Bark Beetles and Willamette Valley Ponderosa Pine: Populations, Geographical Distribution and Management Recommendations

Locations of bark beetle trap sites in the Willamette Valley, 2008-2010By: Paul Oester and Dave Shaw, OSU Extension

Willamette Valley ponderosa pine (WVPP) grows on a wide variety of sites in the Willamette Valley and is well suited for planting on some of the tougher sites, such as droughty and wet sites.  Interest in growing and managing WVPP has been expanding over the past 20 years or so (Fletcher, et al 2003).  Although some knowledge exists and more is being developed regarding insect and disease pests of this species more is needed.  Thus, we thought this study by Flowers and Willhite, would be of value to landowners growing this species and others who might have a more general interest in insects and diseases of Oregon’s trees.

The following summarizes the results from a 3-year study (funded by the U.S. Forest Service) conducted by the Oregon Department of Forestry and U.S. Forest Service to examine the distribution and peak activity (or flight periods) of potentially damaging bark beetles to Willamette Valley ponderosa pine (WVPP). Localized outbreaks of California five-spined Ips bark beetles have thus far been the most significant pests observed, causing patch-kill in young stands and top-kill in older trees. 

From 2008-2010, a series baited “funnel” traps were used to survey for bark beetles and their allies at 11 WVPP sites along a North-to-South transect through the Willamette Valley from April to October (Figure 1).  Trapping efforts were focused on known “tree-killing” bark beetles such as the California five-spined Ips (Ips paraconfusus), as well as historically eastside beetles like the pine engraver (Ips pini), western pine beetle (Dendroctonus brevicomis), and mountain pine beetle (Dendroctonus ponderosae).  The lures contained compounds that attracted “allied” insects as well, that included other bark beetles, ambrosia beetles, woodborers, and their predators.  

Highlights of the study
Over 72,000 bark beetles were collected in all and the overwhelming majority was the California five-spined Ips; the only one found in abundance at all sites.  Pine engraver, western pine beetle, and mountain pine beetle were found in small numbers at a few sites.  Other bark, ambrosia, and woodborer beetles captured were all native to the Western US, with the exception of two long-established non-native ambrosia beetles, Anisandrus dispar and Xyleborinus saxeseni.  

At this time it appears that the California five-spined Ips represents the greatest insect risk to WVPP stands, due to its wide occurrence and demonstrated ability to rapidly increase its populations to levels that cause tree mortality.  

Although some other species can cause significant mortality in Southern and Eastern OR, they were rarely detected in the W. Valley and do not appear to be able to sustain high populations here.  We suspect that in many cases, these were accidentally moved to the Westside by weather events or within untreated wood products, such as firewood, from which they later emerged.  

Overall, there did not appear to be a correlation between the occurrence of California five-spined Ips and W. Valley geography (Figure 2). That is, it was consistently detected at all sites sampled, with a high degree of annual variation by location.  

California five spined ips average trap captures by site.

California five spined ips annual flight perios and 3 year average, 2008-2010

Trap captures indicated 3 peak flight periods (or 2 generations) in the W. Valley, consistent with anecdotal reports.  Peak activity periods were found to occur in early-May, mid-July, and mid-October, with the timing generally similar among years and relative location (Figure 3).  Previous studies indicate that the first flight of beetles usually begins following successive days of >60°F temperatures.  In this study, the first flight period occurred in May, but historic temperature data suggests that this can occur much earlier. This initial flight of beetles is usually fairly weak and breeds mostly in fresh slash from winter damage and very weakened trees.  Also, host trees are usually not drought stressed in spring.  This first flight group then produces the first generation of beetles that year, which under ideal conditions will emerge in 60 days (early to mid-July).  This pattern will continue for as long as temperatures are sufficient for continued development.  

In the W. Valley, it appears that only 2 of these generations occur, with peak emergence at 60-80 day intervals that corresponded to mid-July and mid-October.  It is these latter generations that actually cause tree mortality because they can attack water stressed trees and tops of trees in summer and fall after local population build up in slash.  The only major deviation in the trend, by location, was in the 3rd peak flight, which consistently occurred later in the North-Valley, and by year, in the 2nd peak flight of 2010, which also occurred later.  Each appears to be related to colder temperatures, which can slow down bark beetle development.  

Recommendations
Insect funnel trap used for collections in this studyStudy findings support current recommendations by ODF and the Willamette Valley Ponderosa Pine Conservation Association (WVPPCA) for preventing damage by the California five-spined Ips.

  • Of first priority is limiting the availability and amount of preferred breeding materials (>3” diameter), which can lead to population build-ups.  While a number of strategies can be used, generally any approach that involves cutting and scattering these through the stand (or destroying/removing them) will prevent Ips bark beetle outbreaks.
  • In terms of timing, restricting access of the beetles to preferred, down materials is most critical during the first flight period.  This can be done by early clean-up of storm/winter damage (Feb-June) if possible and/or delaying management activities (thinning, etc.) until later in the summer (July-Dec.).  While bark beetles are still active during this time, it is infestations that occur during the initial flight period that lead to later population build-ups and potential tree mortality.
  • Smaller materials (<3” diameter) from pruning or other operations can be infested, but have not been shown to lead to Ips population build-ups. However, it is recommended that these materials be scattered as well, since stacking/piling can concentrate beetles in one area of a stand.
  • Given the good growing conditions in the W. Valley, maintaining appropriate stand density will also increase resistance to bark beetle attacks and other stressors.
  • Finally, as bark beetles can fly >1 mile, good management practices on the part of neighboring ownerships is critical, and as such we hope to provide these findings to a wider audience in cooperation with the WVPPCA and OSU Extension service.  

For additional information or a copy of the complete study report, contact: Rob Flowers at 503-945-7396 or rflowers@odf.state.or.us

Revised and adapted based on an article written by Rob Flowers, Entomologist, Oregon Department of Forestry and Beth Willhite, Entomologist, USDA Forest Service

References
Fletcher, R. editor.  2003.  Establishing and managing ponderosa pine in the Willamette Valley.  OSU Extension Service publication, EM 8805.  41 p.