Managing Slash to Minimize Colonization of Residual Trees by Ips and Other Bark Beetle Species Following Thinning in Southwestern Ponderosa Pine

Due to high fire hazard and perceived reductions in forest health, thinning of small diameter trees has become a prevalent management activity particularly in dense stands. Creation of large amounts of logging slash, however, has created large quantities of habitat for bark beetles primarily in the Ips genus (Coleoptera: Curculionidae, Scolytinae). Evidence indicates that prior to Euro-American settlement fire played a major role in maintaining ponderosa pine stands in a condition that was much more open in structure than today (Cooper 1960, Covington and Moore 1994, Kolb et al. 1994). In general, lower tree densities led to increased tree growth (Ronco and Edminster 1985) and trees that were more vigorous and presumably less susceptible to insect attack (Kolb et al. 1998, Fettig et al. 2007). Bark beetles are a large and diverse subfamily of insects commonly recognized as the most important biotic mortality agent in western coniferous forests. Most bark beetles feed in the cambium and phloem and some species directly kill the host. These insects influence forest ecosystem structure and function by regulating certain aspects of primary production, nutrient cycling, ecological succession and the size, distribution and abundance of forest trees (Fettig et al. 2007). Attacks reduce tree growth and hasten decline, mortality and subsequent replacement by other tree species. In the southwestern U.S., thinning is advocated by land managers as a means of reducing fuel loads, improving residual tree growth, and as a preventive measure for reducing subsequent amounts of bark beetle-caused tree mortality (DeGomez 2006a). The thinning prescriptions are quite diverse, and their application can result in significantly different stand structures. In most cases large amounts of downed material (i.e., slash) are created and left in the field, due to lack of developed markets for small diameter trees. This material, if left on the ground, has inherent value and ecological functions (e.g., nutrient cycling and wildlife habitat), while at the same time creates host material for many bark beetle species, specifically those in the genus Ips (hereafter referred to as ips). Forest managers and forest health specialists tend to agree that fresh slash left untreated on the forest floor increases risks from bark beetle infestations and eventually wildfire, but those who are managing for other forest attributes are prone to recommend leaving some of the slash untreated to serve as habitat for a variety of fauna that contribute to a healthy forest condition (Brown et al. 2003). This publication presents treatment options and guidelines for managing slash that minimize bark beetle activity in response to thinning treatments. We focus on southwestern ponderosa pine, but borrow knowledge gained from other geographic locations. In addition, while the primary focus is on ips response to thinning and slash management treatments, we also include information on other bark beetle species associated with slash (e.g., Dendroctonus species such as western pine beetle and red turpentine beetle).