Novel Approaches for Artificial Infestations and Early Resistance Screening of Fraser fir against the Balsam Woolly Adelgid

The balsam woolly adelgid (BWA) is a tiny, piercing-sucking insect that was introduced into North America around 1900 and into the Southern Appalachians in the 1950s. It is a major pest in natural Fraser fir stands and Christmas tree plantations. It has caused extensive mortality in native stands and is expensive to the Christmas tree industry. The development of BWA-resistant Fraser fir trees would be a relatively inexpensive solution to a difficult pest problem. The work described here was conducted to assist in this endeavor.;Studies were conducted at constant and ramping temperatures to determine whether young seedlings (1-, 2-, 3-, and 6-year old) could be artificially infested with BWA and whether the infestations could be maintained over a period of time. The studies resulted in at least some of the trees having developed egg-laying adults by the end of the study period. The infestations were sustained over time and supported multiple generations of BWA.;Historically, the technique often utilized for artificial infestations involved cutting pieces of bark from infested trees and attaching the bark to uninfested trees. Because this is a very time consuming technique, studies were developed to test this method with a novel technique mimicking natural dispersal. The new technique involves suspending infested logs over uninfested seedlings and allowing the crawlers to drop onto the material below. The technique was tested on 2- and 7-year old Fraser fir trees. Although there were no differences in infestation levels between the old and new techniques, it took substantially less time to cut the logs and hang them over the trees. An additional study looking at the density distribution of crawlers dropping from the logs revealed that there can be over 50 crawlers per cm2 falling from the log and that there is a relative level of drift in crawler distribution. Suspending logs (10 cm diameter) approximately 30 cm apart may provide ample crawlers to infest trees.;To determine whether seedlings of different fir species showed the same level of BWA susceptibility as mature trees, a study was conducted in which seedlings of 12 fir species of equal age and grown under the same conditions were infested (using the suspended log technique described above) with BWA collected at one time from a single source. Species were categorized into four a priori susceptibility groups (susceptible, tolerant, resistant, unknown). With few exceptions, results for each species fell within expectations for a priori resistance classifications. European silver fir and Veitch fir appear to be somewhat more susceptible in the seedling class than mature trees, however, they generally ranked as less susceptible than Fraser fir and other highly susceptible species. Turkish fir appears to be somewhat resistant to BWA and Trojan fir appears highly susceptible.;A separate infestation study was conducted to compare a (balsam x Veitch) x balsam backcross with other species. The backcross (3-year old seedlings) appeared to be more resistant to BWA infestation than the balsam fir of equal age and grown under the same conditions.;A clonal Fraser fir seed orchard in Avery County was found naturally infested with BWA. Each tree was assessed for infestation level and other variables to determine if there were clonal differences in susceptibility. Over 96% of the trees were infested with BWA and, although there were significant difference noted among clones, every clone was infested at some level. There were highly significant clone effects for infestation level, bark appearance, bark thickness, lichen coverage, apical dominance, crown health, and tree diameter. Heritability calculations were low and it is believed that the practice of spraying the orchard with insecticides each year may be masking any genetic effects associated with BWA resistance.;Although most mature Fraser fir trees in native stands were killed by the balsam woolly adelgid during the first wave of mortality, many trees on mountain peaks in the Great Smoky Mountains National Park survived and are actively growing. Ten sites were visited and data collected on the survivor trees and younger regenerated trees for comparison, to investigate the potential resistance of the survivor trees to BWA by evaluating bark characteristics and to characterize site factors that may aid in the understanding of their survival. Survivor trees exhibited thicker and rougher bark than regeneration trees, and the depth of the outer bark was also greater in the survivor trees. The ratio of bark thickness to diameter at breast height was constant, and the ratio of impermeable tissue to bark thickness was constant, indicating that regeneration trees were developing impermeable tissue at a similar rate as the older survivor trees. This could be linked to genetic similarities between the regeneration and survivor trees.