Leucotaraxis spp. Biocontrol Research
Background
Silver flies may be the key to an effective biocontrol strategy for hemlock woolly adelgid (HWA). Unlike Laricobius nigrinus, which predates only the spring generation of HWA, Leucotaraxis argenticollis and Leucotaraxis piniperda (Diptera: Chamaemyiidae), the two species of silver fly are predators of both generations of HWA. Leucotaraxis (pictured on the right) are abundant specialist predators of HWA on the west coast of North America whose larvae prey on the eggs of both HWA generations. With all three biocontrol species working together, we are hopeful that HWA populations will be suppressed enough to give eastern and Carolina hemlock trees a fighting chance.
While we know that Laricobius nigrinus has established in many parts of New York State and the broader eastern United States, the establishment of Leucotaraxis is still in process. Our current research program focuses on expanding our understanding of the biology, phenology, and ecological interactions of Leucotaraxis argenticollis and Leucotaraxis piniperda to inform effective biocontrol.
We obtain these flies by collecting western hemlock foliage infested with HWA from their native Pacific Northwestern range throughout the winter and spring. Foliage is processed at Sarkaria Arthropod Research Laboratory, the state-of-the-art Cornell University containment facility that allows for the contained collection of target biocontrol agents for research use in the eastern United States. We collect adult flies from the foliage, identify them to species, and release them throughout the HWA egg-laying stages from late winter through spring. Some of them are kept to start colonies or for research into the flies’ biology, behavior, and development. We have been involved in Leucotaraxis biocontrol fly releases since 2015. Our lab fine-tuned the rearing technique to make today’s mass releases possible.
Hemlock “bouquet” for Leucotaraxis collection
A bouquet of hemlock twigs, preserved in floral foam, from which Leucotaraxis will emerge for biocontrol releases
Cage Containing Western Hemlock Bouquets
A cage in our containment facility for Leucotaraxis collection
Leucotaraxis Collection
NYSHI Technician J. Schmidt collects Leucotaraxis from a cage in our containment facility
Leucotaraxis Release
Leucotaraxis being released at Mexico Point State Park in April of 2024. Photo by M. Serviss, NYS Parks
These flies were in the genus Leucopis prior to 2021, when a genetic review described a new genus, “Leucotaraxis,” into which these two species were placed (Gaimari and Havill 2021). Throughout our website, when we refer to silver flies or Leucotaraxis we are talking about one or both of the species Le. argenticollis or Le. piniperda.
Silver Fly Overwintering and Development
Biocontrol success hinges on understanding the life histories of predators, especially when exposing them to a new environment. In the case of Leucotaraxis spp., we continue to gain new insights through ongoing research. Field research shows that Leucotaraxis argenticollis is able to survive New York’s cold winter temperatures and emerge as adults at the appropriate time (Dietschler et al. 2023; Preston et al. 2023). The two species of Leucotaraxis have developmental and ecological differences, and our research is working to shed light on these life history differences to improve implementation of biological control.
We recently completed a development study tracking the life cycle of individual Le. argenticollis and Le. piniperda flies from egg to adult fly at seven different temperatures in the lab. We closely monitored each temperature treatment to understand the effects of temperature on timing of the flies’ life cycle stages from egg to adult. Knowing each species’ development rate will allow us to model the timing of their life cycles in any given climate. A more thorough understanding development rates, including similarities and differences between the two species, will help us to employ them effectively in the field for biocontrol of HWA.
Silver Fly Resource Partitioning
One challenge from working with silver flies is that our two target species, Le. argenticollis and Le. piniperda, are closely related and difficult to tell apart as immatures without genetic analysis. Working with these species in the lab has allowed us to explore subtle differences in their life cycles and use these differences to identify them by the timing of their adult emergence (Dietschler et al. 2021). We now deploy Leucotaraxis fly biocontrols in groups of the same species at ecologically appropriate times. This strategy aims to synchronize the predators with their HWA prey, reduce potential competition, and increase field mating success to improve biocontrol establishment.
A recent experiment, now in the data analysis phase, focused on better understanding the niche these species occupy in their native Pacific Northwest communities. Over several years from late winter through summer, when Leucotaraxis were feeding on HWA eggs, we collected infested foliage samples from sites throughout the Puget Sound area of Washington State. We recorded the location and abundance of HWA and our three specialist predators (these two flies and Laricobius nigrinus) and saved all specimens for identification. This allowed us to track how predators shared HWA prey over time and space and how they interacted with each other in their native range. A better understanding of their place in their native ecosystem has important implications for our biocontrol strategies in New York and the east coast.
Silver Fly Establishment
Monitoring released predators’ establishment success is a critical step in the biocontrol research process. Since Leucotaraxis flies are small (think fruit fly) and mobile, post-release detection is challenging. We are developing and testing survey techniques which are showing promise in the search for Leucotaraxis in the field. One method is using “Lari-Leuco” containers (Mayfield et al. 2021); we collect eastern hemlock foliage from release sites from at least 3 years prior and keep it in containers. We check these containers regularly for the three predators. If they emerge from the foliage, it shows that they have been able to persist in the environment. So far, new generations of flies have been recovered in Virginia.
In 2020, NYSHI began investigating the use of environmental DNA, or eDNA, to confirm HWA predator establishment in the field and understand where our releases have been successful. We are making direct comparisons of eDNA methods against more traditional methods, such as revisiting our release sites multiple years post-release to set up sticky traps (depicted in the photo to the right) and collecting foliage for rearing. The goal is to learn which method(s) yield the most reliable early detection information. If you’d like to learn more about the eDNA process, check out our eDNA page.
