Electronic noses are sensitive to a vast suite of volatile organic compounds that every living organism emits. A new USDA Forest Service study shows that e-noses can detect emerald ash borer (Agrilus planipennis) larvae lurking under the bark – an early, noninvasive detection method.
For 25 years, emerald ash borers (EAB) have rampaged through the sapwood of ash trees. In North America, EAB attacks all ash trees, healthy or not. The U.S. has three major eastern ash or Fraxinus, species affected: white ash (F. americana), green ash (F. pennsylvanica), and black ash (F. nigra).
Infested trees are stressed. Eventually, the signals are easy to see – half the branches die, the bark splits, and epicormic branches sprout. On the tree-rating scale Wilson and his team used, stress signals like these would put a tree in a decline class of 3 or 4. The decline class scale ends at 5, when the tree is dead.
Long before any of these distress signals are visible, the tree’s smellprint is profoundly altered. EAB larvae only tunnel shallowly into the sapwood, but they cause significant damage that changes the physiology of the tree. The physiological changes result in changes of the volatile organic compounds (VOCs) released. Whether in trees, bats, deer, or humans, disease and tissue damage changes the pattern of VOCs emitted.
“Electronic noses have revolutionized the discipline of pathology,” says Wilson, who has published extensively on e-noses and their potential to non-invasively detect disease in plants, animals, and humans.
The recent study combines lab and field methods and uses ash samples from Arkansas, where EAB is spreading. Benjamin Babst and Mohammad Bataineh, assistant professors at the Arkansas Forest Resources Center at the University of Arkansas coordinated the field and sampling components of the study. Bataineh and his team established plots in a green ash plantation. The trees were 25 to 40 years old, and EAB had been detected nearby. Some of the ash trees were still healthy, but others had been infested with EAB and were declining.
In 2017, the scientists bored into the tree trunks to collect sapwood samples. They also collected bark samples. In all, they collected 86 samples and recorded the decline class of each sampled tree.
The samples were frozen and shipped to Wilson’s lab at the Center for Bottomland Hardwoods Research. Wilson and his colleagues Lisa Forse and Babst conducted a number of chemical analyses using ultra-high performance liquid chromatography, mass spectrometry, and an e-nose technology to analyze solvent extracts and VOC emissions from the sapwood and bark.
To identify the e-nose smellprint patterns, the scientists used a dual-technology device. These devices combine the power of e-nose sensor arrays with gas chromatography, allowing the scientists to identify smellprint patterns and some of the VOCs emitted.
The e-nose has hundreds of sensors, each differentially sensitive to a wide range of VOC classes. Wilson and Forse identified 32 sensors that could best discriminate between healthy and infested samples. Healthy sapwood emitted VOCs that produced a strong response in most of the 32 sensors. Just two of the sensors showed no response. Eight VOCs were emitted only from healthy sapwood – a finding which could be potentially used as a land management tool to identify healthy ash trees.
The smellprints of infested and healthy sapwood were vastly different. Overall, infested samples had fewer sensor responses (see photo 2), which means fewer VOCs were being emitted. Five VOCs could be detected only in infested samples, including an aliphatic aldehyde at peak 9, a monoterpene at peak 10, and so on.
In all infested samples, sensors responded to peak 7. The scientists think the sensors were responding to a bicyclic monoterpene, which might be emanating from the EAB galleries. The scientists also identified five VOCs that were similar in healthy and infested samples.
Their findings have important implications.
“E-noses can detect emerald ash borer infestations early, when control treatments will be most effective,” says Wilson. “The e-nose technique would also be very useful in quarantine efforts.”
Other methods for detecting emerald ash borers have proven costly and time-consuming. Adults can be lured into traps, but the traps must be properly located and close to an EAB population. Canopy decline can be observed, and so can the D-shaped exit holes adults leave behind in the bark as they emerge, but by then the infestation has already resulted in extensive damage. The biochemical response in ash bark tissues doesn’t change significantly after trees are infested, so lab-based chemical analyses are likely impractical.
“EAB-infested trees should be marked for immediate harvest,” says Wilson. “Immediate harvest would preserve the monetary value of the lumber by avoiding damage from secondary insect borers, wood-decaying fungi, and other decomposers.” Immediate harvest of infested trees could also help slow the spread of EAB.
One of the most important prevention strategies is to not move firewood.
Ash is a member of forest communities, and it was once a common street tree. In some areas, ash comprised up to 20 percent of the urban canopy. Now, billions of untreated ash trees in forests and urban areas have died. The loss of urban canopy is costly. Since 2009, municipalities and homeowners in the U.S. have spent over $10 billion to treat or replace 45 percent of the ash trees in urban areas.
For more information, email Dan Wilson at firstname.lastname@example.org.