In the summer of 2004, forest pest detection traps set up by California Department of Food and Agriculture (CDFA) detected the Mediterranean pine engraver beetle (Orthotomicus erosus Wollaston) at two sites near Fresno (Lee et al. 2005; Devorshak 2005). The beetle was quickly discovered to be widespread in the Central Valley. Within a few years, several factors led scientists to conclude that the beetle had been introduced perhaps three years before it was detected. These factors included the beetles’ abundance throughout the eastern half of the southern Central Valley; the distance between outbreaks; and symptoms of earlier beetle infestation in isolated pine trees in remote locations (Seybold and Downing, 2009; Seybold, Penrose and Graves 2016).
In the years since, the Mediterranean pine engraver beetle has continued to spread. By 2015, it was known to be established throughout the Central Valley as far north as Shasta County. In 2012 a beetle was trapped at an entrance to Yosemite National Park in Tuolumne County. The beetle has also been detected as far east as Clark County, Nevada (near Las Vegas) (Seybold, Penrose and Graves 2016); nearby St George, Utah; and Phoenix (Graves, 2021) and Tucson, Arizona. Trapping efforts continue in the periurban forested areas throughout southern Arizona and New Mexico where native pines occur (Graves pers. comm.). The beetle is moving at about 50 km / year (Graves, 2021). Transport of firewood appears to be one the important contributors to this movement. It has been noted that Interstate 10 provides a direct link between the southwestern and the southeastern United States – an area where many pines grow and where pine forestry is an important economic resource (Graves, 2021)].
The location of the first (apparent) establishment of O. erosus – the California Central Valley – does not overlap with the distributions of native pines in California. The pines on which the beetle initially established were artificial plantings, primarily of Eurasian pine species that would have been familiar hosts to the beetle. These included Aleppo pine (Pinus halpensis), stone pine (P. pinea), Canary Island pine (P. canariensis), Scots pine (P. sylvestris), and Turkish pine (P. brutia).
However, some native pines are grown in the Valley and were quickly discovered to host O. erosus. These included Monterey pine (P. radiata D. Don), in a planting of Christmas trees in Tulare Co.; and gray pine, (P. sabiniana Douglas ex D. Don) (Seybold, Penrose and Graves 2016). Before 2020, outbreaks had reached regions of native coniferous forest in the Sierra Nevada and near Phoenix (Seybold, Penrose and Graves 2016; Graves 2021) although as of 2021, O. erosus has not been found infesting native pine trees in either California or Arizona (Graves pers. comm.).
Mode of Introduction
Introduction of the Mediterranean pine engraver beetle was not surprising, as this species had been the second most frequently intercepted scolytid species in wood packaging at U.S. ports during the period 1985 and 2000. O. erosus was intercepted 385 times during this period. Only a small proportion of those interceptions – 15 – were in California (Seybold, Penrose and Graves 2016). The climatic match between southern California and the Mediterranean region may explain the beetle’s successful establishment in California but not elsewhere in the U.S.
The Mediterranean pine engraver beetle is native to Europe, the Middle East, northern Africa; Central Asia and China. It had previously been introduced to Fiji, Chile, South Africa and Swaziland (Lee et al. 2005; Devorshak 2005). The large native range is reflected in the detection record: between 1985 and 2000, O. erosus was intercepted in packaging from at least 19 different countries, especially Spain, Italy, China, and Turkey. It was most commonly found in crates holding imported tiles, marble, and granite (Haack, 2004). Before 1995, the U.S. did not require any treatment of wood packaging but relied on inspection. In that year, USDA APHIS adopted regulations requiring that wood packaging be stripped of bark. Even after that action, however, shipments from Italy often still had pests (Campbell and Schlarbaum, 2002).
The Mediterranean pine engraver beetle is usually considered to be a secondary pest of pines. However, several experts considered the risk to North American pines to be significant for several reasons. Indeed, Aukema et al. (2010) considered O. erosus to be one of 62 “high impact” nonindigenous, invasive forest insects in the continental USA (Seybold, Penrose and Graves 2016).
Two factors guided these heightened concerns. First, the beetle has caused significant damage to stressed pines on other continents (Seybold and Downing, 2009). Second, the beetle’s wide host range includes 11 pines native to North America: eastern white pine (Pinus strobus), gray pine (P. sabineana), jack pine (P. banksiana), Jeffrey pine (P. jeffreyi), loblolly pine (P. taeda), Monterey pine (P. radiata), ponderosa pine (P. ponderosa), red pine (P. resinosa), lodgepole pine (P. contorta), singleleaf pinyon pine (P. monophylla) and sugar pine (P. lambertiana). The beetles also feed and reproduce on several non-pine conifers, including Douglas-fir (Pseudotsuta menziesii); black spruce (Picea mariana) and white spruce (P. glauca); and tamarack (Larix laricina). Relatively few progeny are produced on the non-pine hosts (Lee et al. 2008).
In addition to direct feeding, the insect can transmit fungi, some of which might be pathogenic (Lee et al. 2005; Devorshak 2005). Documented examples include Ophiostoma ips (Rumb.) Nannf., causal agent of bluestain fungus; Leptographium lundbergii Lagerb. & Melin; Graphium pseudormiticum Mouton & Wingfield; and the pitch canker fungus, Fursarium circinatum Nirberg and O’Donnell (Seybold and Downing, 2009). In California, it has been determined that Ophiostoma ips dominates the mycoflora of O. erosus overwintering in exotic pines (Seybold, Penrose and Graves 2016). A third factor is that the Mediterranean pine engraver beetle probably completes three generations per year in California’s Central Valley. It possibly even begins a fourth generation in November that overwinters in the immature stages (Seybold, Penrose and Graves 2016). This short life cycle allows rapid buildup of the insect’s population.
Despite California’s pines hosting many native woodborers and natural enemies thereof, so far O. erosus populations in California have been able to build up extremely large populations and there is not much evidence of either natural enemies or interspecific competition (Seybold, Penrose and Graves 2016).
The Mediterranean pine engraver beetle usually infests recently fallen trees and slash, but it also attacks stressed living trees. Scientists agree that the greatest economic damage would probably occur in pine plantations established in conditions under which the trees suffer stress, particularly drought. Native North American pine forests are already often subject to drought. Climate change is expected to increase both the frequency and intensity of drought in California. The Southeast is expected to experience more extreme weather events. These and other stress factors make both natural pine forests and plantations in these regions vulnerable to O. erosus (Eglitis, 2000; Devorshak, 2005). Furthermore, the Mediterranean pine engraver beetle can kill trees following various disturbances, including thinning followed by drought, forest thinning alone, and fire (Seybold and Downing, 2009) and likely most natural disturbances.
Areas with Mediterranean climates – including California and Mexico – and the southeastern United States are considered to have climates suitable to the Mediterranean pine engraver beetle (Eglitis, 2000; Devorshak, 2005; Seybold 2006 pers. comm.). As noted above, pines native to both regions – e.g., lodgepole, ponderosa, loblolly — are hosts for the beetle. Monterey pine is a narrowly endemic species listed by the World Conservation Union as endangered (Beckman et al. 2021), and it is already under severe stress from the pitch canker fungus (Fusarium moniliforme var. subglutinans Wollenw. & Reinking) (Seybold and Downing, 2009; Devorshak, 2005). Studies of the Mediterranean pine engraver beetle’s cold tolerance indicate it could probably survive as far north as Maryland (Hayes and Lundquist, 2009). In the East, O. erosus might have to compete against the introduced pine shoot beetle, Tomicus piniperda L. In its native and intro range, the Mediterranean beetle competes poorly against this beetle (Seybold, Penrose and Graves 2016).
The potential for damage to urban forests is significant. As noted above, four pines from the beetle’s native range in Eurasia – Aleppo, Canary Island, Italian stone pine, and Scots pine – are often used in landscaping.
Expansion of its range into the Sierra Nevada, Coastal, and Transverse mountain ranges would likely lead first to feeding and reproduction in native California foothill pine (Pinus sabiniana Douglas ex D. Don), which encircles the Central Valley at foothill elevations (500–1000 m). At higher elevations, the Mediterranean pine engraver beetle might attack potentially Sierra lodgepole, Jeffrey, sugar, and ponderosa pines — although the beetle’s cold hardiness limits might restrict its invasion into these ecotones. O. erosus has already spread into parts of the Los Angeles Basin and Inland Empire regions, to the Tehachapi mountain range (where the native single-leaf pinyon pine is found) and to adventive stands of Monterey pine in the coastal locations to the west of the Central Valley. Urban Las Vegas, Nevada (Clark Co.) has a comparable habitat and urban host type as the Los Angeles Basin. Orthotomicus erosus is generally considered widespread in urban Las Vegas, NV (Graves pers. comm.). In all of these instances, collection of O. erosus in these California native hosts has not yet occurred and the impact of O. erosus on these trees in native forest areas has not been quantified. (Seybold, Penrose and Graves 2016).
Neither California nor USDA APHIS has instituted any controls on the movement of potentially O. erosus infested wood or nursery stock. Scientists continue to study the insect (USFS Research & FHTET); and monitor its spread in California. APHIS considers the Mediterranean pine engraver beetle to be an “actionable” pest – which means that steps are taken when the insect is found infesting imported items or packaging (Seybold and Downing, 2009).
The United States began applying the international standard requiring treatment of crates, pallets, and other forms of wood packaging in 2006. Studies indicate that this regulation has reduced the “approach rate” of pests in wood packaging by 36 – 52%. The infestation rate is estimated at 0.001% (1/10th of 1%) (Haack et al. 2014). Given the tens of millions of shipping containers that enter the U.S. every year, this apparently small risk multiplies into actual arrival at our shores of potentially tens of thousands of containers harboring pests associated with wood packaging each year- presenting the risk of introduction of additional O. erosus populations.
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