A new study, done by scientists at the University of Georgia, suggests that one of the secrets to controlling the hemlock woolly adelgid may be at the top of the tree.
That’s where adelgid populations are highest, the study found. Results of the study were published in the December issue of the Journal of Economic Entomology.
To combat hemlock woolly adelgid (Adelges tsugae), many researchers have released two species of beetles, Laricobius nigrinus and Sasajscymnus tsugae, which eat the adelgid. The beetle releases have been fairly successful at controlling the adelgid each year, but populations must be replenished at the beginning of each season, and the beetles are expensive to rear.
Scientists raise the beetles on infested hemlock branches that are kept alive with a mixture of insecticide and fertilizer. When a significant population of beetles is on the branches, the branches are tied to the lower third of infested hemlock trees.
That, the study says, may be one of the reasons why beetles do not survive from year to year. Their prey—the adelgids—seem to populate the upper third of the tree instead. Scientists have not studied how the beetles move to find their food, but they say that the larvae remain rather stationary, and many of them do not survive.
The beetles face a number of challenges depending on the location of the forest. Hemlocks die quickly after the initiation of an attack—often no more than three years—and as tree health fades, adelgid populations diminish. In areas with colder winters, adelgid populations often die back in winter, leaving little food for beetle predators. Finally, unlike adelgid populations, which seem to stay massive despite declining forest health, beetle populations have not been able to reach a mass significant enough to help them overwinter.
Other pest management strategies for hemlock woolly adelgid include imidacloprid injections, which are not always appropriate and are prohibitively expensive, and resistance breeding, research on which is being conducted by the Alliance for Saving Threatened Forests.
The research team observed two of the hemlock woolly adelgid’s three generations: the progrediens—wingless females that lay eggs and feed on the needles—and sistens—nymphs that develop from the eggs and eventually overwinter into adults. (see the Canadian Food Inspection Agency website for a more detailed description of the adelgid’s life cycle)
Results showed that adelgid ovisacs (egg sacs) were more abundant in the upper third of the tree crown than in the lower crown. Researchers found fewer progrediens and their eggs in the lower crown than in the upper and middle crown. Sistens populations were significantly greater in the upper crown than in the lower crown. Sistens densities were also greater in the upper crown when adelgid infestations were low or had just begun.
“Most predator releases have been made on the lower third of the hemlock tree crown,” quotes the article. “Although adult predators can fly, their ability to locate and colonize branches based on adelgid density is not well-understood. Because larval mobility is limited, predator larvae might have a greater chance of survival if placed on shoots having a patch of high adelgid density.”
When they compared the location of new growth to the area of the tree, the team found that new growth was not concentrated in the upper third. This finding made the location of adelgid densities more remarkable, since other studies have concluded that adelgids prefer the new growth over mature branches.
Concentrating predator releases in the sector where more adelgids are likely to live may increase the chance of survival and reproduction for the beetles, which co-author Kris Bramen says are like “diamonds over the canopy.”
The University of Georgia press release is available online at http://news.uga.edu/releases/article/uga-study-offers-hope-for-hemlock-attack/.
The Journal of Economic Entomology article is online at http://www.bioone.org/doi/full/10.1603/EC11022.
Filed under: Insects, Invasive species Tagged: | Adelges tsugae, hemlock biocontrol, hemlock woolly adelgid, Laricobius nigrinus, Sasajscymnus tsugae, University of Georgia hemlock study
