From Attack to Emergence: Interactions between Southern Pine Beetle, Mites, Microbes, and TreesThis article is part of a larger document. View the larger document here.
Bark beetles are among the most ecologically and economically influential organisms in forest ecosystems worldwide. These important organisms are consistently associated in complex symbioses with fungi. Despite this, little is known of the net impacts of the fungi on their vectors, and mites are often completely overlooked. In this chapter, we will describe interactions involving the southern pine beetle (SPB), among the most economically damaging of North American forest insects. We examine SPB interactions with mites, fungi, and other microbes, following the natural temporal progression from beetle attack to offspring emergence from trees. Associations with fungi are universal within bark beetles. Many beetle species possess specialized structures, termed mycangia, for the transport of fungi. The SPB consistently carries three main fungi and numerous mites into the trees it attacks. One fungus, Ophiostoma minus, is carried phoretically on the SPB exoskeleton and by phoretic mites. The mycangium of each female SPB may contain a pure culture of either Ceratocystiopsis ranaculosus or Entomocorticium sp. A. The mycangial fungi are, by definition, transferred in a specific fashion. The SPB possesses two types of gland cells associated with the mycangium. The role of these cells and their products remains unknown. Preliminary studies have observed yeast-like fungal spores in the mycangium and several surrounding tubes that presumably carry secreted chemicals from gland cells (or bacteria) to the mycangium. The degree to which there is selective activity of the glandular chemical secretions remains to be seen. While O. minus may play some role in tree killing, none of these three fungi are highly virulent in their pine hosts. All three fungi grow within the phloem, sporulating heavily in beetle tunnels within which the SPB larvae graze. Though their ecological roles are complex and context-dependent, these three fungi can be divided into an antagonist (O. minus) and two mutualists (both mycangial fungi, though Entomocorticium sp. A appears to be of greater benefit to the beetles than C. ranaculosus). Naturally, all three of the fungi compete for access to uncolonized pine phloem. The results of these competitions can have significant impacts on their beetle and mite hosts, and ultimately on the population dynamics of this destructive pest.