New Container Aids in Biocontrol of Hemlock Woolly Adelgid
By John P. Roche, Ph.D.
The hemlock woolly adelgid (Adelges tsugae) is a small invasive insect that feeds on tree sap of eastern hemlocks (Tsuga canadensis), causing loss of tree health or tree death. Biological control using predatory insects holds promise for the control of woolly adelgids. Two options for biological control are predatory beetles of the genus Laricobius and silver flies of the genus Leucopis.
Laricobius nigrinus beetles from the Pacific Northwest have been released for hemlock woolly adelgid (HWA) control since 2003, and Laricobius osakensis beetles from Japan have been released for HWA control in the U.S. since 2012. In addition, since 2015, two silver fly species under evaluation have been released in the eastern U.S., but the status of their populations there is unknown.
To help inform biocontrol with these predatory species, Albert Mayfield III, Ph.D., of the USDA Forest Service and Nicholas Dietschler and Mark Whitmore, Ph.D., of Cornell University designed and tested a two-chamber collecting device to assess the presence of Laricobius beetles and Leucopis silver flies in hemlock trees in the wild. A report of their study was published in September in the Journal of Economic Entomology.
The hemlock wooly adelgid is native to Japan. The first record of HWA in the eastern U.S. was in Virginia in 1951, and its arrival may have been via imported ornamental hemlocks. The insect spread slowly for decades, but it began to spread rapidly in the 1980s, and it is now found in 17 eastern states. Because the adelgid’s survival is reduced by cold winters, climate change is making it easier for adelgids to survive, which may have assisted its expansion.
The host of the HWA, the eastern hemlock (Tsuga canadensis), is found from northern Georgia to Maine. Eastern hemlock are the dominant trees on over 2 million acres of forest in the eastern U.S. Often growing along streams, hemlocks help regulate air temperature, water flow, and nitrate levels in streams. These conifers also provide habitat for many game and wildlife species. The survival of these hemlock forests—and the ecosystem services they provide—is threatened because the invasive HWA causes extensive mortality to eastern hemlock trees in over 50 percent of the tree’s range.
Biological control of HWA depends on an understanding of their life cycles and their enemies. Adelgids produce two generations per year, a late winter/early spring generation and a late spring/early summer generation. Eggs hatch into tiny “crawler stage” individuals that settle on hemlock twigs, cover themselves with the woolly white fibers for which the species is named, and feed on sap from the tree.
Laricobius beetles produce ovisacs on twigs in the winter, and their larvae feed on eggs of the winter generation of HWA. Then they fall from the foliage to the soil where they pupate. In contrast, after feeding on HWA eggs, Leucopis silver fly larvae pupate on hemlock twigs, become winged adults, and disperse upward from the tree.
And silver fly larvae offer a profound advantage: They feed on HWA eggs of both the winter generation and the spring/summer generation. As Mayfield explains, “A major constraint to control of the hemlock woolly adelgid is the current absence of an effective natural enemy in eastern North America that feeds on adelgids during the late spring and early summer, when adults of the second adelgid generation are producing eggs. A potential solution to this constraint would be successful introduction, establishment, and spread of Leucopis silver flies from western North America that target hemlock woolly adelgids during this time period.”
Entomologists are therefore eager to gather information about the presence of Laricobius beetles and Leucopis silver flies in areas where their presence is being encouraged. To fill this need, Mayfield and colleagues designed a container to collect and separate Laricobius beetles and Leucopis silver flies with the same device. They call it the Lari-Leuco container.The container consists of nine parts, held together with hot glue. It includes a 500-millileter round container on the bottom and a 3.8-liter round container on the top. The bottom collection jar is for Laricobius beetle larvae dropping from foliage. The top collection jar is for Leucopis silver fly adults rising in the foliage.
Mayfield and colleagues conducted two lab trials with their Lari-Leuco container. For their trials, they made bouquets of foliage of western hemlock, Tsuga heterophylla, and stuck them in green horticulture foam. The foliage contained hemlock woolly adelgids along with Laricobius nigrinus beetles, and Leucopis argenticollis and Leucopis piniperda silver flies. Then they hung Lari-Leuco containers on the foliage and counted the presence of Laricobius larvae and Leucopis adults every one to two days. In the second trial, they tested a modified device with an inverted funnel added to the top collection container to see if it helped capture more Leucopis adults.
Mayfield summarizes their device, saying, “The container was designed to efficiently collect two different insect species that disperse from a hemlock branch; one species that falls (the beetle larvae) and one that flies (the adult silver flies). The container has a cup at the bottom to catch the fallers, and a cup at the top to catch the fliers.”
In Trial 1, 174 Laricobius beetle larvae were captured, and 100 percent of them were found in the bottom container. Six Leucopis silver fly adults were trapped, half in the top container and half in the bottom container.
In Trial 2, the investigators modified the initial container design by adding an inverted funnel to the top container. They used eight bouquets of hemlock: four bouquets had a funnel on the top container and four did not.
In all, 343 Laricobius beetle larvae were captured in Trial 2, 100 percent of them in the bottom container. And 124 Leucopis silver fly adults were captured in Trial 2. In containers with no funnel, 60 percent of the silver flyadults were found in the top container, but, in containers with a funnel, 94 percent of the silver flyadults were found in the top container. This difference was statistically significant.
Therefore, in both trials, all Laricobius beetles were found in the bottom container, as desired, and in the second trial, the presence of a funnel significantly increased the proportion of Leucopis silver flies recovered in the top container.
In conclusion, the investigators observe, “The Lari-Leuco container with the improved top design was effective for collecting descending Laricobius larvae and ascending Leucopis adults dispersing from the same T. heterophylla foliage sample.”
With previous methods of collection, individual insects usually had to be collected individually. But this new container overcomes the need for individual collection, saving time and money. When implemented in the field, the Lari-Leuco container offers promise for providing data on the distributions of Laricobius beetlesand Leucopis silver flies in hemlock stands, providing entomologists with invaluable information for optimizing biological control of the devastating hemlock woolly adelgid.
Journal of Economic Entomology
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.