The hemlock woolly adelgid remains near the top of the list of harmful pest insects in eastern forests. It has significantly damaged the hemlocks in 2.5 million acres of hemlock-dominated forests in 18 states, from Vermont to Georgia. And because hemlocks play a crucial role in the health of surrounding ecosystems, the disappearance of this species could lead to a cascade of changes in soil chemistry, stream temperatures, and species diversity. Faced with this prospect, a great deal of research has been directed at testing potential biocontrols to combat the adelgid; a University of Vermont project may be the most promising.
The same adelgid is also present in hemlock forests of the Pacific Northwest, but there two species of native silver fly are keeping it in check. So UVM ecologist Kimberly Wallin joined scientists from Oregon State University to study whether the flies will also eat the adelgids on eastern hemlocks. “This family of fly includes the only predator that has effectively controlled adelgids anywhere in the world,” said Wallin, who spent five years conducting host specificity tests to ensure that the flies do not become pests if they are introduced in the East. In laboratory studies, she found that the flies will feed on the adelgids on eastern hemlocks and that the insect can complete its life cycle and successfully reproduce. While it does occasionally feed on a native pine bark adelgid, it much prefers hemlock woolly adelgid.
After obtaining the necessary permits, Wallin placed flies inside sleeves that were hung over adelgid-infested hemlock branches in private forests in Tennessee and along Skaneateles Lake in central New York. Some of the sleeves received four flies, some ten, and some were left empty as a control. “Rather than go where adelgid populations are really high now (or used to be), we did our release at the leading edge of the infestation, where adelgids are expanding,” Wallin explained. “We wanted to see if the flies were going to feed on the adelgid and suppress it. And after a month it was very successful. That was a huge step, and we’re super happy.”
The next step is to expand the study to additional sites and to extend it for a full year to see whether the flies can survive cold winters in the North and hot summers in the South. That work will begin in the spring.
After the success of the first phase, Wallin and her colleagues are optimistic that the silver flies could play an important role in the fight against the hemlock woolly adelgid. The U.S. Forest Service is testing a Laricobius beetle as another possible biocontrol against the adelgid, and because the beetle is active in different seasons than the flies, the researchers believe the insects could complement each other. “Our hope is that this strategy will stop the expansion of the adelgid,” Wallin said. “The adelgid hasn’t touched the entire range of hemlock trees yet, but in areas where it is already established, we hope it will reduce those populations so it won’t kill the trees.”