Hidden Threats: How eDNA is Transforming Forest Pest Detection

Invasive species remain one of the most urgent ecological and economic threats of our time. They disrupt ecosystems, reduce biodiversity, and cause billions in damage. Among them is the emerald ash borer (Agrilus planipennis, or EAB), a small, metallic-green beetle that has devastated ash tree populations across North America and is now advancing through parts of Europe.

Detecting pests like EAB early, when they are still rare and relatively contained, is critical. But this is easier said than done. Much of the EAB life cycle occurs beneath tree bark, making it almost invisible until the damage is already done. Traditional survey methods struggle to pick up early infestations. Recent research, however, suggests that environmental DNA (eDNA) could offer a much-needed breakthrough.

Why eDNA Offers a New Hope

The emerald ash borer has caused more than $2 billion in damage in the United States alone. Its larvae bore through ash trees, disrupting the transport of water and nutrients and ultimately killing the host. Since its accidental introduction in the 1990s, EAB has left millions of dead trees in its wake. This loss carries broader ecological consequences, as ash trees play a vital role in supporting wildlife and stabilising soil.

The key to limiting the spread of invasive pests is early detection. But conventional tools—visual surveys, baited traps, and girdled “trap trees”—often prove too slow or imprecise. These methods are labour-intensive, seasonally restricted, and frequently miss low-density populations.

Environmental DNA offers an alternative. All living organisms shed genetic material into their surroundings through skin, faeces, saliva, or, in the case of EAB, larval feeding under bark. By collecting samples from tree tissue, soil, or water, researchers can screen for a species’ DNA and confirm its presence without needing to find the actual organism.

Testing the Method: Tree Cores and DNA

In the study, scientists trialled an eDNA-based detection method for EAB at two contrasting field sites: one with high infestation levels in New Jersey and another with low-level presence in New Hampshire.

In New Jersey, green ash trees (Fraxinus pennsylvanica) showing clear signs of decline—such as canopy dieback—were sampled, although visible symptoms like bark splits or beetle exit holes were deliberately avoided. This allowed the method to be tested in trees that were visibly unwell, but not obviously infested.

In New Hampshire, where EAB populations were thought to be low, white ash trees (Fraxinus americana) were categorised into three levels of visible damage: none, light, and moderate. This site provided a tougher test of the method’s sensitivity.

Tree core samples were taken using a Haglöf increment hammer, which extracts small wood cylinders with minimal harm to the tree. Two cores were collected from each tree—one from the north side, one from the south—at chest height. To prevent contamination, the tool was flame-sterilised between uses, and control samples were taken from nearby non-host species such as oak and birch.

Samples were frozen and later analysed in the lab using a highly specific qPCR (quantitative polymerase chain reaction) assay designed to detect EAB DNA.

What the Results Showed

In New Jersey, the eDNA method successfully detected EAB DNA in 64% of sampled trees during peak summer months—an encouraging result for an early-stage technique. It confirmed that eDNA from larvae feeding inside the tree can be recovered and identified from small core samples.

In contrast, no positive detections were made in New Hampshire, despite some trees showing signs of light or moderate damage. This may reflect genuinely lower pest densities, but it also highlights the influence of seasonal timing. The New Hampshire sampling was done earlier in the year, when larvae were less active and DNA concentrations were likely lower.

Despite these limitations, the study represents a significant step towards the use of eDNA for forest pest surveillance.

Why eDNA Matters for Forest Managers

Compared to traditional methods, eDNA offers a number of advantages:

• Non-destructive: Collecting tree cores causes far less damage than methods like girdling, which kill the tree to attract beetles.
• Sensitive: eDNA can detect small traces of DNA, making it suitable for identifying early-stage or low-density infestations.
• Efficient: Sampling is quick and can be done by a single person. The material can be processed later, providing flexibility in field operations.
• Extended detection window: eDNA signal is strongest later in the growing season, offering a longer time frame than some conventional methods.

Together, these advantages could make eDNA an essential tool for large-scale forest monitoring, particularly when surveillance resources are limited.

As promising as eDNA is, there are challenges to overcome before it can be widely adopted in forestry settings:

1. Seasonal variation: DNA concentrations within the tree fluctuate depending on larval activity and sap flow. Sampling outside the optimal window risks false negatives.
2. Low-density detection: As seen in New Hampshire, detecting sparse populations remains difficult. More work is needed to refine sampling protocols and increase sensitivity.
3. Understanding DNA movement: It’s still unclear how far EAB DNA travels within the tree. If it remains localised near feeding sites, sampling strategies will need to account for that.
4. Head-to-head comparisons: Rigorous studies comparing eDNA with traditional survey methods will help determine when, where, and how each should be used.

Final Thoughts: Beyond Ash Trees: A Broader Role for eDNA

While this study focused on EAB, the implications are much broader. The same approach could be adapted to detect other invasive wood-boring insects, such as the Asian longhorn beetle, or even fungal and microbial pathogens.

For biosecurity agencies and forest managers, eDNA could be the difference between containment and costly, long-term control. With global trade and climate change increasing the risk of biological invasions, having fast, sensitive tools like eDNA will be vital for early warning and response.

It’s important to see eDNA not as a replacement, but as a complement to existing tools. No single method will suit all species or scenarios. But used together, eDNA and conventional approaches can offer a more complete, layered surveillance system—capable of both broad screening and targeted follow-up.

The use of environmental DNA to detect hidden tree pests like emerald ash borer marks a powerful shift in how we monitor our forests. It opens new doors for early detection, rapid response, and smarter resource allocation.