[Bark Beetle Management Guidebook Table of Contents]

Introduction

The group of insects known as bark beetles are members of the family Scolytidae in the order Coleoptera. Most bark beetles cause little or no economic damage as they normally infest branches, stumps, and stems of standing dead, severely weakened trees or downed material. A relatively few species will attack and kill living, apparently healthy trees. Bark beetles are integral components in forest ecosystems and therefore can be viewed as beneficial or detrimental depending on the management objectives.

The major bark beetle species that have histories of causing significant damage to forests in British Columbia are listed in Table 1.

Table 1. Major damaging bark beetles of British Columbia

These bark beetle species periodically reach outbreak levels in British Columbia. During outbreaks, they kill large numbers of apparently healthy trees over extensive areas in many parts of the province. All of these beetle species attack and kill large diameter, mature and overmature trees. Outbreaks of bark beetles may be threats to sustainable harvest levels; in the short term, infestations can significantly disrupt five-year management plans.

A variety of approaches are available to reduce the amount of mortality caused by bark beetle outbreaks. Detection methods, treatment selection and application, and other management activities vary for individual beetle species. This guidebook is designed to provide a background to bark beetle management and specific practices for managing mountain pine beetle, spruce beetle, and Douglas-fir beetle. While the western balsam bark beetle causes substantial mortality in many areas of the province, few proven management tools exist. Therefore, this guidebook does not provide information on the western balsam bark beetle.

Where possible, biological features and activities that are common between the three beetle species are included in the first sections of this guideline. Those features and practices which differ are described under specific beetle species sections. Beetle biology has been summarized; complex beetle life cycles and population dynamics have not been included. Management practices are discussed in a “how to” manner; extensive background information or the rationales for certain items have not been included in this guidebook. Key references for more information on specific beetle species have been listed in the last section of this guidebook.

General description of bark beetles

All three species of bark beetles discussed in this guidebook are similar in appearance through all their life stages. Main differences between the species are in size, tree species attacked, and timing of parts of the life cycle. Life stages for bark beetles are described below:

Egg: pearly white, oblong, 0.75–1.0 mm long. Eggs are laid in individual niches or elongate grooves on alternate sides of the egg gallery.

Larva: creamy white, cylindrical, stout, wrinkled, legless grub, 6–7 mm long at maturity (fourth instar). Head is pale tan to pale rusty.

Pupa: creamy white, approximately 6 mm long, becoming pale tan near maturity. Pupae have legs and maturing wings that are visible.

Adult: immature, or callow, adults after transforming from pupae are initially white and then medium brown. Mature adults are mostly all black or have rusty coloured wing covers. Adults are approximately 6 mm long.

Distribution and host range

In general, the distribution of each species of beetle follows the distribution of its primary host. Figures 1 to 3 indicate the distribution of each species of bark beetle in the province.

Figure 1. Distribution map of mountain pine beetle in British Columbia.

Figure 2. Distribution map of spruce beetle in British Columbia.

Figure 3. Distribution map of Douglas-fir beetle in British Columbia.

Life cycles and dynamics

The life cycles of the beetles are similar with some variation in timing and effect. Also, while the mode of action in killing individual trees is the same for all three species, affected trees vary in their display of symptoms. It is necessary to know the differences when identifying the causal agent, establishing where the infestation is in its cycle, and for selecting the most appropriate treatment prescription.

Life cycles

 All three beetle species have similar life cycles, progressing from egg, through four larval instars, to pupa, and finally to adult. The life stages for each insect closely resemble each other; however, differences in timing and duration affect the selection and application of management tools. The timing of various components of the life cycles are summarized in Table 2. The timing of life cycle events for any species of bark beetle will vary from year to year and from location to location due to variations in climate and local weather.

Table 2. Timing of life cycle events

A brief synopsis of the life history of all three beetle species is as follows:

  1. Adult females emerge when ambient air temperature exceeds approximately 16* C, find new suitable host material, and emit aggregating pheromones to initiate mass attack.

  2. Males join the females and each pair construct an egg gallery under the bark, parallel to the grain.

  3. The sapwood is inoculated with spores of a blue stain fungus as the egg gallery is built.

  4. Eggs are laid and hatch into larvae which feed on the phloem in feeding channels constructed at right angles to the egg gallery.

  5. After four instars, larvae pupate and develop into adults under the bark.

  6. Young adults pick up blue stain fungal spores while in the pupal chamber.

The action of the larval feeding in the phloem and fungal colonization of the sapwood completely blocks all translocation tissues and kills the infested tree. In some cases only one side of a tree will be successfully attacked (strip attack); this tree will survive unless living portions are reattacked in subsequent years.

Symptoms of successful attack

Successfully attacked trees display symptoms that are utilized in ground and aerial surveys. These symptoms are summarized in Table 3 and 4 below. The information presented in the tables are generalized and will vary from year to year depending on weather patterns, and will vary in different areas of the province due to local conditions. Refer to management descriptions for specific bark beetles (later in this guidebook) for further information on survey procedures and interpretations.

Other bark beetle species, such as Ips spp., may be found under the bark of trees. Identification of these usually lesser important beetles is done by examining egg gallery patterns and adult morphology which differ significantly from the major bark beetles. Adults and larvae of wood borers may also be found under the bark or on the boles of trees. Proper identification of the insect is necessary to ensure that the appropriate management strategy is selected.

Table 3. Bole symptoms of successful bark beetle attack

Table 4. Foliage symptoms of successful bark beetle attack

Population prediction

Two types of surveys can be conducted annually in areas of infestation in order to determine beetle population trends. The trend of an infestation in a specific stand may be measured in a number of ways, primarily through ground surveys. The trend can be estimated by calculating the ratio of currently attacked trees to one-year-old attacked trees. A ratio substantially >1 indicates an increasing population; substantially <1 indicates a declining population. This estimate, however, should be viewed in light of bark sampling in the early spring after attack when the potential new brood can be compared to the initial attacking population.

Overwintering brood assessments surveys (r value) are conducted in the spring. They are used to estimate brood mortality and determine health and vigour of progeny. These results are supplemented by the fall surveys which determine current attack and help estimate volume losses and infestation trends. In this case, the number of entrance holes on the bark sample represents the number of attacking female beetles. One half the number of brood under the bark estimates the numbers of females that will emerge to attack new host trees. Bark sampling should be done in the spring following attack to account for overwinter mortality and losses to parasites and predators, particularly woodpeckers.

The equation below details how to calculate and interpret “r” values.

Interpretation of “r” values to determine population status:

General impacts

The impact of bark beetles is not limited to timber loss. There are also impacts upon a variety of other resource values, which include: However, timber loss, and more importantly, the disruption of long-term forest management plans, are major concerns to forest managers and planners.

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