Douglas-firs usually do not tolerate extreme drought conditions well, and the effects of the exceptionally low moisture are often evident following drought years. Crowns are thin, tops and branches have died outright because the roots they were associated with were not able to obtain water, and large trees may have died. Insects and canker fungi have colonized weakened portions of trees, girdling branches and stems.
The most common insects in the drought stressed or dead Douglas-fir include:
- Douglas-fir twig weevils (Cylindrocopturnus furnissi) and other twig beetles that cause death of twigs and small branches.
- Douglas-fir engraver beetles (Scolytus unispinosus) that kill patches of cambium on the stem; this may cause branch or top kill.
- Roundheaded wood borers that infest the branch collar area and main bole, typically of trees that are dead or dying. The larvae of roundheaded and other wood boring beetles are often seen tunneling in logs or firewood. With the exception of the flatheaded fir borer, wood boring beetles do not represent a significant threat to standing trees.
- Flatheaded wood borers (Buprestids), particularly the flatheaded fir borer (Melanophila drummondi) that infests the entire tree, the top, or individual branches and has caused a substantial part of the large tree mortality observed recently. Flatheaded fir borer infestations often involve groups of trees in pockets that may expand from year to year when conditions are right for the insect. Trees infested by flatheaded fir borers often exhibit extensive evidence of woodpecker feeding. Adults fly from May to September. Stocking control and other vigor-enhancing treatments in host stands on high-risk sites may be worthwhile for reducing impacts by the flatheaded fir borer.
- Douglas-fir beetles (Dendroctonus pseudotsugae) that can kill large diameter (8" diameter or greater) trees. Douglas-fir beetles have an earlier fight schedule; the adults emerge and find new host material as early as April at lower elevations or after mild winters. Douglas-fir beetles prefer weakened or recently downed trees. Their impacts can be reduced by maintaining tree vigor and by reducing levels of Douglas-fir blowdown to fewer than four, ten inch or greater diameter Douglas-fir trees per acre. The Douglas-fir beetle is most often encountered following extensive blowdown of mature trees, and is seldom an issue at lower (<3,000’) elevations in interior SW Oregon.
- Several canker-causing fungi, including Diaporthe lokoyae and Phomo spp., can infect and kill stems and branches of drought-stressed Douglas-firs. These fungi are weak pathogens that cause few if any impacts on vigorous trees. Canker disease occurrence usually increases after drier than normal weather and subsides during periods of normal or above normal rainfall.