Thursday, December 12, 2013

Pruners and Girdlers

Twig Girdler - Oncideres cingulata - Dave Rintoul
This is the time of year when we are out pruning the trees in our riparian plantings, encouraging their growth upward and removing extra undesirable branches.  Also pruning trees during the year are twig girdlers and pruners, the long-horned beetles of the Cerambycidae family.  Fall and early winter is a good time to look for their handiwork while the leaves are off the trees.

I have never caught these beetles in action, but their evidence is especially easy to find on young trees whose branches are at eye level.  They work in two separate ways, each an interesting method of reproduction.

Twig Girdler
Twig Girdler

Twig girdlers feed on the tender bark at the tips of branches, the rapidly growing portion which must be juicy and tender.  After mating, the female works from the outside in, making a smooth cut around the branch, somewhat V-shaped while the center remnant remains intact for a while.  Next she deposits an egg under the bark in the distal or outer half of the branch.  Her larva requires dead wood to develop and the cut effectively blocks the branch's circulation.  

The larvae of various species are white legless grubs that reach 5/8 to 1 inch long.With time the branch falls to the ground while the larva emerges and bores into the dead branch.  After over-wintering in a resting phase, it consumes the rest of the branch in the spring, creating a chamber filled with wood shavings and frass.  Following pupation, the adult beetle emerges in the fall, mates and starts the process again.

Twig Pruner -USFS
Twig pruners on the other hand work from the inside out.  The beetle deposits its egg at the tip of the twig under a leaf axil.  The larva tunnels down the center of the twig, growing larger as it moves to the thicker portion of the proximal branch.  Later that summer it makes circular cuts from the inside out, leaving only a thin rim of bark to support the twig until it eventually falls.  The pruned ends are smooth inside and jagged on the outside, the opposite of the twig girdlers marks.  The larva plugs each end of its chamber with frass and pupates over winter, emerging the next spring as an adult.

Twig Pruner - Anelaphus villosus- tom murray
Twig girdlers and pruners hosts in Missouri include persimmon, pecan, elm, hickory, oak, honeylocust, hackberry, poplar, linden, redbud, basswood, dogwood and various fruit trees.  They can be a commercial problem in pecan and fruit orchards and may require treatments in that setting.
Along Bull Creek they do not produce serious damage.  If you had a heavy infestation on your urban tree, it would pay to pick up and destroy them by burning the pruned branches to decrease next year's infestation.  Pruning the cut branches before they fall is also an option.  Meanwhile, nature has a way of maintaining a balance with parasites such as Eurytoma magdalidis, Iphiaulax agrili, and Horismenus sp., and a checkered flower beetle, Cymatodera undulata.
Twig Girdler
Twig Pruner

Drawings from MU Extension Brochure
There are lots of species shown in the twig girdler identification guide.

1 comment:

  1. wonderful explanation - it's something else - mother nature! We are fortunate to have this knowledge passed on!