White, Woolly, and Worrisome: Detecting the Hemlock Woolly Adelgid with eDNA

Original article: Kirtane, A., Dietschler, N.J., Bittner, T.D., Lefebvre, M.B., Celis, S., O’Connor, K., Havill, N. and Whitmore, M.C., 2022. Sensitive environmental DNA (eDNA) methods to detect hemlock woolly adelgid and its biological control predators Leucotaraxis silver flies and a Laricobius beetle. Environmental DNA.

A Woolly Invasive

Insects, especially small ones living in trees, can be difficult to detect visually. This becomes problematic when an invasive insect begins to cause disturbances in ecosystems, such as forests. The hemlock woolly adelgid is an insect native to the Pacific Northwest and Asia but has unfortunately been introduced on the American East Coast. The consequences are severe; susceptible forest trees, including eastern hemlock and Carolina hemlock are dying at an alarming rate. The woolly adelgid damages conifers by sucking nutrients from the needle base. This in turn results in loss of the needles, the photosynthetic powerhouses of the trees. Rapid early detection of this pest as it continues to spread is critical. Identifying new areas of infestation will inform management and control options for the adelgid, but scientists need a more reliable way to detect this insect’s presence. Additionally, the adelgid does have some natural enemies, mostly native to the Western United States. These have potential as biological control, which is using a pest’s natural predators to reduce population sizes. The two adelgid predator groups are wasps belonging to the genus Leucotaraxis and beetles, specifically, Laricobius nigrinus. However, the predators are small and flighty, which also makes them hard to visualize and quantify. Regardless, knowing if predators of the pest are in the area is important for management decisions as well.

Hemlock woolly adelgids secrete a white, waxy substance that serves as protection as the insect feeds and grows. Photo credit: Connecticut Agricultural Experiment Station Archive License: Creative Commons 3.0

The Invisible Clue

Environmental DNA or eDNA could be the answer to detecting insect presence. Any DNA that is shed by an organism, whether it be skin cells or feces, is considered eDNA. Kirtane et al. wanted to investigate if they could detect hemlock woolly adelgid (HWA) eDNA on foliage taken from its eastern host trees. Additionally, they collected foliage from three various heights on the trees to see if there was a difference in effectiveness of eDNA detection. Finally, they also sought to quantify eDNA from three different predators: Leucotaraxis argenticollis, Leucotaraxis piniperda, and Laricobius nigrinus. The ultimate goal was to determine if visual investigation of foliage or eDNA analysis of foliage was more efficient in detecting the presence of the target organism.

Laricobius nigrinus is a predator of the hemlock woolly adelgid. It feeds on the insect’s eggs as well as adults. Here, you can see the beetle larvae attacking the eggs of another insect that are surrounded by white fuzz. Photo credit: Tom McAvoy, License: CC0 1.0 Universal.

The scientists collected foliage samples of eastern hemlock for HWA experiments and western hemlock for the predator experiments. Samples were collected at different points throughout the year. This allowed the scientists to determine if the three biological predators spatially overlapped at any point. After the foliage samples were collected and inspected for their respective organism, they were thoroughly rinsed. The liquid was saved, and different chemicals and filtration was used to capture any DNA that was in the solution. The next step was to amplify any traces of the target DNA found in the sample. In other words, the scientists cloned the target DNA. To capture only the target DNA, they used special cloning molecules that have a DNA sequence unique to the target species. After there are many DNA copies, they can then use sophisticated machinery to quantify the DNA present.

For the HWA and all three predator species, eDNA analysis resulted in a significantly higher positive detection rate than visual inspection. This highlights the inefficiency associated with manual inspection of foliage. Perhaps the insect was briefly there before moving on to another branch. If just the first branch is collected and visually inspected, then the results will return negative. However, the insect was there and likely deposited eDNA, which can be amplified and detected. eDNA can also travel, sometimes even by hitchhiking on a different organism. While this can result in false positive results, it signifies that the target organisms are close by. There was no significant difference between the three different tree heights form which samples were taken. This suggests that sampling can be accomplished with foliage within arm’s reach, overall simplifying the process.

This table from the publication demonstrates the higher positive detection rate when eDNA analysis was used. This trend is seen for both the pest (Adelges tsugae) and all predators. Source: Kirtane et al., 2022.

Strength in Numbers

If more than one predator species cohabits a space at any given time, greater population control of the prey can exist. Asynchrony of adult emergence between the different predator species could result in a timing mismatch though. In only 2 tree samples, both species of Leucotaraxis were visually detected in May and October samplings. However, eDNA analysis found evidence of both species co-occurring on 18 samples. The scientists acknowledge that more research is needed on this topic, but the results suggest that there is a time where both species occupy the same habitat. Interestingly, during the May sampling, eDNA of the third predator was also detected! Visual inspection of foliage, however, did not detect the presence of all three predator species concurrently. This emphasizes the importance of eDNA in investigating predator-prey relationships, and whether life cycles match the prey’s cycle.

Rapid detection and response is critical to mitigating impacts from invasive species. Small insects pose a challenge to first detectors, as their presence can go unnoticed until the infestation reaches damaging levels. Environmental DNA holds promise as a potential avenue for more effective and timely detection. While continued research is needed to perfect methods and implications, advancements in this technology will benefit and protect our forests in years to come. In addition to detecting invasive species, eDNA monitoring can provide information on threatened or endangered species presence as well. It is a useful tool for monitoring any species existing in a given location, as well as their predators and prey. Ultimately, eDNA can provide a snapshot into the inhabitants of an ecosystem that would be difficult to achieve by just visual observation.

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Maria Marlin

I am a plant pathologist living in Oregon and working for Oregon State University extension. I study soilborne and foliar pathogens that attack ornamental crops, but the vast majority of my time is spent conducting outreach! I train nursery workers in scouting and detecting signs and symptoms of plant disease. I love to write and share my love of science with others! In my free time, I love to horseback ride and adventure through the magical Pacific Northwest that I am so fortunate to call home. Whether it is chasing mountain summits, exploring the rugged coast, or basking in the silence of the mossy, misty, and moody forests, I am my happiest and most inspired when surrounded by nature.

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