Bark Beetles
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Roundheaded Pine Beetle
Dendroctonus adjunctus (Blandford)

Host:  Ponderosa pine

Figure 108. Figure 108. Egg gallery of roundheaded pine beetle. Note that galleries are vertical, but more snake-like in appearance than mountain pine beetle egg galleries.

Symptoms/Signs:  Reddish brown boring dust is present in bark crevices and numerous pitch tubes are observed on the bark at entrance holes. In heavily attacked decadent trees, pitch tubes are often missing or very small. Later, the foliage fades from green to yellow or bright red, and eventually a dull brown. Woodpecker activity on the trunk is also a good indication that bark beetles have attacked the tree.

Biology:  Adult roundheaded bark beetles attack and colonize susceptible trees in October and November. Females initiate the attack producing an aggregation pheromone drawing large numbers of beetles to the tree. The beetles chew through the bark and excavate a chamber in the moist tissue beneath the bark. Males enter the chamber and mating occurs. Female beetles then excavate horizontal tunnels in the inner bark region for 25 to 50 mm and then longitudinally with the grain an average distance of 300 mm. Eggs are laid individually in niches on alternate sides of the egg galleries, which are packed with frass. The larvae mine across the grain in the inner bark until in the third instar and then bore into the outer bark to complete development.

Figure 109. Figure 109. Pitch tubes caused by roundheaded pine beetle are typically larger and more cream colored than those formed by western pine beetle

Effects:  Roundheaded pine beetles attack, colonize, and kill trees in the southwestern United States and Mexico. Periodic outbreaks have killed large numbers of ponderosa pine in southern New Mexico and southeastern Arizona. In 1950, 16,000 pole and sawtimber-sized trees were infested on 2,500 acres near Cloudcroft and 400,000 pole-sized ponderosa pines were killed on 150,000 acres from Mayhill to Ruidoso in 1971. Similar outbreaks occurred during the 1990s in the Sacramento Mountains in southern New Mexico and the Pinaleño Mountains in southeast Arizona.

Similar Insects:  The roundheaded pine beetle is similar to other Dendroctonus beetles, Ips beetles, and other members of the subfamily Scolytidae. Dendroctonus beetles are distinguished from each other mainly by their unique egg galleries and, in the case of the Douglas-fir beetle and spruce beetles by the host species they attack. Dendroctonus beetles are distinguished from Ips beetles by their rounded posterior ends and galleries, which are packed with frass. In contrast, the posterior ends of Ips beetles are slightly concave with and have several to many small spines and the runways (egg galleries) are free of frass.

References:  8, 24, 61

Figure 110. Figure 110. Ponderosa pine mortality caused by roundheaded pine beetle in the Pinaleño Mountains, Arizona.
Figure 111. Figure 111. Aerial view of ponderosa pine mortality caused by roundheaded pine beetle in the Pinaleño Mountains, Arizona.