and Natural Resources and UC Cooperative Extension. In addition, pesticide-manufacturing companies, pest control advisers and arborists provided materials and labor to help offset the cost of research.
“UCI was the perfect testing ground to determine integrated pest management strategies for this beetle/disease complex,” said John Kabashima, UCCE environmental horticulture advisor emeritus. “Our research was multifaceted, delving into early detection, monitoring and sampling, and cultural and chemical management.”
Kabashima said UCI provided the researchers with “a lot of freedom” to try a variety of approaches and study the results over time.
“We could cut down and sample trees or leave selected infested trees alone; we explored a variety of pesticide/fungicide combinations and application techniques,” he said. “That freedom resulted in many of the management solutions that are used today to effectively control this pest.”
UCI and the researchers also established a full inventory of affected trees on campus, evaluating severity of infestation by the number of entry/exit holes and signs of dieback. One important key to management is getting rid of “amplifiers” – heavily infested trees that are both hazardous and a source of beetles to spread to other trees.
“Typically, shothole borer infestations begin with just a few trees that for some reason are highly attractive to the beetles – perhaps based on tree species, tree spacing, irrigation conditions or other factors,” Kabashima said. “Over time, the beetles and fungus multiply largely undetected in those few trees. When the beetle population reaches a critical point and the trees begin to die, the female beetles fly to adjacent trees in a secondary invasion, eventually infesting many trees over a large area.”
An opportunity to diversify UCI's urban forest
At UCI, that initial invasion took place in landscaped areas containing many large, majestic sycamores that were planted when the campus began operations in the mid-1960s.
Over several years, UCI removed 700 heavily infested trees, including many of those historic sycamores, and replaced them with other tree species.
Today, the forest at UCI is very different than it was in 2015. While shothole borers have not been eliminated completely, their presence is reduced significantly, and UCI now has the tools to manage them effectively. Reforestation efforts resulted in a diverse treescape that is not only more sustainable but also beautiful.
“Managing a 1,500-acre campus with 30,000 trees is a never-ending process,” said Richard Demerjian, UCI's assistant vice chancellor, Campus Physical & Environmental Planning. “Our forest continues to evolve, with an ongoing focus on increasing diversity and plant health.”
Demerjian also noted that UCI is now starting to consider planting new sycamore trees on a limited basis.
A primer on effective shothole borer management
Whether managing a forest of thousands of trees or just a few trees, landscape managers and residents can apply many of the lessons learned at UCI to control invasive shothole borers and other tree pests.
- Avoid monocultures. Tree diversity provides beauty and resiliency.
- Keep trees healthy. Proper irrigation and maintenance will keep trees strong and help protect them from shothole borers and other pests.
- Check trees. Look for the common signs and symptoms of infestation such as beetle entry/exit holes. Regular monitoring ensures that infestations are managed early, before they cause dieback or tree death.
- Confirm suspected infestations. Use the detection tool at www.ishb.org.
- Review management options. For trees with low infestation, prune the infested branches and monitor the tree's health over time. In non-riparian, urban settings, consider treating low and moderately infested trees with pesticides/fungicides demonstrated to be effective against the pest-disease complex (A licensed professional will be needed to apply the treatments). Severely infested trees may require removal.
- Call in a professional. A certified arborist or pest control professional would be able to provide recommendations based on the tree's condition. The local county Agricultural Commissioner's Office and UC Cooperative Extension office may have additional knowledge about current shothole borer monitoring and management programs in your area.
- Take care of green waste. The beetles can survive in cut wood for weeks or even months. Proper disposal of green waste includes chipping infested wood, followed by solarizing or composting the chips.
- Replant wisely. Begin planting new trees only after removing all amplifiers and establishing an ongoing monitoring program. Consider the current concentration of tree species when deciding what type of trees to plant.
Shothole Borer Seminar at Long Beach Landscape Expo
UCCE's Dr. Beatriz Nobua-Behrmann will present "Using an Integrated Pest Management Approach to...
New Video on Sampling for ISHB
A new video outlining best practices in monitoring and sampling for invasive shot hole...
Tea Shot Hole Borer - PSHB, KSHB, ISHB
A “boring” problem that's generating
big interest in southern California
By Deena Husein, UCR Entomology, Ph.D. Candidate, Stouthamer Lab, UC Riverside
It has been a little over a decade since the first sighting of the polyphagous shot hole borer (PSHB) in Los Angeles County, yet a reliable control method has not been established. Part of the setback was caused by the misidentification of the species due to the beetle having a similar appearance to a low-risk pest — the tea shot hole borer. Aside from Los Angeles County, the PSHB is now established in Ventura, Orange, Riverside, San Bernardino and Santa Barbara Counties. We should be very alarmed at the rate the beetle has spread in southern California! The PSHB bores tunnels, also called galleries, in the xylem of the plant host and compromises the water transportation system. Tissue damage on the plant host is not only caused by the boring action alone, but also by the symbiotic, yet pathogenic, fungi the beetles carry to inoculate the gallery walls and utilize as a food resource. Consequently, the fungal buildup obstructs more water from reaching the branches, which can lead to dieback symptoms and eventually the mortality of the tree. The underlying issue lies in the gradual increase of plant species susceptible to being attacked. Currently, there are over 300 plants, 60 of which the beetle can reproduce on, including several native oaks, maples, sycamores, and willows along with ornamental plants and major agricultural crops (avocado) (Eskalen et al. 2013). A study by McPherson et al. 2017 showed that 23.2 million trees (32.8%) of southern California's region are susceptible to attacks by the PSHB beetle. Should half become infested, the approximate cost of removing and replacing those trees will be $15.9 billion with an annual accruement of $616.9 million over the next 10 years. The damage and loss the beetles have already costed the state will only continue to rise without the establishment of an effective management program.
We understand that this pest is a major threat to the forestry and agricultural industry, but what efforts have been made to stop it? So far, conventional methods such as pesticides and mate disruption through pheromone traps have been rendered ineffective due to the beetle's cryptic lifestyle. The PSHB mates prior to leaving its natal gallery and is only outside for a brief period of time spent entirely on finding a suitable plant host. With that in mind, our laboratory has taken a different course of action that focuses entirely on biological control. Analyses of beetles sampled worldwide paved the path to a foreign exploration trip to Taiwan in the hopes of finding promising natural enemies of the PSBH. Since we are dealing with a wood boring beetle that remains hidden inside of its host, the best approach to encounter any natural enemy is by collecting beetle-infested plant material such as logs. The collection sites can be seen on the map below, all of which were in the district of Tainan City. Infested logs were shipped to the Insectary and Quarantine facility located at the University of California, Riverside (Figure 1A). From there, logs were individually placed in separate containers and monitored daily for anything that emerged (Figure 1B). After three trips and many logs later, we were able to identify three promising parasitoid wasps and two fungal-feeding nematodes associated with the PSHB.
The first parasitoid wasp we have encountered belongs in the family Bethylidae (Figure 2). A closely related species in the same genus has been documented to parasitize the coffee berry borer, another invasive pest with a similar lifestyle to the PSHB. While breaking apart some of the logs, we were able to find parasitized beetles with silk cocoons extending from their heads. Inside one of these cocoons was a bethylid pupa, which looks very promising (Figure 3). The second parasitoid is a braconid with an interesting method of attack. This novel species in the genus Sinuatophorus has prominent and large mandibles to 1) remove any excess material, such as frass and sawdust, at the entrance of the beetle gallery then 2) clasp the head of the beetle and eventually parasitize it. However, most taxonomic descriptions of wasps in this genus always cite Seitner and Notzel's publication from 1925, which mainly focuses on the development and lifecycle of another closely related wasp in a different genus. This poses as a challenge since our understanding of the lifecycle of this novel wasp is limited. Despite this setback, we were able to rear 4 generations by reintroducing every newly emerged wasp to multiple PSHB-infested castor bean logs.
The third parasitoid belongs to a very small, yet unique, group. Not only does this wasp attack beetles in their adult stage, but it also lays two eggs: one male and one female. Once fully developed, the female chews a hole from the inside of the abdomen and creates an exit path for herself and the male wasp. But the excitement doesn't stop there! While rearing beetles that emerged from the shipped logs, we were able to come across multiple nematodes. While most of them appeared harmless, some were categorized as fungal feeders. By introducing these fungal-feeding nematodes to beetle galleries, we could potentially use them as an indirect approach to mitigate the population of the beetles by attacking their food supply.
We have come pretty far in the past three years and are now able to narrow our efforts on these natural enemies. Though, as is the case with most biological control programs, there are many challenges and failures that will be faced. Our biggest hurdle to overcome is the cryptic lifestyle of the beetle. Once we introduce newly emerged wasps to beetle-infested logs, any control we have over the rearing process is taken away. Basically, this becomes a black box with no way of observing any parasitization occurring inside without breaking into the logs and jeopardize killing the insects by accident. We are currently trying to improve our rearing methods in the hopes of increasing our wasp colony size. Eventually with large numbers, we can move forward with non-target testing and come to a decision on whether or not we can utilize these natural enemies as a biological control agents to suppress the PSHB population.
Map: collection sites of PSHB beetle-infested avocado logs from Tainan City, Taiwan.
Figure 1A (left): Bag with beetle-infested avocado logs from Taiwan. Figure 1B (right): Temperature-controlled room with logs separated in plastic containers in the Insectary and Quarantine facility at UCR.
Figure 2:Image taken of Bethylid adult that emerged from beetle-infested avocado logs collected from Taiwan. Credit: Iris Chien
Figure 3 (top): Parasitized PSHB found with a silk cocoon extended from the head of the beetle. Bottom:A cocoon was dissected, adjacent from the intact cocoon, with a visible bethylid pupa.
Read and see more at:
http://ceventura.ucanr.edu/newsletters/Topics_in_Subtropics80946.pdf
Literature Cited:
Eskalen, A., Stouthamer, R., Lynch, S. C., Rugman-Jones, P. F., Twizeyimana, M.,
Gonzalez, A., and Thibault, T. (2013). Host range of Fusarium dieback and its
ambrosia beetle (Coleoptera: Scolytinae) vector in Southern California. Plant Dis,
97(7), 938–951.
McPherson, G., Xiao, Q., Van Doorn, N. S., De Goede, L., Bjorkman, J., Hollander, A., Boynton, R. M., Quinn, J. F., and Thorne, J. H. (2017) The structure, function and value of urban forests in California communities.Urban Forestry & Urban Gardening 28: 43-53.
Seitner, M. and Notzl, P. (1925). Pityophthorus Henscheli Seitner und sein Parasit
Cosrnophorus Henscheli Ruschka.
During California Invasive Species Action Week, learn about invasive species
Californians can help in the fight against invasive species by learning and participating during California Invasive Species Action Week, June 2–10.
During the week, the UC Statewide Integrated Pest Management Program invites the public to spend lunch learning about invasive tree killing pests, aquatic nasties like quagga mussels and nutria, and how the invasive weed/wildfire cycle is altering our ecosystems. http://ucanr.edu/sites/invasivelunch/
The invasive species killing trees is causing sugar volcanoes to erupt on avocado trunks and branches that might be infected with Fusarium dieback. Fusarium dieback is a invasive, beetle-vectored disease that causes damage on avocado and more than 39 other tree species. The disease has spread in urban forests and wild lands in the Los Angeles basin since early 2012, and in Orange and San Diego counties since early 2013 and Ventura County in 2015.
The symptoms — staining, sugary exudate, gumming and beetle frass — are often noticed before the tiny beetles (1.5–2.5 mm) are found.
As its name suggests, these beetles bore into trees. Near or beneath the symptoms, you might notice the beetle's entry and exit holes into the tree. The female tunnels into trees forming galleries, where she lays her eggs. Once grown, the sibling beetles mate with each other so that females leaving the tree to start their own galleries are already pregnant. Males do not fly and stay in the host tree.
Shothole borers have a special structure in their mouth where they carry two or three kinds of their own novel symbiotic fungi. Shothole borers grow these fungi in their tree galleries. It's these fungi that cause Fusarium dieback disease, which interrupts the transportation of water and nutrients in the host tree. Advanced fungal infections will eventually lead to branch dieback.
Early detection of infestations and removal of the infested branches will help reduce beetle numbers and therefore, also reduce the spread of the fungus.
- Chip infested wood onsite to one inch in size or smaller. If the branch is too large to chip, solarize them under a clear tarp for several months
- Avoid movement of infested firewood and chipping material out of infested area
Avocado is one tree host. Shothole borers successfully lay eggs and grow fungi in many tree hosts, with some of these trees susceptible to the Fusarium dieback disease. For more information about tree host species, where the shothole borer is in California, and what symptoms look like in other tree hosts, visit the UC Riverside Eskalen Lab website or the Invasive Shot Hole Borers website.
Content in this post taken from the UC IPM Avocado Pest Management Guidelines. Faber BA, Willen CA, Eskalen A, Morse JG, Hanson B, Hoddle MS. Revised continuously. UC IPM Pest Management Guidelines Avocado. UC ANR Publication 3436. Oakland, CA.