Date of Award

8-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Forestry and Natural Resources

Advisor

Hedden, Roy L

Committee Member

Culin , Joseph D

Committee Member

Adler , Peter H

Abstract

To better understand the tritrophic interaction involving Sasajiscymnus tsugae Sasaji and McClure, hemlock woolly adelgid (Adelges tsugae Annand (HWA)), and Tsuga (hemlock) tree hosts, an experiment was designed to determine the fitness of S. tsugae while feeding on HWA infesting either eastern hemlock (Tsuga canadensis (L.) (Carriere)) or Carolina hemlock (T. caroliniana (Engelmann)). The predator in this interaction is S. tsugae, a possible biological control agent for HWA. In the eastern United States, preliminary work on S. tsugae for potential as a biological control agent for HWA was started in 1995. It was determined that S. tsugae was a promising biological control agent for HWA infestations because both larvae and adult S. tsugae are prey specific on all developmental stages of HWA and typically avoid other insects on hemlock trees. S. tsugae longevity, reproductive capability and synchrony with its prey suggest it could be an effective HWA management tool. S. tsugae has two overlapping generations while preying on three generations of HWA in the northeastern United States.
My project aspired to answer questions about the optimal survival response of S. tsugae while feeding on one of two HWA-infested endemic Tsuga species in the eastern United States. Prior to this study, the vast majority of HWA research has been with HWA-infested T. canadensis and not T. caroliniana. Previously, little was known about HWA infestations on T. caroliniana. My study was designed to better understand how HWA-infested T. caroliniana differs from HWA-infested T. canadensis.
In 2005 and 2006 four lab experiments were conducted in Clemson, South Carolina, and two field experiments in Brevard, North Carolina. Overall, the results suggest that S. tsugae has a similar impact on controlling both HWA-infested T. canadensis and HWA-infested T. caroliniana. Again, S. tsugae seemed to have equal effectiveness on both endemic species of HWA-infested Tsuga.
In both 2005 and 2006 in an oviposistion study, no significant difference was found in S. tsugae fecundity rates while restricted to HWA-infested T. canadensis or HWA-infested T. caroliniana. This experiment consisted of counting S. tsugae eggs on each HWA-infested T. canadensis and HWA-infested T. caroliniana bouquet within each oviposition jar.
The rearing-container studies found a significant statistical difference between the two HWA-infested endemic species of Tsuga as a food source for maturing S. tsugae. The experiment consisted of placing S. tsugae eggs from the first experiment into rearing containers. Within the rearing containers, S. tsugae eggs matured to adults. The ratio of S. tsugae adults to eggs represented the survivorship of S. tsugae when feeding on a restricted diet of HWA-infested T. canadensis or HWA-infested T. caroliniana.
To further investigate the difference observed in 2005, I counted HWA adults on oviposition branches in 2006. When HWA population size was observed, the number of HWA on infested T. caroliniana was consistently lower. I believe this HWA population difference strongly influenced the survivorship of the S. tsugae while restricted to either HWA-infested T. canadensis or T. caroliniana. When in a rearing container with HWA-infested T. canadensis, maturing S. tsugae had a greater food supply. Conversely, S. tsugae restricted to a HWA-infested T. caroliniana rearing container, with fewer prey, experienced reduced survivorship.
In 2005 and 2006, field studies I found similar results to the oviposition study. No statistical significance was found between S.tsugae hatch rates on HWA-infested T. canadensis and T. caroliniana. This experiment took place on Tsuga spp. located on the property near Brevard, North Carolina. A sample plot was chosen on randomly selected hemlock branches. Each sample area was a placement for S. tsugae eggs. After S. tsugae eggs were placed in field sample areas, S. tsugae larvae were observed within sample areas. Both HWA-infested endemic Tsuga species appeared to be a similar source of nutrition for HWA, both in the lab and field.
However, since there were inherent uncontrolled sources of variation in these experiments, such as HWA densities on individual branches, a definitive test of the hypothesis of a difference in S. tsugae fitness or different HWA-infested host species was not possible. Regardless, the results of these experiments may be used to design a study to more precisely evaluate the effect of host-tree differences on HWA predation by S. tsugae.

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