Pine borers

Posted by Aaron Rosenbluth. Aaron is a sophomore from Cleveland, Ohio studying Natural Resource Planning and Economics. He loves being in the outdoors and is happily a member of the UVM outing club!

What: Two months ago I was in Centennial Woods leaning against a fallen tree taking notes. The sun was out and I couldn't have been happier listening to the summer grasshoppers sing and birds above me chirp. After almost a half an hour out there, I looked behind me at the tree I was leaning against and realized I was leaning against a phenomenon that I've seen countless times over the years, but have never really thought twice about. What I saw was a simply massive maze of tunnels/passages in the wood. The tunnels weren't very deep down - just under the bark layer - about 1cm deep, but it was a fascinatingly huge and sprawling system. I figured these tunnels were created by some sort of insect species, but which one and how and why?

After some preliminary research I quickly realized what type of species I was starting to investigate; species within the realm of wood-borers, or species which eat and destroy wood (Xylophagous species, from Greek "xylon" for wood and "phagus" for eater). These species range considerably in their characteristics (as well as richness, with over 10,000 species of wood-boring insects identified).

• All make unique patterns or “galleries” where they live and lay their larvae, 
• all produce and leave behind unique types of frass (excrement from eating wood), 
• all prefer different species of wood to bore into, 
• and all prefer different levels of health in the tree's which they choose to bore into. 

Once larvae have completed development, new adults will chew an exit hole through the bark and emerge. These exit holes are either round, D-shaped, or ovular. Typically, these wood-borers are insects and arthropod species (beetles, moths, ants, bees) whose larvae are laid in and subsequently mature under the bark layer of trees.

After realizing the tree I was leaning against was once home to massive borer community, I looked around the area (a stand of conifers, mostly white pine and hemlock) and was surprised to find that most of the downed trees in the area had once been host to this same borer species. After researching with Eiseman and Charney's Tracks and Sign of Insects and many hours of poring over internet databases, I was unable to come to a conclusion on exactly what species of borer did this, however I did come to the conclusion that it was some sort of bark beetle of the Curculiondae family within the sub-family Scolytinae (linked at the bottom of this posting is an official Vermont State report on insects which likely contains the correct species). These beetles feed on the cambium layer of trees, creating distinguishable patterns in the inner bark and outer sapwood. It is unlike bark beetles to bore deeper than 3mm into the sapwood layer of the tree, which matches what I observed in my tree. Most commonly, bark beetles will attack a tree that is already injured or diseased, often killing it in the process.

The most basic bark beetle gallery begins with the creation of a mating chamber, from which a female excavates a linear tunnel of uniform width either perpendicular or parallel with the grain of the tree (mine went parallel). On either side of the tunnel, the female creates small little niches and lays an egg in each one. I was able to see these niches in my downed tree species (these were in white pine, Pinus strobus); they look like tick marks that are evenly spread out following the main tunnel (see image above).

Ecological Notes:
Wood borers serve the ecologically keystone task of facilitating tree decomposition by creating holes and channels in the woody tissue, phloem, and bark of the stressed, diseased, or dying trees which they feed on. This allows for fungi to enter the tree and begin decomposition.
http://www.vtfpr.org/protection/documents/FullVersion2009ConditionReport.pdf