Agrilus auroguttatus

Schaeffer, 1905

Goldspotted Oak Borer

Agrilus auroguttatus, the goldspotted oak borer, is a to southeastern Arizona that became in southern California around 2004. In California, it has caused extensive mortality in oak forests, killing an estimated 80,000 trees, primarily affecting red oaks. The exhibits significantly higher in its invasive range compared to its native range, where it coexists with a more diverse of native borers. It was previously treated as a of Agrilus coxalis but is now recognized as a distinct species.

Agrilus auroguttatus by Mike Lewis. Used under a CC BY 3.0 license.

Pronunciation

How to pronounce Agrilus auroguttatus: /ˈæɡrɪləs ɔːˌroʊɡʌˈteɪtəs/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

distinguished by combination of dull dark metallic green coloration with three spots per ; D-shaped exit holes approximately 4 mm wide are uniquely diagnostic among California oak subcortical . Extremely difficult to distinguish from Agrilus coxalis, which occurs in Mexico and Guatemala; separation requires examination of male or molecular analysis.

Images

Appearance

is dull dark metallic green, approximately 10 mm long, with three characteristic spots on each (). is , legless, and reaches 18 mm in length. Adults produce D-shaped exit holes approximately 4 mm wide on the outer bark of trees.

Habitat

Oak woodlands and mixed oak-pine forests in Mediterranean climate zones of southern California and semi-arid mountain ranges of southeastern Arizona. In California: elevations 900–1800 m with hot summers, winter precipitation, rocky well-drained sandy loam soils. In Arizona: elevations 1400–1750 m in semi-arid conditions. Larval development occurs in phloem and wood of lower main stem (bole) of oak trees.

Distribution

to mountain ranges in southeastern Arizona (Coronado National Forest). in southern California, first detected in 2004 at Cuyamaca Rancho State Park; confirmed as cause of oak damage in 2008. Current centered in Cleveland National Forest, San Diego County, with expansion to six of seven southernmost California counties. Modeled environmental suitability indicates potential for establishment in northern Sierra Madre Occidental and northwestern Mexican Plateau.

Seasonality

captured in from May through mid-October, with peak flight activity July through August. and or mature adults present within trees from April through August. present throughout most of the year. time likely one year for majority of individuals, though possibly longer or shorter for some.

Diet

Phloem and wood of oak trees (Quercus spp.), feeding in subcortical tissues of the lower main stem (bole).

Host Associations

Quercus kelloggii - primary California black oak, Section Lobatae; most affected in California
Quercus agrifolia - primary coast live oak, Section Lobatae; preferred for seasonal development studies
Quercus emoryi - Emory oak, Section Lobatae
Quercus hypoleucoides - silverleaf oak, Section Lobatae
Quercus chrysolepis - canyon live oak, Section Protobalanus; exhibits similar to red oaks despite taxonomic intermediate position
Quercus engelmannii - Engelmann oak, Section Quercus; colonized at very low

Life Cycle

Development occurs in phloem and wood of lower bole. Four larval identified based on peristoma measurements. Prepupal mortality increases significantly from winter (3%) to spring (36%). holes indicate completion of development. Recently dead trees (died within 1–3 years) sampled for studies.

Behavior

Occurs most frequently in lower portion of stem (≤1.52 m). Exit hole greater at root collar than at mid-boles. patterns on lower boles random at low densities, aggregated at high densities. activity peaks mid-summer.

Ecological Role

Primary biotic agent causing tree mortality in southern California oak forests. As , may alter borer structure; in California, significantly outnumbers native borers on red oaks (9.6±0.7 versus 4.5±0.6 holes per 0.09 m²). Extensive tree mortality increases wildfire risk in fire-prone region. Native borer community in Arizona includes diverse and that may be affected by competitive interactions.

Human Relevance

Major forest pest causing significant economic and ecological damage in southern California, with approximately 80,000 trees killed. Dead trees increase wildfire hazard. Management recommendations include avoiding transport of oak firewood to prevent spread. Subject of research, though candidate agents (including Pyemotes tritici ) have limitations. US Forest Service advises guidelines similar to those for emerald ash borer and bronze birch borer.

Similar Taxa

Agrilus coxalisExtremely similar ; previously treated as (A. coxalis auroguttatus). Occurs in Mexico and Guatemala. Separation requires examination or molecular analysis.
Agrilus planipennisEmerald ash borer; similar biology as Agrilus pest of , but attacks Fraxinus (ash) rather than Quercus, and occurs in eastern North America.
Agrilus anxiusBronze birch borer; similar biology as Agrilus pest that attacks living trees, but specializes on Betula (birch).

Misconceptions

Originally misidentified as Agrilus coxalis when first detected in California; early tree mortality attributed to drought stress until surveys revealed damage. Early estimate of 15,000 trees killed was later corrected to 80,000.

More Details

Taxonomic History

Originally described as Agrilus coxalis auroguttatus Schaeffer, 1905; elevated to rank as Agrilus auroguttatus. Much literature prior to 2008 refers to it by the synonym A. coxalis or A. coxalis auroguttatus.

Population Density Patterns

In California, on red oaks average 9.6±0.7 holes per 0.09 m², versus 0.9±0.2 in Arizona range. This 10-fold difference suggests release from or other factors in range.

Sampling Methodology

Three randomly placed 0.09 m² on lower boles provide reliable estimate of exit hole with <25% error from true mean, facilitating survey and efforts.