Interactions Between the Invasive Balsam Woolly Adelgid ( Adelges piceae) and a Native Host, Fraser Fir (Abies fraseri )

Adelges piceae, balsam woolly adelgid (BWA), is an invasive insect in the United States where it kills natural populations of Fraser fir (Abies fraseri) and causes economic damage in Christmas tree plantations. Fir tree defensive reactions to BWA infestations vary between species and include a loss of apical dominance, gouting, rotholz production, and eventually tree death. Fraser fir is highly susceptible to BWA infestations; however, putative resistance to BWA has been suggested for natural populations. Tolerance to BWA exists in Turkish fir (A. bornmuelleriana (Mattf.)), a European fir where BWA are native. In the following studies we examine both Fraser and Turkish fir in a variety of studies.;It has been suggested that the reactions observed in susceptible Abies species, i.e. swelling, gouting, rotholz production, are caused by BWA digestive enzymes introduced into the tree during feeding. Whole body homogenates of balsam woolly adelgid (Adelges piceae) adults, crawlers, and eggs were tested for various types of enzyme activity. All samples showed amylase, protease, peroxidase, and polyphenol oxidase activity. Occurrence of these enzymes in crawlers as well as adults indicate they are not a byproduct of feeding but are produced in the insect and most likely introduced into the tree during feeding. Whole body homogenates of BWA have also been shown to possess pectinase, an enzyme important in the digestion of the middle lamella of plant cell walls. If BWA introduce amylase, pectinase, protease, peroxidase, and polyphenol oxidase into xylem ray parenchyma when feeding, it would explain, in part, the local swelling of cells, disruption of phenylalanine ammonia lyase synthesis, and the abnormal formation of heartwood, termed "rotholz", observed in susceptible Fraser fir hosts. Here we present the possible mechanism for rotholz production in Fraser fir by the action of digestive enzymes present in BWA.;In relation to physical changes, chemical changes and the production of defense compounds were studied in healthy and infested Fraser fir. Comparisons between bark, mature wood and juvenile wood samples from healthy and BWA infested trees were analyzed using gas chromatography with a flame ionization mass spectrophotometer (GCMS) focusing on a known plant compound, juvabione, along with other defense chemicals in the bark. As expected, juvabione concentrations were higher in infested trees compared to uninfested trees, and it was more concentrated in juvenile wood compared to mature wood. Several important insect and fungal defense compounds were differentially regulated in bark samples. Samples were tested in a bioassay using mealworms and compared to pure juvabione and another known insect growth regulator, Methoprene. The IC50 for pure juvabione was lower than that calculated for extracts from juvenile and mature wood, indicating a possible antagonistic effect from unknown compounds.;In addition to chemical defenses, energy relations were studied by examining carbohydrate and phenolics changes in Fraser and Turkish fir 5 year old trees before and after infestation with BWA. Plant health was assessed by examining total soluble carbohydrates, lipids, and phenolics in conjunction with anatomical characterization of xylem ray parenchyma cells in Fraser and Turkish fir. Lipids were significantly different between species and infestation level with higher lipid concentrations in Fraser fir and infested samples. There were positive correlations between TSC and TSP for both species, but only infested Turkish fir maintained this relationship, while infested Fraser fir did not. More XRP cells were present in Turkish fir and positive correlations between XRP cells and lateral pits between cells existed in Fraser fir. Other correlations between anatomical characteristics for each species are discussed.;Finally, methoprene was tested as a possible alternative to current chemical BWA control methods. Chemical control is currently the only form of management for this invasive insect pest. Broad-spectrum insecticides not only reduce natural predator populations, but eliminate competition for other pests which subsequently also require chemical control. Methoprene has a lower toxicity, and negligible impacts on predatory/beneficial insects, and the environment. This study evaluates methoprene as a control treatment for BWA. Methoprene was shown to be a successful growth regulator and its potential use in combination with augmentative biological control is discussed.;Our overall goal is to reduce or eliminate chemical pesticide inputs on Fraser fir Christmas tree farms. This can be accomplished through breeding resistant Fraser fir, use of biological control, and the use of alternative biopesticides. This study focused on determining the mechanisms by which BWA cause deleterious effects in Fraser fir possibly by the introduction of digestive enzymes. Fraser fir chemistry and comparative physiology are discussed as potential traits for selecting BWA resistant fir. The link between digestive enzymes and reactions observed in Fraser fir are particularly useful in determining mechanisms for potential areas for future resistance research.