IMPORTANCE OF OAK AMBROSIA BEETLES IN PREDISPOSING COAST LIVE OAK TREES TO WOOD DECAY

Journal of Arboriculture, Nov 2004 by Svihra, Pavel, Kelly, Maggi

Abstract. Attacks by the oak ambrosia beetle (Monothrum scutellare) accelerated and increased the amount of wood decay in stems of downed coast live oak (Quercus agrifolia) trees. When permethrin insecticide was sprayed on oak bark surface, the ambrosia beetles produced only one-fourth as many galleries in the sapwood as compared to sapwood beneath the unsprayed bark surface. Although decay fungi initiated infection in oak in the absence of beetles, the amount of wood decay was much greater in trees with beetle activity. The ambrosia beetles deeply penetrated the sapwood and constructed extensive galleries that hastened wood decay by decay pathogens. The insecticide inhibited beetle attack and formation of egg galleries and greatly decreased the amount of wood decay.

Key Words. Coast live oak; Quercus agrifolia; sudden oak death; Phytophthora ramorum; ambrosia beetles; decay progression.

Coast live oak trees (Quercus agifolia) are injured by as many as 297 insect species and 148 fungus pathogens in California, U.S. (Swiecki et al. 1997). Phytophlhora ramorum is a major causal agent of sudden oak death disease (SOD) in coastal California forests (Rizzo et al. 2002; Rizzo and Garbelotto 2003). The conditions that predispose several oak species in the red oak group (Erythrobalanus), including Q. agnfolia, Q. parvula var. shrevei, and Q. kelloggii, to infection by P. ramorum are not completely understood. However, sequential symptoms of SOD [(exudation of sap on lower stems, attacks by oak bark beetles (Pseudopityophthorus pubipennis) and oak ambrosia beetles (Monothrum scutellare) near bleeding cankers, and production of Hypoxylon thouarsianum basiodiocarps that follows P ramorum infection] have been well described (Svihra 1999a, 1999b; McPherson et al. 2000; Svihra 2001; Storer et al. 2002; Svihra 2004). Stem breaking of coast live oaks with SOD symptoms often occurs when the leaves are still green (Swiecki and Bernhardt 2003; Svihra et al. 2004). Such trees may become hazards. Tree wounding may precede wood decay (Filip 2001), with the extent of decay influencing the time of stem breakage. Decay is influenced by several factors, including wood and site characteristics, as well as a variety of physical disturbances (Kaank 1974).

Ostry and Anderson (1998) studied the role of insects, woodpeckers, and Hypoxylon canker (H. mammatum) in the infection and decay processes on aspen (Populus tremuloides). Both the amount of infection and wood decay were favored by (1) deep wounding in the xylem and (2) frequent wounding by cerambycid beetles, cicadas, tree hoppers, and woodpeckers. Although P. ramorum cankers typically do not penetrate the xylem of coast live oaks by more than 3 cm (1.2 in.) (Rizzo et al. 2002), galleries of oak ambrosia beetles penetrate the xylem to depths of 5 to 15 cm (2 to 6 in.) and may play an important role in wood decay. By comparison, P. pubipennis beetles penetrate the xylem much less deeply but enough to impede water transport in xylem vessels (Svihra et al. 2004). When the stems of coast live oak trees break, dense egg galleries of oak ambrosia beetles are often found deep in the sapwood, together with extensive early wood decay adjacent to such galleries. Near the height of oak stem breakage, fruiting bodies of Hypoxylon thouarsianum often are present on bark surfaces. The fruiting bodies of Inonotus andersonii may appear several months later.

With the foregoing considerations in mind, a study was conducted to determine whether construction of galleries and brood development by oak ambrosia beetles affected the infection process as well as the rate and progress of wood decay in coast live oak. Specifically, we wanted to compare decay progress and its severity after the infestation and subsequent brood development of M. scutellare was prevented or retarded by permeihrin insecticide.

METHODS

The study was conducted in Miwok Park, located in the City of Novato, California. The park is situated in a rolling landscape covering 10 ha (24.7 ac) of forest area at elevations between 21 and 84 m (69 and 277 ft). The dominant overstory forest trees are coast live oak and California bay laurel (Umbellularia calijornica). California black oak (Q. kelloggii), madrone (Arbutus menziesii), and California buckeye (Aesculus californica) are scattered among the dominants. Associated shrubs and vines include manzanita (Arctostaphylos manzanita), toyon (Heteromeles arbutifolia), poison oak (Toxicodendron diversilobium), and honeysuckle (Lonicera hispidula). A survey conducted in the fall of 2001 showed that 17 coast live oaks and four California black oaks displayed SOD symptoms. Six coast live oaks near park trails were dying or dead and had to be removed, and the stems of eight trees broke in the forest some distance from park trails. The presence of the SOD pathogen was confirmed on live sampled coast live oaks in laboratories at the University of California Davis and the California State Department of Food and Agriculture, Sacramento.