Bark insects are those that attack and consume the nutritional inner bark, or phloem of trees. The most important group in this category is the bark beetles (Order Coleoptera, Family Curculionidae, Subfamily Scolytinae), but other insects also utilize this resource. In this lab, you will look at those insects that complete their development in the inner bark.

 

As you work through this material and the lab exercises, review the life cycles of the different species in your field guide or through the links provided, and make note of the major characteristics.

Bark Beetles (Order Coleoptera, Family Curculionidae, Subfamily Scolytinae)

Bark beetles are extremely variable, and many innocuous species can easily be mistaken for economically important species (and vice versa!) unless you gain some general familiarity with different types. Bark beetle larvae cannot be differentiated easily from each other or from other weevil larvae (bark beetles are a subfamily of the weevils). For diagnosis, you normally have symptoms and signs from the host tree, which makes identification less difficult than it may seem. On occasion, someone will bring adult beetles to you, or you may be involved in trapping for monitoring purposes, and you should then be able to at least make an educated guess at an identification before sending specimens off to experts.

 

You should note the differences between polygamous and monogamous gallery patterns.

 

The most aggressive species are usually monogamous. Monogamous species (in the genera Dendroctonus, and Scolytus) have a single, usually straight (however, note the sinuous pattern of the western pine beetle) egg gallery that usually follows the grain of the wood, with larval mines radiating out perpendicularly to the egg gallery. These beetles frequently pack frass (a mix of feces and wood fragments) in the gallery. In monogamous species the attack is initiated by the female, with a single male joining. Polygamous species (e.g. Ips, Dryocoetes [pronounced dry-o-seetes]) have a central nuptial chamber constructed by the attack-initiating male, with a number of egg galleries, each excavated by one female, radiating out in a more or less species-specific pattern (Figure 1). The nuptial chamber is sometimes not visible, i.e., it may be within the bark. In Ips perturbatus and Ips tridens, both spruce engraver beetles, the egg galleries are parallel to each other and the grain, and may be mistaken for straight monogamous galleries. Ips galleries do not have frass in the galleries. Note also the difference in width between Dendroctonus galleries and those of secondary beetles.

The spruce beetle, Dendroctonus rufipennis, generally lays eggs in batches on alternating sides of the gallery, and the larvae usually feed communally for the first while. Thus, you can see areas of feeding close to the egg gallery, followed by single larval mines radiating out from these communal feeding areas. In the mountain pine beetle, Dendroctonus ponderosae, the larvae tend to excavate chambers when they reach areas with blue stain. Thus, their larval mines tend to be short, with a chamber where they feed largely on fungus-infected phloem at the end. The Douglas-fir beetle, Dendroctonus pseudotsugae, has long larval mines. Douglas-fir beetle and western balsam bark beetle larvae turn into the bark at various stages, i.e., the gallery ends as if there is no successful brood. However, brood development is completed inside the bark. In the western pine beetle, Dendroctonus brevicomis, larval mines may not visible on the inside of the bark at all, because the larvae mine into the bark, possibly as a predation avoidance strategy.

 

You should note characteristics (size, colour, shape) of the insects covered in this lab, including larval forms, and the damage they cause. You will have an opportunity to see them more closely in the laboratory. Note that it is only important to be able to differentiate among species within a host species, not between different hosts, because the diagnosis uses the host as well as gallery pattern. For example, focus on the gallery patterns of Dendroctonus ponderosae in pine, and compare this to secondary bark beetles in pine. **Note that this means it is important for you to be able to identify the tree species from a bark sample.

Hylastes spp. and Hylurgops spp.

Beetles in these genera generally attack dead or dying conifers in contact with, or near the ground, e.g., down trees and root collars. After thinning, they can appear on stumps and down trees in large numbers. Some species in these genera are primary vectors of black stain root disease, Leptographium wageneri. They are generally black, brown or reddish brown, and some species superficially resemble the mountain pine beetle. These insects breed in bark of dead conifers near the ground, or in direct contact with the ground. They do their maturation feeding on bark of live conifers, and may vector spores of black stain root disease while doing so. For a reference picture check https://www.fs.fed.us/r3/resources/health/field-guide/rd/blackstain.shtml.

Dendroctonus spp.

The genus name means “tree killer”, and this genus includes some of the most economically and ecologically important bark beetle species. Most are North American, but one species occurs in Eurasia. Several species are displayed in the Petri dish. The mountain pine beetle is always black when mature, but adults may be brown or pale yellow before sclerotization of the cuticle is complete (callow or teneral adults). Note that there can be great variation in size, so you cannot reliably use size to differentiate bark beetle species. The spruce beetle, which tends to be a bit larger than mountain pine beetles, is all black, but can also have reddish-brown elytra (the colour of elytra is genetically determined). The Douglas-fir beetle always has reddish brown elytra, and a black pronotum and head, and is superficially very similar to the spruce beetle, but their host association differentiates them. Several other species resemble the spruce beetle and Douglas-fir beetle, but are found in different hosts, and are of minor importance. Host identification is critical to diagnosis.

Ips spp.

These bark beetles are generally referred to as engraver beetles, because their egg galleries tend to score the sapwood very distinctly. Several species are important and, in Eurasia, the European spruce beetle, I. typographus, is the most important bark beetle species. Ips species can be recognized by their concave elytral declivity (the sloping part at the posterior end of the elytra), the edges of which have species-specific numbers of spines or teeth. Most Ips beetles avoid live trees. Thus, you almost never see pitch tubes associated with Ips attack. On down trees they often occupy the upper, exposed part of the tree.

Scolytus spp.

These bark beetles are easily recognized by the straight shallow elytra, and the abdomen sloping up to the elytra, rather than the elytra sloping down. Several species are important, notably the non-native European elm bark beetle, S. multistriatus, a vector of Dutch elm disease.

Predatory beetles

Adult predatory beetles are often found on trees or logs attacked by bark beetles. The majority of these are in the Family Cleridae, the checkered beetles. The adult beetles are typically dark grey, with a wavy light grey band across the elytra, and the thorax and underside is often red, e.g., the red-bellied clerid. Adults feed on adult bark beetles, and oviposit on attacked trees. Larvae of the checkered beetles live in the galleries of bark beetles, where they feed on the bark beetle larvae. They are easily recognized by their purple to pinkish colour and sclerotized double “horn” at the end.

Important Bark Beetles by Hosts

Bark beetles in pine

There are two important species in western North America. One is the mountain pine beetle, Dendroctonus ponderosae, which occurs throughout western North America, and attacks almost all pine species. The other is the western pine beetle, Dendroctonus brevicomis, which is restricted to ponderosa pine in British Columbia. It is an extremely important species in California, and has been locally important in BC.

 

In the southeastern United States, the southern pine beetle, Dendroctonus frontalis, is of major concern, particularly in loblolly pine, which is the major commercial species there.

 

Secondary species of occasional concern are the lodgepole pine beetle, Dendroctonus murrayanae, a close relative of the spruce beetle, and the red turpentine beetle, Dendroctonus valens. These species attack stressed or damaged trees low to the ground, and generally create large, brownish-red pitch tubes. The red turpentine beetle is the largest of the Dendroctonus species, reaching 9 mm or more in length. It is found low on the trunk, or in the roots, of stressed or dead pines, and it is red, rather than black. Small turpentine beetles can potentially be confused with callow (teneral) adults of other species (What is a callow or teneral adult?).

 

Common engraver species in pine are Ips pini, Orthotomicus (Ips) latidens, and Pseudips (Ips) mexicanus. Pityogenes spp. are very small beetles often associated with other bark beetles. P. knechteli is commonly found with Ips on pine. On occasion they kill immature pine, especially following thinning. They have spines on the elytral declivity, but the declivity is not concave with well-defined edges as in Ips.

Bark beetles on spruce

The spruce beetle, Dendroctonus rufipennis is second only to the mountain pine beetle in economic importance in temperate forests. It is distributed throughout the range of spruce in North America. Outbreaks have occurred in the east, but are particularly devastating in the west from Alaska to Colorado. The spruce beetle normally requires 2 years to successfully complete development, but warm summers will promote a 1-year cycle. The boreal spruce beetle (also called the Allegheny spruce beetle), Dendroctonus punctatus, is a minor species that attacks weakened spruce trees, but attacks do not kill the tree. Generally, single attacks are found at ground level, particularly in stands affected by Inonotus tomentosus. Ips perturbatus has caused mortality of spruce in Alaska, but is secondary in northern BC. Ips tridens and Ips borealis are also found on spruce. The four-eyed spruce beetle, Polygraphus rufipennis, is a small secondary beetle usually found in the thin-barked portions of down spruce. Beetles in this genus have their eyes completely separated into two parts, giving this beetle its common name. In Newfoundland, it has caused significant mortality.

Bark beetles in Douglas-fir

The Douglas-fir beetle (Fig. 3), Dendroctonus pseudotsugae, kills older, stressed Douglas-fir, particularly in dry environments. It is a significant problem east of the Cascade Mountains in the United States. In British Columbia, mortality tends to be associated with other problems, e.g., root disease, defoliation or other stress. This insect is not well adapted to breeding in living hosts, and outbreaks tend to subside within 2-3 years. The Douglas-fir engraver, Scolytus unispinosus, is common on thin-bark portions of dead trees, and occasionally kills severely stressed younger trees.

Bark beetles on true firs (Abies spp.)

The most important species is the western balsam bark beetle, Dryocoetes confusus. This insect has been largely ignored until recently. As commercial interest in high elevation forests increases, the need for management options for the western balsam bark beetle increases. Mortality is generally scattered over large areas, and weakened or declining trees (sometimes associated with defoliation) are preferred. The fir engraver, Scolytus ventralis, kills Abies spp. on occasion. Grand fir has been susceptible in Idaho and south central BC, as well as on the south coast of BC (Sechelt and the east coast of Vancouver Island), usually associated with stress events. Many other secondary species attack true firs.

Bark beetles on other trees

Most trees have some bark beetles associated with them. In general, these are secondary and of little importance. Exceptions are those species that vector diseases, e.g., the European elm bark beetle, Scolytus multistriatus (Fig. 5), and the native elm bark beetle, Hylurgopinus rufipes, both of which vector Dutch elm disease, caused by the pathogen Ophiostoma ulmi. The former occurs in BC and Washington State, but the disease is not yet present in these populations.

Pitch moths (order Lepidoptera)

In addition to the bark beetles, there are also some moth species associated with large pitch masses on suppressed, diseased (stem rusts) or damaged conifers. The Douglas-fir pitch moth, Synanthedon novaroensis, is the main conifer-infesting clearwing moth (Family Sesiidae) species found in northern BC, while in southern BC the sequoia pitch moth, Synanthedon sequoiae, is more common. Both primarily attack lodgepole pine, contrary to what their common names might suggest. The latter is slightly sturdier, and has yellow and black banding, rather than orange and black. The sequoia pitch moth tends to confine attacks to the lower bole, whereas Douglas-fir pitch moth attacks may be found anywhere on the bole. Pitch masses of these species can be very large, and repeated attacks often occur, usually oriented horizontally on the stem, and sometimes resulting in stem breakage. Larvae of the two species cannot be differentiated. In eastern Canada, Synanthedon pini causes similar symptoms.

Adults of the western pine moth, Dioryctria cambiicola, show the “W” pattern on the forewings typical of this large family (the snout moths, Family Pyralidae). The pitch masses of this species are smaller, and numerous pitch masses are often found along the vertical edges of stem rusts.

The larvae of the pyralid (western pine moth) and the sesiid (Douglas-fir and sequoia pitch moth) species can be differentiated best by the presence of dark spots on the back of the western pine moth, and a different arrangement of the crochets (the little hooks on the prolegs, which are the fleshy abdominal legs). In sesiid larvae, the crochets are arranged in two opposing bands, while in the pyralid larva they form a complete circle.

Suggested additional reading

Safranyik, L. and B. Wilson. 2006. The mountain pine beetle. A synthesis of biology, management and impacts on lodgepole pine. Natural Resources Canada, Victoria, BC, 304 pp.

Lindgren, B.S. and K.F. Raffa. 2013. The evolution of tree-killing by bark beetles: Trade-offs between the maddening crowds and a sticky situation. The Canadian Entomologist 145: 471–495, doi 10.4039/tce.2013.27.

Bark beetle management guidebook available at https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/forest-health/bark-beetles/bark_beetle_management_guidebook.pdf

https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/forest-health/forest-health-docs/field_guide_to_forest_damage_in_bc_web.pdf