AT THE INTERSECTION OF ECOLOGY AND GENOMICS
Hemlock Woolly Adelgid (HWA) Monitoring
Eastern hemlock (Tsuga canadensis), is a foundation species in eastern North American forests and its decline due to the introduction of the tiny invasive insect, hemlock wooly adelgid (Adelges tsugae, HWA), has had significant impacts on the health of forest ecosystems. HWA was first detected in Michigan (Emmet County) in 2006, and since that time it has been identified in Allegan, Ottawa, Muskegon, Mason, Oceana, Benzie, and Antrim Counties. One of the main focuses for HWA in Michigan is early detection and this is accomplished through visual surveys. However, visual monitoring is an enormous undertaking given the roughly 170 million hemlock trees in the state.
One technology that may aid in early detection efforts is environmental DNA (eDNA). Environmental DNA is DNA that is collected from the environment (air, water, or soil) and this approach is commonly used in aquatic systems for monitoring and management of various species. Airborne eDNA approaches have been used to detect fungal spores (Quesada et al. 2018) and pollen (Johnson 2017), and it can be applied to a wide range of species with wind dispersal. The use of continuous eDNA monitoring technology for HWA could lead to earlier detection than current monitoring methods. eDNA monitoring could also be coupled with visual monitoring protocols so that in areas where HWA is identified using this technology, more complete and focused visual surveys can be conducted to assess the exact location and severity of the infestation.
Our Work
Through a collaborative effort with Michigan Department of Natural Resources, Huron-Manistee National Forest, Sleeping Bear Dunes National Lakeshore, local townships, and private citizen we are using eDNA-based approaches to monitor for HWA. We are targeting two distinct areas: (1) high-risk areas where HWA is likely to invade but infestations have not been identified, and (2) sites that have been previously treated for HWA, but visual follow-up surveys have not been fully conducted. Previous work from our lab found these traps to be effective in catching both HWA crawlers and ovisac material at sites with known HWA infestations. Crawler are extremely small (0.5 mm), and the most mobile of the HWA life stages. Individuals are so small they can even be moved by wind!! So an airborne eDNA-based sampling approach is a great monitoring resource.
Image Credit: Kelly Oten, N.C. Forest Service
Our Traps
Through a USDA Special Technologies Development Grant, our lab developed a low-cost trap that allows for the collection of mobile HWA crawlers or airborne HWA material (i.e., ovisacs). With funding through the Michigan Invasive Species Grant Program, we have deployed 100 of these traps in coastal hemlock forests in Michigan. These traps include a centralized disk with glass microscope slides affixed to the disk and positioned parallel with the ground. Prior to deployment, the microscope slides are immersed in a 1:1 solution of petroleum jelly and water, which creates a thin, gelatinous adhesive to prevent crawlers or ovisac material from escaping once contacting the trap. The glass slides from each trap are exchanged every other week. Each trap slide is visually assessed for the presence of adelgids and the number of adelgids are counted.
For eDNA analysis, the petroleum jelly and all particles captured on each slide are removed and any DNA captured is extracted using a Zymo Quick-DNA Insect Microprep kit. The DNA is then amplified through quantitative PCR using markers specifically designed for HWA. This also allows us to distinguish HWA from any other adelgids that may be present in the area.
The Data
The efficiency of our traps were tested in 2021 in a moderately infested site in North Ottawa Dunes in Ottawa County, Michigan. We deployed 105 traps in a 90-acrea area from May – July, 2021. We collected slides from each trap every two weeks, and identified (1) if a trap captured an adelgid, and (2) how many adelgids were captured with each trap. Over the 16-week period, 96% of the deployed traps captured at least one adelgid. At the peak of the crawler period, some traps captured over 600 adelgids within a two-week period!!!! The traps also contained very little by-catch of other larger insects and this greatly enhanced our ability to accurately count the number of adelgids captured, when compared to other types of insect traps.​
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In 2022 & 2023, we deployed 100 eDNA traps throughout high-risk and invested areas throughout Michigan. Thus far, these traps have helped detect multiple newly infested areas, with one of those being the first detection of HWA in Huron-Manistee National Forest. By monitoring infested sites we are aiming to understand how HWA is responding to current treatment strategies and assess how HWA is being influenced by increased winter temperatures.