Gambel oak borer, Agrilus quercicola
The Gambel oak borer is a native species of wood boring beetle found in forest stands of Gambel oak (Quercus gambelii). This pest can also attack oaks in urban areas and is particularly problematic in English oak (Quercus robur). Gambel oak borer is not usually considered a pest since its impact is typically limited to newly transplanted trees or established trees with damaged, stressed, or overshaded limbs. However, large populations can develop in areas with Gambel oak stressed by drought or late spring freezes, which destroy newly emerging foliage. As rainfall increases, populations of Gambel oak borer naturally decline.
The adults have small antennae and elongated, oval shaped bodies that measure a little less than 1 cm (1/4-1/3 inch). The thorax tends to be copper or bronze colored and the abdomen is usually green with black elytra (wing covers). The copper or bronze thorax is a distinguishing feature of this species when compared with other wood boring beetles (Agrilus spp.) in Colorado. The larvae are legless and have pale, soft, elongated worm-like bodies with dark jaws and a slightly flattened body region just behind the head.
- Gambel oak borer is native to Colorado and is in the same genus as the invasive emerald ash borer (Agrilus planipennis). Although Gambel oak borer is rarely a significant pest, in 2003 this wood boring beetle infested oak trees in landscapes and caused significant financial losses in nurseries. In landscapes, English oak and its hybrids are particularly susceptible to attack.
- Forest stands of Gambel oak are most susceptible to attack during periods of drought stress, late frosts, and after transplanting.
- The primary signs of an infestation include D-shaped exit holes on the bark surface and galleries underneath bark.
- Properly caring for susceptible oaks in landscapes is recommended to reduce susceptibility to Gambel oak borer attack. There are also several chemical control options available to protect susceptible trees, including soil drenches, and injections.
Adult of Gambel oak borer. Note the copper-colored thorax and black elytra covering the green abdomen. The coloration of Gambel oak borer is a distinguishing feature from other Agrilus spp. inhabiting Colorado. Image credit: Hanna Royals, Screening Aids, USDA APHIS PPQ, Bugwood.org
Gambel oak borer emerging from the trunk of a Gambel oak. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Galleries with larvae of agrilus spp. Image credit: James Solomon, USDA Forest Service, Bugwood.org
D-shaped exit hole produced by emergence of Agrilus spp. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Life history and habits
Larvae of Gambel oak borer overwinter under the bark of a host tree. The following May, adults emerge through D-shaped exit holes and begin dispersing to new hosts to feed and lay eggs, with peak flights occurring in mid-June. Very few adults are still active by mid-August. After emerging from a host tree, adults mate and feed on oak leaves for a couple of weeks. Over the next few weeks, females lay several dozen eggs in bark crevices on trunks and branches. After two weeks, newly hatched larvae tunnel into the cambium where they feed and develop through the summer. Mature larvae cease feeding and excavate shallow chambers deeper in the wood where they likely overwinter. One generation is produced each year.
In 2003, oaks in Colorado landscapes and nurseries were infested, with the most severe damage observed in nurseries in Douglas County and the southern Denver metropolitan area. In the Denver Metro area, infestations of Gambel oak borer have been confirmed in burr oak (Q. macrocarpa), northern read oak (Q. rubara), southern red oak (Q. falcate), swamp white oak (Q. bicolor), Shumard oak (Q. shumardii), scarlet oak (Q. coccinea), and English oak (Q. robur).
Infestations tend to begin in the upper crown region of a host tree and progress downward. Tunnelling of larvae results in meandering galleries in zig-zag patterns under bark or one or two rings of wood in the host tree. Extensive tunneling can restrict nutrient exchange in the plant and cause thinning of the canopy, browning of foliage, and die back of branches. The emergence of adults produces small D-shaped exit holes in bark that measure approximately 1.5 mm (1/16 inch).
Trees can be inspected for D-shaped exit holes in bark, and galleries can be exposed by removing small portions of bark. Areas of wet or gummy bark may be observed on some infested trees. Portions of raised bark above galleries may be observed on trees healthy enough to fight the infestation. Thinning of the canopy, browning of foliage, and dieback of branches can also be observed on infested trees.
Newly transplanted oak trees are more vulnerable to attack than established trees. It can take several years for a tree to become established, but adequate tree care with proper irrigation and fertilizing practices can help expedite the process. Detailed information on Gambel oak care in landscapes is available here.
One species of chalcid wasp, Phasgonophora sulcate, is a confirmed parasitoid of Gambel oak borer and likely helps maintain low populations under normal environmental conditions. In one site sampled during the Gambel oak borer outbreak in 2003, P. sulcate was recorded in over 80% of borers.
Several chemical control guidelines have been established for other pestiferous Agrilus spp. and may be effective for Gambel oak borer.
Colorado State University. (n.d.). Gambel oak borer. Colorado insect of interest. Available https://webdoc.agsci.colostate.edu/bspm/Hexapoda%20(Insects)/GambelOakBorer.pdf
Colorado State University. (n.d.). Oak borers. Plant Talk Colorado. Available https://planttalk.colostate.edu/topics/insects-diseases/1477-oak-borers/
Sever, A., W. Cranshaw, and R. Brudenell. 2012. Southwestern Entomologist. 37(2): 147-150. Available https://bioone.org/journals/southwestern-entomologist/volume-37/issue-2/059.037.0207/Agrilus-quercicola-Fisher-Coleoptera–Buprestidae-the-Gambel-Oak-Borer/10.3958/059.037.0207.full
USDA. 2011. Agrilus quercicola. Forest Service – Forest Health Protection. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5347769.pdf