Mountain pine beetle

Distribution

Saskatchewan, Alberta, British Columbia

Present in most lodgepole pine forests of British Columbia, adjacent, central and northern Alberta, and in isolated locations in southwestern Saskatchewan. The geographic range has been expanding northward and eastward for the past decade.

Damage, symptoms and biology

Biology

The mountain pine beetle has a life cycle that normally lasts one year. In late summer, the adults emerge from the trees in which they fed and developed and fly off in search of new hosts, into which the females bore waiting for males to come to them. The females bore vertical galleries just under the bark, in which they lay their eggs. The larvae that emerge from the eggs spend the winter feeding under the bark. Adult emergence takes place between July and September.

A key stage in the life cycle occurs when the beetle transmits a blue stain fungus to the tree. Attacking beetles carry the spores of the fungus, which gain entry to the tree and eventually overcome its defence system and its ability to withstand beetle attack.

• Adults are cylindrical, 3.7 to 7.5 mm long; teneral adults are light creamy tan in colour, changing to black when mature.
• Eggs are pearly white, about 1 mm in size, and are laid singly in niches on both sides of the parent gallery.
• Larvae are white legless grubs with red-brown heads, about 5 mm long in the fourth (final) instar.
• Pupae are white at first, changing to light brown, about 5 mm long, with the external characteristics of the adult beetle visible.

The mountain pine beetle and associated blue stain fungi (Ascomycetes) act together to kill trees. Adults transport spores of the blue stain fungi to new trees within a specialized sac (mycangium) on the maxillary cardine. These fungi are believed to stop water transport in the stem and thus kill infected trees.

Although the mountain pine beetle has many natural enemies including insect predators, parasitoids, and woodpeckers, these do not have sufficient impact on incipient and outbreak populations to exert effective control.

Damage

Trees undergoing attack by the mountain pine beetle are first detectable only from the ground, and identification of symptoms requires close examination of trees. Infested trees can be detected through crown and external symptoms. The first signs are boring dust and resin on the bark associated with the attacking adults, but the mountain pine beetle can only be positively identified (and the success of an attack can only be positively determined) by looking under the bark.

At low (endemic) populations the mountain pine beetle survives in weakened or stressed trees. As populations increase or more trees become stressed because of drought or other causes, the population may quickly increase and spread. Healthy trees are then attacked and huge areas of mature pine stands may be threatened or killed. Warm summers and mild winters play a role in both insect survival and the continuation and intensification of an outbreak. Adverse weather conditions (winter low of -40°C or high winds during dispersal period) can reduce the beetle populations and slow the spread, but insect populations may recover (not the individual insects) and resume their attack on otherwise healthy forests.

Aerial detection of successfully attacked trees is possible as early as late spring (more typically mid-summer) in the year following attack. Aerial surveys are useful to detect stands where mountain pine beetle activity is probable; however current monitoring approaches are not optimized for detecting new populations in an expanding range, where founder populations may be initiated from immigration from afar.  Detection of small groups of red-topped trees should be followed with ground inspection to verify cause. If mountain pine beetle is confirmed, ground surveys may be implemented to look for newly attacked trees in the vicinity.  These ground surveys may need to be extensive if they are intended to detect founder populations arising from immigration from afar.

The current outbreak in BC is starting to wane. MPB populations are likely to continue to spread eastward through jack pine and are unlikely to be stopped by an occasional cold winter.  Over the last decade the insect's range in northern Alberta has expanded annually, despite several cold winters. 

Symptoms

Accumulations of pitch or sawdust are conspicuous around entrance holes bored into the bark of trees by adult beetles from mid-July to early September. Sawdust is quickly blown or washed away, but abundant pitch tubes may remain for more than a year after attack. Pitch tubes may be much less evident on trees under severe drought stress prior to attack. During the fall and winter after attack, woodpeckers feed on bark- and wood-boring insects on infested trees. Trunks of trees foraged on by woodpeckers are easily visible as much bark is stripped off and bark fragments accumulate in piles on the ground at the base of trees. Removal of bark from infested trees reveals adult egg galleries, larval feeding galleries, and one or more life stages (eggs, larvae, pupae, adults), depending on the time since attack. Egg galleries are 10–41 cm (average, 28 cm) long, oriented vertically on the stem, and have a short curved or diagonal section at the bottom. Grayish blue staining of sapwood, caused by colonization of ray parenchyma cells by blue stain fungi transmitted by adult beetles, provides a conspicuous symptom shortly after successful attack. Various fungal fruiting structures (such as synnemata and perithecia) and mycelia of blue stain fungi and other fungi are often evident in beetle galleries and pupal chambers.

Tree foliage begins to dry out as soon as the conduction of water up the tree is interrupted. As a result, the colour of the foliage on infested trees gradually changes from bright to dull green. This early symptom in the lower crown will often become visible 2-3 months after attack. However, more distinct colour changes occur during the onset of the growing season the spring following attack when tree foliage turns brick red. The needles of infested trees first turn a faint yellow and then a reddish brown by late summer, which allows easy detection; however, by the time trees prominently display these symptoms, they are typically vacated by the mountain pine beetle, which has moved on to attack other trees. With time, retained foliage colour becomes more dull, and most of the foliage drops in 2-3 years; this will vary from species to species and with weather conditions. These rapid and distinct colour changes are used to schedule aerial mapping of recently attacked trees.