MCH

Female Douglas-fir beetles initiate an attack in the spring when ambient air temperature exceeds approximately 18ºC (65ºF). They are about 4-7 mm (1/6 – 1/3″) long when they bore into the outer bark and through into the inner bark on the host tree. Once she creates a galley for her eggs, the female beetles release a powerful attractive pheromone that attracts both males and females to the tree. Once mated, female beetles produce less of the attractive pheromone and male Douglas-fir beetles start to produce an anti-aggregation pheromone called MCH. The growing plume of MCH from the males signals to other beetles around the area that the tree is full. The change in semiochemical signal from being attractive to repulsive is the natural mechanism beetles use to avoid overcrowding under the bark. In this way, MCH acts like a “no-vacancy” sign and the mass attack ceases.

Eggs hatch in 2 weeks and the larvae bore away horizontally from the egg gallery. They feed in the inner bark for 2-3 months until winter and go through the pupal stage and become young adults. The adults do not emerge until the following spring. Generally the duration of the life cycle is one year. If conditions are favorable, some parent beetles emerge the same summer for a second flight and will attack again to establish a second brood. Prolonged temperatures of -40°C / °F can kill over-wintering adults.

Part of the destructiveness of Douglas fir beetles is the transmission of blue-stain fungi from the beetle to the tree. The spores of these fungi are brought into the tree by adult beetles during colonization. Fungi grow within the tree while the beetle larvae feed, which weakens and eventually kills the tree. This mutual network of beetle galleries and blue stain fungi disrupts the movement of food and water between the roots and needles and rapidly kills the tree. The fungi give a blue-grey appearance to the sapwood.