Can combining chemical and biological treatments save eastern hemlocks from the hemlock woolly adelgid?
Recently published research by U.S. Forest Service scientists and collaborators offers hope that integrated management can provide sustained protection for an iconic tree.
In an article published in the journal Forest Ecology and Management, Forest Service and university researchers provide findings based on data collected from a natural stand of hemlocks in Georgia that suggest chemical and biological control can be successfully integrated to help prolong hemlock health.
“Our study demonstrated that populations of beetle predators can successfully establish and increase on hemlocks previously treated with insecticide after the chemical protection wears off and adelgids infest the trees,” said Bud Mayfield, research entomologist with the Forest Service Southern Research Station and lead author of the article.
First found in the eastern U.S. in the 1950s, the nonnative hemlock woolly adelgid now infests hemlock trees from the U.S.-Canada border south to Georgia, in some areas killing 95 percent of eastern and Carolina hemlocks. This is particularly devastating to the Southern Appalachians, where eastern hemlock is considered a “foundation” species because of the important role it plays in streamside ecosystems.
Mayfield and fellow researchers used a stand of 60 trees in northern Georgia that were treated in 2006 with low rates of the insecticide imidacloprid (10 and 25 percent of label rate) to improve crown health while allowing a low rate of adelgid infestation needed to establish predator beetles.
In February 2008 and October 2010, they released Laricobius nigrinus, predator beetles originally from the U.S. Northwest found to be effective in controlling hemlock woolly adelgid, on the same trees. In December 2012, they also started a predator exclusion study in the crowns of selected trees to study the impact of predator beetles on adelgid populations at that level.
The researchers were interested in finding out:
- The longer-term (5 to 7 years after treatment) effects of imidacloprid treatments on tree health, adelgids, and predator beetles;
- The effect of the established predator beetle populations on the density of adelgids on the hemlocks; and
- The extent of hybridization between predator beetles and their local counterparts.
Hybridization adds a new wrinkle to the introduction of predator beetles. In the years since the 2003 introduction of Laricobius nigrinus, researchers at various locations have found evidence of the insect hybridizing with Laricobius rubidus, an eastern native that feeds primarily on the pine bark adelgid found on white pines.
Finding out the extent of hybridization is important. “For the predator beetle releases to be successful, their populations cannot be replaced or diluted by the native insect,” said Mayfield.
Researchers started sampling adelgids, predators, and hemlock health in 2011, using hydraulic lift trucks to access the tops of trees. They painstakingly counted insects and larvae and genetically analyzed predator beetles to determine levels of hybridization.
Findings from the study are encouraging, but cautionary.
By year seven, the hemlocks treated with 25 percent imidacloprid rate lost their insecticide protection, but had better crown health and supported as many predator beetle larvae as untreated trees, and larvae showed no sign of insecticide residues. Most (77 percent) of the predators collected on study trees were identified as Laricobius nigrinus, 12 percent as the native species, and 11 percent as hybrids, with the hybridization rate remaining the same over time.
Importantly, the cage exclusion studies showed twice as many undisturbed adelgid egg sacs on the branches protected with cages as on unprotected branches, clearly showing the effectiveness of the predators in the crowns of the trees. Nearly 70 percent of the uncaged branches harbored predator beetle larvae.
“To our knowledge, ours is the first published study to quantitatively document the impact on hemlock woolly adelgid populations of Laricobius predator beetles established on eastern hemlocks in a natural forest setting, as well as the first to examine predation throughout the canopies of mature trees,” said Mayfield.
“Our results show that chemical and biological control of hemlock woolly adelgid can be integrated to promote hemlock health and reduce numbers of adelgids,” said Mayfield. “That said, the Laricobius beetle only feeds on adelgids that are active from fall through early spring. Sustained protection of hemlock through integrated management in the Southern Appalachians may require the addition of an effective predator on the second adelgid generation that is active in late spring and early summer.”
For more information, email Bud Mayfield at firstname.lastname@example.org .