Oak Twig Gall Wasp

Cynipid gall wasps on oak- Bassettia ligni, Andricus californicus, Besbicus mirabilis, and Neuroterus saltatorius

There are several cynipid gall wasps that make galls on oaks in the PNW. One potentially damaging cynipid gall wasp in nursery production is Bassettia ligni, a native insect that causes twig galls on Oregon white oaks.

Biology
This species has alternate asexual and sexual generations. B. ligni wasps overwinter in stem galls and adult wasps emerge in spring (by mid-April in 2016) leaving tiny holes in the gall. These wasps, all female, lay their eggs in oak leaves causing inconspicuous galls. A sexual generation hatch from the leaf galls in June through mid-July. The mated females from this generation lay eggs in oak twigs, which form twig galls later in the fall.

Damage
In forest landscapes, this damage becomes obvious as squirrels debark white oak twigs in search of a wasp larvae meal. This debarking girdles the oak twigs causing a striking dieback of white oak branches. This damage generally will not kill an oak tree but can be an aesthetic issue.

In nurseries, the damage is more impactful. Twig galls caused by the B. ligni, cause the affected terminal to die, stimulating lateral branches to grow. While in some cases, this might provide a preferred bushier tree, sometimes this damage is a less appealing witches broom or other distortions, requiring retraining of another terminal.

Resources:

Abraham, Melika, G. 2007. Review of the Nearctic gallwasp Species of the Genus Bassettia Ashmead, 1887, with Description of New Species (Hymentoptera: Cynipidae: Cynipini). Acta zool. hung. 53, 2007

Larew, H. and J. Capizzi. 1983. Common Insect and Mite Galls of the Pacific Northwest. Oregon State University Press.

Shaw, Dave. 2015. Summer 2015 DROUGHT! And heat. A Forest Health Report. Tree Topics. Oregon State University Extension. Posted September 2 2015. <18 July 2016>

Shaw, Dave. 2013. Oregon white oak in the Willamette Valley, 2013 . . . Some in your face issues. Tree Topics. Oregon State University Extension. Posted 2013. <18 July 2016>

Garry oak leaf and twig gall. PNW Insect Management Handbook.

Original version: <18 July 2016)

Last revision <19 July 2016>

Author: R.L. Rosetta, Extension Nursery Integrated Pest Management, Department of Horticulture, Oregon State University/NWREC.