Dominican Scientists Combat Invasive Beetle

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Researchers from Dominican University of California are turning up the heat on controlling the spread of an emerging invasive beetle threatening Northern California’s oak trees, particularly trees already weakened due to changing environmental conditions, including drought and high temperatures.

The National Ornamentals Research Site at Dominican University of California (NORS-DUC) has received  a grant from the United States Department of Agriculture (USDA) Forest Service to test heat treatment of wood infested by the Mediterranean Oak Borer (MOB) beetle, an invasive ambrosia beetle attacking mature oaks in Northern California, particularly in Napa County and the Sacramento valley.

A collaboration between Dominican, the USDA, and California Department of Food and Agriculture CDFA, and NORS-DUC allows scientists to study emerging and invasive quarantine pathogens of ornamental and native plants in an open, nursery-like environment.

Steaming and solarization are two potential alternative methods to treat heavily infested trees and can be used as part of an integrated pest management plan. This technology was originally developed by NORS-DUC to decontaminate research plots in which experiments with Phytophthora ramorum (the pathogen that causes Sudden Oak Death)  or other quarantine pathogens were conducted.

Stopping the spread of MOB is critical to prevent widespread infestation.

“In its native range in Europe, the Middle East and North Africa, the MOB does not seem to cause significant damage in its host trees, which include chestnuts, beeches and oaks,” says Dr. Wolfgang Schweigkofler, research associate professor and lead scientist at NORS-DUC.

“However, in the new environment in the U.S., infested trees show symptoms including wilting, defoliation, and broken branches. The insect seems to prefer trees which are already weakened by water stress or other pests or pathogens. Treatment of felled, but still infested trees, is necessary to limit pest spread and population size, but can be expensive, difficult, or have negative impacts on the environment.”

The NORS-DUC researchers have found that heat treatment at a temperature of 50⁰ C for 30 minutes will kill the Sudden Oak Death causing P. ramorum. Based on research done at NORS-DUC, the USDA’s Animal and Plant Health Inspection Service accepts steaming as a treatment for nursery soil infested with P. ramorum.

“Steaming is now an established part of the best management practices in several nurseries in and outside of California,” Dr. Schweigkofler says. “Steaming can be used to mitigate existing soil beds in situ, but also potting mixes, pots, and other nursery equipment and will reduce the risk of establishment and transmission of plant diseases.”

In a pilot experiment on oaks infected with MOB, NORS-DUC researchers treated wood in Calistoga, Napa Valley. Three size classes (wood chips, mid–size logs and large-size logs) were steamed, with the time required to reach the target temperature ranging between one and 12 hours.

The research team was able to reach the 50C target temperature in the wood. In order to confirm that the temperature killed the beetles, they have installed cages above the steamed wood and untreated control wood with some insect traps. Entomologists from the U.S. Forest Service are observing to confirm that the steam has killed the beetles.

The NORS-DUC team plan to do additional heat treatments with wood infested with the MOB beetle next winter. This will happen just prior to the mature beetles leaving the wood to mate and infest new trees.

For the past several years, NORS-DUC has offered steaming services for nurseries and other interested parties in the San Francisco Bay Area and surrounding counties concerned with the spread of P ramorum. NORS-DUC brings its own equipment, including a SIOUX SF-20 steam engine and temperature sensors, to the treatment site, and will plan, set up, conduct, and monitor the steaming event.

“Recently, we helped a well-known producer of strawberries and other berries,” Dr. Schweigkofler says. “The company grows huge numbers of plant rootstocks in Northern California before moving them to a production field in Central California for out planting. Although no Phytophthoras were found in the used pots, the company traditionally had a policy of ‘single use’ for their planting pots to prevent the movement of potentially contaminated soil and plant debris.”

After a first successful trial in 2021, the company asked NORS-DUC to steam 175,000 used pots in winter of 2022. Vernon Huffman, NORS-DUC nursery manager and steaming specialist, was able to carry out the task during a three day ‘steaming marathon’. The treated pots can now be reused for growing new plants, which not only saves the company a considerable amount of money, but also helps to reduce plastic waste and is environmentally friendly.

The NORS-DUC team also studies ways to improve the steaming technology. The traditional ‘top down’ approach, with heat penetrating the treated material from steaming socks placed on top of it, works well for many situations, but the technique can be slow and inefficient when treating large amounts of soil or potting mix.

“For this purpose, we use a ‘bottom up’ approach where the steam moves from a release point located below the material vertically to the top,” Dr. Schweigkofler says. “A manifold was designed, which is basically a steel scaffolding consisting of a central pipe connected at a right angle to a number side pipes. The steam is released from exit holes drilled at the bottom of the pipes. We compared the two steaming methods at our NORS-DUC facility and treated 7.5 cubic yard soil mix: using the ‘top down’ approach the target temperature of 50°C was not reached after seven hours on the bottom of the soil pile, whereas using the ‘bottom up’ approach with the manifold the whole pile reached the target temperature after 1.5 hours.”

Using the manifold not only resulted in a much faster and efficient heat treatment, but also reduced the costs and environmental impact by considerably reducing the fuel needed to run the steamer engine. Manifolds can be designed in different sizes and shapes based on the needs of the stakeholder using it. NORS-DUC has helped several nurseries to build manifolds, including a nursery which grows plants for the huge development project at the San Francisco Treasure Island.

Each year, NORS-DUC organizes the annual “Sudden Oak Death Blitz” in Marin. The SOD Blitz is an annual “citizen science” event organized by Prof. Matteo Garbelotto and his team UC Berkeley’s Department of Environmental Science Policy and Management, together with local partners at locations along coastal California. The SOD Blitz is designed to inform and educate the community about SOD, get local residents involved in detecting the disease, and produce a detailed web-based map of disease distribution. The map can then be used to identify those areas where the infestation may be mild enough to justify proactive management. This year’s Marin SOD blitz is from April 23-25. For more information, email Wolfgang Schweigkofler at

Photo above of Vernon Huffman, research nursery manager at NORS-DUC on campus, posing with a SIOUX SF-20 Steam Generator that can convert water into steam.

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