In the last 10 years, an estimated 47 million ha of forest have been affected by bark beetle driven tree mortality in the western U.S. and Canada. It has long been thought that a mutualism between bark beetles and their closely associated fungi is the key reason these insects are so successful. Both beetles and their fungi require tree death for success; however, despite considerable study, controversy remains about the importance of each individual agent in causing tree mortality and whether the association is a true mutualism. Clearly fungi benefit from the association by gaining transport to a new host but the benefit to the beetle is less clear. The fungi are variously thought to benefit the beetles by assisting in depleting tree defense, disrupting water conduction in the tree, and/or providing nutrient supplementation to the beetles themselves. Due to the almost ubiquitous presence of fungi with the beetles it has been virtually impossible to truly separate the action of the two agents. This project will investigate the impact of fungicide on tree physiology and the potential effectiveness of fungicide at reducing the growth of bark beetle-associated fungi at the whole tree level. In addition, this project will examine the impact of fungicide on bark beetle activity/success. If fungicide proves to be effective at stopping or slowing fungal growth, with minimal impacts on tree function and beetle activity, bark beetle researchers will be able to investigate the respective role of each agent in tree mortality and ultimately the impact of fungi on beetle success. This will have broad implications for informing our understanding of the nature of bark beetle/fungus relationships. This project also has potential impacts for mitigating future tree mortality from bark beetles.
The ability of several bark beetle species to attack and kill trees is overwhelmingly evident in North America where several species have or are developing outbreaks of exceptional size; however, despite many decades of research, the processes involved in tree death after beetle attack are still poorly understood. To effectively resolve this issue, scientists need to understand the respective role of bark beetles and their symbiotic fungi in the processes that lead to tree mortality. More specifically, are bark beetles dependent on the fungi to kill trees? A better understanding of the nature of the symbiotic relationship between beetles and their associated fungi will advance understanding of other insect/fungal relationships. Our study was a pilot study, essentially a proof of concept study, which was designed to assess the potential of a systemic fungicide in stopping, or significantly slowing, the growth of the bark beetle associated fungi in the tree tissue. Due to the close association of bark beetles and their fungi; i.e. the fungus is carried within specialized compartments and externally on the body of the beetle, previous experiments to isolate the impact of the beetle from that of the fungus have been notoriously problematic. Proving the effectiveness of the fungicide would allow us to initiate a full-factorial experiment that would separate the beetle activity from the activity of the fungus. By examining tree physiological processes that are disrupted during and after bark beetle attack, with and without fungus, we would be able to determine which agent plays the primary role in tree death and answer the larger issue surrounding the nature of the relationship between beetles and their fungi. Or study was a three part study. The first part investigated the efficacy of the fungicide in stopping fungal growth. Study number two examined the effect of the fungicide on tree physiology and study three examined the effect of the fungicide on the beetles themselves. These additional studies were included to ensure that if the fungicide were used in the larger study the differences in tree physiology and beetle performance among the treatments could be attributed to the presence or absence of the fungus and were not an artifact of the fungicide itself. To examine the efficacy of the fungicide (study 1) we used both artificial inoculations of bark beetle associated fungi and beetle attacks (via baiting of the trees with a pheromone lure). Our results found no difference in fungal penetration into the phloem or xylem among the treatments (two levels of fungicide and the control); however, it should be noted that fungal growth in all treatments was minimal suggesting that the tree defenses alone were able to stop the growth of the fungi. In addition, one full year after inoculations no tree mortality was noted. Thus, due to the natural defenses of the tree, it is difficult for us to assess the contribution of the fungicide in reducing fungal growth. For study number two, the impact of the fungicide on tree physiology, analysis to date has found no impact of the fungicide on tree water relations or photosynthesis rates. There does appear to be a slight decline in resin exudation (the primary tree defense after beetle attacks) in those trees injected with fungicide. For study three, the impact of the fungicide on beetle performance, we found a significant effect of the fungicide on beetle size, with beetles emerging from the trees treated with fungicide being larger than those emerging from the control tree. Sample size was small for study number three and we suggest that further research is necessary to confirm the effect of the fungicide on beetle size. This study was used to enhance the classroom curriculum at two universities and in two disciplines, forest entomology and tree physiology. The project employed one Latino undergraduate student from the REU program. In addition, the research was used as the basis for a research proposal which generated funding from McIntire-Stennis for a new PhD student, an additional undergraduate student, and a second field season for the REU student. We were able to provide training on the use of the sapflux system, photosynthesis machine, and techniques for fungal inoculation to the newly recruited PhD student during his first field season. In addition, the research funded a female early career scientist. Science understanding was be enhanced by providing results to participants in the Master Gardner Class offered via the University of Arizona extension agency and a manuscript is in preparation to be published in peer reviewed journal in 2014.
