Pine bark beetles cause significant economic damage through timber loss, soil destabilization, and forest fire suppression costs, making them the most destructive pests of coniferous forests in North America [Drooz, 1985; Rozell, 1997]. Among them, the mountain pine beetle, Dendroctonus ponderosae, is particularly destructive [Amman, 1989]. It is distributed throughout western North America, where it preferentially infests lodgepole and ponderosa pines, but can attack any pine, with the exception of Jeffrey pine, particularly during outbreak conditions [Wood, 1982].
An historically unprecedented outbreak in Alaska and British Columbia has devastated millions of acres of mature forests, with 80% of pines in the Canadian Rocky Mountains predicted to be lost within a decade [Jay, 2007]. Locally, nearly a third of the coniferous forest in the Lake Tahoe basin was killed by a bark beetle outbreak nearly two decades ago, a condition that contributed to recent wildfire problems in the area. Mountain pine beetle outbreaks are expected to worsen as global climate change favors outbreak conditions, and there is alarming news that the beetle is expanding its host range to include new trees [Brown, 2004].
Like all bark beetles, mountain pine beetles are physically protected from sprayed pesticides because they spend the majority of their lives beneath the bark of the very trees they destroy, emerging only for a short, pheromone-mediated flight to a new host tree. Current control measures are thus restricted to silvicultural practices and small-scale trapping, with limited effectiveness [Bordon, 1997; Goyer et al. ,1998].
A long term goal of my research is to develop new ideas to deal with bark beetle infestations, at least on a local geographic scale, by exploiting unique biochemical characteristics as potential targets. For example, populations may be managed if reproductively-essential semiochemical biosynthesis is inhibited. This may be achieved by delivering an interfering RNA to feeding beetles to disrupt semiochemical production. Feeding-mediated RNAi delivery has been demonstrated in other herbivorous beetles [e.g. Baum, 2007] and should be applicable to bark beetles.
Before these measures can be developed, we need to better understand the metabolic steps involved in semiochemical production. Two potentially important, chemically related compounds are trans-verbenol and p-cymene [Pureswaran et al., 2000].
trans-Verbenol is a bicyclic monoterpene alcohol. It is produced by females shortly after they begin feeding on a new host and acts as an aggregation pheromone to attract males (and more females) for mating. It almost certainly arises from specific hydroxylation of host-produced a-pinene [Greis et al 1990].
p-Cymene is a monocyclic monoterpene whose biological role is not clear. Indeed, because p-cymene levels in adult male and female tissues are initially high, and then fall rapidly when adults feed, it is not considered a pheromone component. However, it is thought to be a degradation product of trans-verbenol [Pureswaran et al., 2000], and may also be a metabolite of host tree terpenine [Foti and Ingold, 2003]. Biologically, p-cymene inhibits beetle-vectored blue stain fungi [Hofstetter et al., 2005]. Preliminary experiments in my lab show p-cymene titres in insect tissues gradually increase as early instar larvae progress to pupae, reach a peak in unfed adults and then fall rapidly upon feeding. Thus, this chemical may protect the beetle from blue stain fungus, act as a sink for trans-verbenol, and/or be a metabolic intermediate of resin detoxification. Curiously, p-cymene is toxic to insects and vertebrates [Fukumoto et al., 2006; Cheng et al., 2008 ], so its accumulation in D. ponderosae is interesting.