Naturespeak A naturalist's view of the world

July 30, 2008

The Horntail Cometh

Filed under: Uncategorized — wykes @ 7:55 pm

  I’ve been concerned about the health of my backyard Silver Maple as of late. The tree has slowly been falling apart and whole limbs have died back to the main trunk.  One section of the upper main trunk has rotted to the core – much to the pleasure of a family of Red Squirrels that raised a brood in the cavity this summer.  Yet my tree continues to produce fresh green leaves and branches like a faithful old white-faced dog.

  I, like a faithful old dog owner, have chosen to ignore what I know to be true – the tree is in decline. A recent visitor, in the form of a Pigeon Horntail, has clarified the situation like a visit from the Angel of Death.

  Horntails are large wasp like insects that lay their eggs in dying trees.  Females are known to prefer silver maples, ash, cottonwood, and elm and they seek weakened wood that is sufficiently low in moisture content. They do not bring about the death but simply take advantage of, and encourage, previously sickened trees. The horntails are so named because of the pointed stinger-like “horn” that terminates the end of their abdomens. This structure is not a stinger, however – the creatures are stingless.

  The mission of a visiting female is to deposit her eggs deep into the sapwood.  To accomplish this task she has a very long ovipositor with which to “sting” the wood. I watched the female slowly examine the surface of my tree before she finally settled on one location.   She raised the middle portion of her abdomen, un-sheathed her ovipositor, and directed its point at a right angle into the bark.  In essence, she sets herself up like an oil drilling rig (see photo above – note ovipositor pointing down from the center of the body and the sheath pointing back). Since the ovipositor has the consistency of a stiff hair, it seems near impossible that she could drill this thing into hardwood, but she can.

  I can best describe her technique by referring to the human action of running a plumber’s snake down a drain. You need to twist and turn one end in order to push the other end around the bends and curves of the pipe. This horntail performed the same action with her body as she drove the drill tip into the fine spaces of the wood fiber.  First a twist to the left (see here) and then a twist to the right (see here) drove the thing home. These creatures can penetrate solid wood up to 20 mm deep with nothing more than a robust hollow thread!

  Once the egg chamber is drilled, anywhere from 2-7 eggs are laid.  The female adds a hefty dose of a white rot fungus to the chamber – the spores of which are stored within two sacs located at the base of her ovipositor. This fungus accelerates the wood decay process and softens the wood for the hatchling larvae. Without it, the larvae can not survive.

  As the Horntail ended her egg-laying session, a process that took about 5 minutes, she clumsily fell to the ground.  Instead of seeking revenge upon the messenger, I helped her on her way. I carefully offered a twig for support and watched as she launched into a slow departing flight.

  Young Horntails take up to two years to mature. The wood munching larvae will eat tunnels into the sapwood and grow to be 2 inches in length before exiting the tree as adults.

  In a way, the Pigeon Horntail’s visit offered some assurance.  My Silver Maple will probably hang on for at least a few more years since it takes all of that to complete the larval growth process. Horntail females don’t invest in their future without some innate knowledge of the odds.

  Whether I will need to have a “vet” put my tree down before that time is the future I will need to ponder.

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