Thursday, November 25, 2010

Southern Pine Bark Beetle; It's a "Southern Thang"

Adult Southern Pine Bark Beetle
A:Egg, B:Larva, C:Pupa
    Southern pine bark beetles (Dendroctonus frontalis) like the one pictured above destroy millions of pine trees from Texas to Virginia every year.  Adult southern pine bark beetles (SPB) are reddish-brown to black and measure about 1/8 inch long.  Infestation typically begins in trees weakened by lightening strikes, storms, disease, or old age.  Once the beetles are established in a damaged host, they seem to have no trouble infesting healthy trees.  These insects go through four life stages.  Females burrow under the bark and lay their eggs along s-shaped channels that eventually girdle the tree and damage the phloem, the living layer of the tree that transports water and nutrients.  Beneath the bark, eggs hatch and the legless white larva or "grubs" rear their little reddish-brown heads and burrow into the tree trunk where they gorge themselves.  When they pupate, they remain white but take on a shape that closely mimics an adult.  The process takes about a month.  Given optimum conditions, SPB may raise seven or eight generations during their annual season from April to September.  When the host tree inevitably dies, adult beetles take to the wing to find a living host where they start the cycle again.
     SPB show a preference for particular species of pine.  They target the various yellow pine species, as well as loblolly, shortleaf, and Virginia pine.  They appear to avoid white, sand, slash, and longleaf species.  This preference may be related to a trees production of resin.  Pine trees respond to any damage by patching the wound with resin.  When SPB attack, resin oozes from the holes in the bark to produce pitch tubes.  Trees that produces a stronger response to attack appear to be less susceptible to infestation by SPB.  Research indicates that the trait is heritable. It may be practical to develop genetic lines of pines that have greater resin flow and more inherent resistance to SPB attack.
     Needles on trees that cannot repel an attack change color from yellow to red and finally to brown.  The images below show the damage caused by SPB and the pine tree's defensive response to attack.
     In an interesting footnote, SPB has a commensalistic (symbiotic) relationship with a fungus known as "bluestain".  The beetles introduce the fungi which then cause further damage to the conductive capacity of the trees phloem layer.   

Primary Direction of Infestation (bottom to top)
Defensive Response: Dozen of Pitch Tubes
Trunk Bearing "Bluestain" Fungi
An Adult Beetle and the Holes It Bores
Pitch Tubes Often Resemble Popcorn

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