Life in a Dead Tree

Bark beetle galleries

With an Arctic vortex receding, we emerged to get some sunshine and hopefully a little dose of nature.  A fifty foot tall dead tree was standing beside the trail, with a few edges of loose bark on one side.  As I prepared to fell it to keep the road open, I peeled off some of the bark and found lots of evidence of life.  There were bark beetle tracks and two cockroaches attempting to scurry to safety.   A lone crab spider retreated into a crack in the wood beside some unidentified insect legs and elytra, the hardened forewings of a beetle, suggesting a past spider meal.

Bark beetles, members of the subfamily Scolytinae are actually specialized "true weevils."  They are small, usually only 1/16 to 1/4 inch.  The female lays its eggs just beneath the outer bark in an egg gallery.  After hatching, the tiny larvae tunnel out their own galleries, branching out from their former home.  As they grow their tunnels increase in diameter.  The tunnels end where pupation occurs and then the adults emerge through holes of their own to start the cycle all over.

Like most insects, our opinion of them depends on how they are encountered.  If they are attacking your landscape or fruit tree, they are a pest or worse.  Some may only cause limb damage while others such as the mountain pine beetle can decimate forests of the west.  Thousand cankers is caused by bark beetles and others can carry plant pathogens such as the Dutch elm disease. 

A word in the defense of the bark beetle.  Unless they are attacking in epidemic proportions, they do serve a useful purpose as food for woodpeckers and other birds and insects.  Most live in dead, weakened, or dying hosts and aid in the decomposition of dead wood while renewing the forest by killing older trees.  Some "ambrosia beetle" species farm fungi, eating the products of their crop, even carrying the fungus with them and inoculating new host trees.

Frass filled galleries of bark beetles

My personal interest in winter is peeling off the decaying bark from firewood, exposing the beautiful tunnels that they have produced.  Sometimes the patterns are etched in the inner bark layer, but more commonly they are on the tree trunk itself.  Many of these galleries are packed with powder which can be scraped out to reveal the intricate engraved patterns.  The powder, as you have guessed, is frass, also known by the technical 5th grader's term of "insect poop."  The intricate patterns have a beauty all their own, and my barn has a lot of decorated logs that I haven't had the heart to burn.

Bark beetle galleries, some with brown frass left in

Bark beetle galleries and white slime mold- click to enlarge

The larvae below are probably a from different genus, a woodboring beetle.  Most of these attack dead or dying trees thus making way for new growth.  They serve as primary decomposers, recycling nutrients locked away in wood that would otherwise take many years to decay.  Again, there are a few rogue species that can damage household wood and furniture or even destroy forests - think of the emerald ash borer. 

This specimen probably was just working on dead wood, trying to make a living.  Either way it had a rough day, first with a chainsaw cutting its log home in half, exposing its tunnel through the heartwood, then the pounding of a splitting maul.  It crawled out only to encounter a freezing night.

Cockroach, possibly of the genus Cryptocercus

The cockroaches I found are insects of the order Blattodea, which includes termites now thought to have evolved directly from the roaches.  Though more common in the tropics, cockroaches can flourish in any environment where there is sufficient food and warmth. Most North American cockroach species live in woodlands and are not pests.  As humans have created comfortable habitats, cockroaches have moved in, finding all the warmth, food and amenities of an upscale insect hotel.

Because of their location, I suspect these roaches were of the genus Cryptocercus which are wood-eaters.  Like termites, they don't have the enzymes to digest wood and depend on protozoans and bacteria that they host in their guts to digest the cellulose into sugars that they can then metabolize.

There is a lot of life in a dead tree.  If there weren't, we would no longer have forests, just a hundred foot deep pile of dead trees.