Sunday, October 13, 2019

Twig Girdlers

Twig girdler, Oncideres cingulata - Dave Rintoul
This is the time of year our forest floor becomes littered with small tree branches with a distinctive break at the end.  They look like they have gone through a blunt pencil sharpener, ending in a jagged tip.  This is the work of twig girdler beetles, Oncideres cingulata.  These are long-horned beetles of the Cerambycidae family.  

In the summer after mating, the female straddles a small branch and chews a V-shaped groove in the outer portion around the twig, leaving the center portion intact.  Her feet grip the tender bark, leaving a series of horizontal grooves in the distal branch.

Hole for egg - Chris Barnhart (CB)
Next she chews a small hole in the bark of the outer half of the branch and deposits an egg under the bark.  
The eggs are long and narrow, fitting into a tiny hole usually at the site of a side branch.
Egg in its chamber - CB  Click to enlarge

The number of eggs per twig normally ranges from 3 to 8 but may range up to 40. Adults live 6 to 10 weeks. 

Each female deposits 50 to 200 eggs which hatch in about 3 weeks.

Larva writhing around in its frass chamber - (CB)

This will be her offspring's home until next summer, slowly enlarging its chamber while living in its woody poop (frass for insect connoisseurs).  Her larvae require dead wood to develop and the cut effectively cuts off the twig's circulation.  They cannot fully develop in green twigs with high moisture content. 

The legless grub will live in the twig all winter whether it falls to the ground or occasionally dangles from the tree. The chamber hollows to provide a winter home as the grub rests.  It starts feeding again in the spring and grows up to an inch long, living in a world of its frass which provides some insulation.  Eventually it chews an exit hole for the future, then pupates in the frass before emerging as an adult beetle in late summer.

You can see some of the action in Dr. Chris Barnhart's video here.  Meanwhile a couple of beautiful final portraits from Chris of the emerged adult, Oncideres cingulata.  
Note: 2013 blog updated with video.

Mating twig girdlers - Sid Vogelpohl