The world is home to about 6,000 bark beetle species – known to biologists as the subfamily Scolytinae. One of the most famous examples is the elm bark beetle (Scolytus schevyrewi), which transmits the fungi responsible for causing Dutch elm disease. However, the most important bark beetles for North American trees (particularly conifers) are undoubtedly the bark beetles of the genus Dendroctonus.
Basic Bark Beetle Life Cycle
Most – if not all – pine beetles form large aggregations, consisting of thousands of adult beetles. Mating usually follows the mass arrival at a single tree, and the females begin tunneling through the tree’s tough outer bark soon thereafter. Once under the outer bark, the females usually begin constructing egg galleries – small tunnels cut into the living inner bark (phloem) or the tree. The eggs are laid along the sides of these galleries, where they will remain until they hatch shortly later.
Upon hatching, the larvae begin feeding voraciously on the nutrient-rich phloem. These feeding tunnels are often visible under the bark of afflicted trees. Like many other insects, pine beetles spend most of their lives in the larval, rather than adult, life stage. Nevertheless, they eventually complete their larval life stage and pupate under the bark of the tree.
A short while later, the adult beetles emerge from their pupal state, and seek out other beetles, thus starting the process anew.
Native Pine Beetle Species
The bulk of the economically important bark beetles are members of the genus Dendroctonus. Most bark beetles are small, black to brown insects that are about one-quarter-inch long. But despite their small size, these miniscule bark munchers can cause a lot of trouble for the trees they infest. Biologists clearly understood this when originally describing these pests – their generic name, Dendroctonus, literally means “tree killer.”
As currently construed, the genus Dendroctonus contains nearly 20 species of bark-eating beetles. Most of these species range from the far northern reaches of North America down as far south as Nicaragua; however, two species inhabit northern Europe and Asia. Though they all exhibit the same basic life history, the various species differ with regard to their preferred host species, gallery-creating behaviors and morphology.
Southern California is home to four native bark beetle species.
- Western pine beetle (Dendroctonusbrevicomis) – The western pine beetle is an aggressive bark beetle, capable of killing even otherwise healthy trees. Its primary hosts include the ponderosa and Coulter pines.
- Mountain pine beetle (Dendroctonus ponderosae) – As its name suggests, the mountain pine beetle ranges throughout most of the Rocky Mountain region, but it also spills into the Sierra Nevada range as well. The mountain pine beetle primarily attacks four different pines, including ponderosa, sugar, lodgepole and white.
- Jeffrey pine beetle (Dendroctonus jeffreyi) – Jeffrey pine beetles are so-named for their habit of feeding solely on Jeffrey pines. Often, Jeffrey pines cause a widening circle of dead trees, which may grow in circumference from one year to the next.
- Red turpentine beetle (Dendroctonusvalens) – The red turpentine beetle has the largest range of any of the North American bark beetles. However, this beetle is not usually responsible for widespread die offs. This species has been documented inhabiting more than 40 different conifer species, although it usually feeds on the phloem of pines.
In addition to the native bark beetles that have been co-evolving alongside their hosts for thousands of years, several invasive bark beetle species have been introduced to California and other locations. Currently, three species – the banded elm bark beetle (Scolytus schevyrewi), the Mediterranean pine engraver (Orthotomicus erosus) and the redhaired pine bark beetle (Hylurgus ligniperda) — have been documented to occur within the state’s borders. Note that while these insects are still considered bark beetles, they are not categorized in the same genus as the North American natives are.
In addition to threatening pines and other conifers as native bark beetles do, these exotic invaders may also introduce pathogens, fungi and other harmful organisms to North American ecosystems.
The Defenses of Trees
Conifers do not simply stand by and allow pine beetles to destroy their tissues, they actively defend themselves from beetle attacks. Their primary weapon against the beetles is their thick, sticky sap, which begins flowing from the holes the beetles create in the inner bark. However, pine beetles often attack trees en masse, which often overwhelms the host tree’s ability to repel the beetles.
You can often see the evidence of bark beetle attacks, via “pitch tubes.” Pitch tubes are holes made by the beetles, which have hardened sap that has flowed out of the hole. Sometimes, it is possible to see dead beetles trapped inside the sap.
Like most other native insects and pests, pine beetles rarely cause widespread problems for the ecosystem as a whole. Single trees may often die, but the forests continue, largely unaffected. In fact, the beetles play important roles in ensuring biodiversity and forest succession. However, this historical trend has been disturbed by habitat alteration, increasingly frequent and prolonged droughts and commercial forestry operations.
Accordingly, vast swaths of trees are dying off in portions of the country. This is most pronounced in areas that have suffered through prolonged droughts, as the lack of water reduces the amount of sap available to the trees to use in defense.