Effect on Trees

Indications of Infestation

Many species of pine trees have been affected and many more will be affected by the mountain pine beetle.  Countless field observations have determined that pine trees exhibit certain signs that indicate a mountain pine beetle infestation.

Successful pine beetle inhibition
(Courtesy of the City of Grand Prairie)

Previously mentioned, one sign of infestation is pitch, a resin found at the entrance of burrowing tunnels.  Pitch is an evolved, natural defense mechanism of the pine trees to prevent the burrowing of beetles.  Pitch clogs up the tunnels and inhibits beetle movement (Colorado State University).

Presence of beetles and larvae are another tell-tale sign of an infestation.  The beetles are found beneath the bark along with the tunnels they bore.  The tunnels form a "J" shape that averages 55 to 140 cm in length. (Dept. of Natural Resources, Montana; Furniss and Carolin 1977). 

Host tree color can also change as a result of beetle affliction.  Many trees may maintain green color, but some after eight to ten months of infestation, lose their green color and the needles change to a reddish-orange or yellow color, indicating lack of phloem movement and the eventual death of the tree.  Trees that have been dead for a long time appear grey and lifeless (Colorado State University; Dept. of Natural Resources, Montana).

Blue-stain fungi stain
(Courtesy of the USDA)

If infected trees have been felled, another sign of pestilence may be observed.  A cross-section of an infected, felled tree may exhibit a blue-green stain on the trunk.  The stain is the marker of the aptly named, blue stain fungi (Grosmannia clavigera).  The fungi finds its way onto the mountain pine beetles as they emerge from their trees.  When the beetles land on their new tree, the fungi is transmitted and the fungi produces slimy spores infect the tree.  As the fungus grows through the tree, it produces a blue stain on in the tree which gives the fungus its name.  The fungi kills the tree by blocking nutrient as well as water flow in the tree (Tsui et al. 2009, Powell and Raffa 2011).