Overview and Introduction to North America
The family Siricidae contains more than 120 species distributed across forests of the Northern Hemisphere. In native ranges these species are typically minor or secondary pests (Wilcken et al. 2025). Several members of the family have been transported in international trade; in one study, 84.2% – 97.6% of woodwasps were detected in wood packaging. Widespread adoption of the International Standard on Phytosanitary Measures Number 15 (ISPM#15) has significantly reduced interception, establishment, and damage of Siricids in the United States, Canada, and New Zealand since 2002. However, continued detection of live Siricids in wood packaging demonstrates the need for strict enforcement of the standard’s provisions and potentially adoption of additional mitigation strategies (Nardi et al. 2026).
One woodwasp species, Sirex noctilio, has invaded pine stands (native and nonnative) in nine countries in Oceania, Africa, and South and North America. Four other species—Urocerus gigas, Urocerus flavicornis, Tremex fuscicornis, and Sirex obesus—have been detected in South America (Wilcken et al. 2025). Its associated fungus, Amylostereum areolatum, has numerous strains native to much of the Northern hemisphere including North America, Eurasia, and North Africa (Hajek, Haavik, and Stephen, 2021).
Like other xylem-chewing insects (e.g., ambrosia beetles), siricids have co-evolved with mutualistic fungi that break down this material into compounds that can be readily assimilated. See Hajek, Haavik and Stephen (2021) for a full description of the complicated Sirex/Amelosterium pest complex life cycle.
Introduction to North America
In North America, S. noctilio was repeatedly intercepted in wood packaging before implementation of the ISPM#15 in 2006). Over the period 1993 – 2001, USDA APHIS inspectors found 103 shipments containing wood packaging infested by S. noctilio. The site where the woodwasp was initially detected is along the St. Lawrence Seaway, a major route for imports. Analysis of the woodwasp’s and the fungus’ genetic patterns indicates that S. noctilio was introduced at least twice to North America, once from an unknown location in Europe (the native range) and once from South America (where “bridgehead” introductions have established in the pine plantations of several countries).
The woodwasp was detected in a trap (part of the USDA/CAPS woodborer trapping program) in Oswego, New York near Lake Ontario in 2004. Subsequent delimitation surveys revealed that the woodwasp had been present for many years; eradication was thus considered infeasible. By 2020, the woodwasp and associated fungi were found in nine Northeastern states (New York, Pennsylvania, Vermont, Connecticut, Ohio, Michigan, New Jersey, Massachusetts and New Hampshire) and two Canadian provinces (Ontario and Quebec). The woodwasp appears to be spreading but data are sparse since neither country operates a coordinated survey effort.
Because of the well-documented impact of S. noctilio in the Southern Hemisphere (see below), upon detection the USDA undertook several efforts to determine the level of threat posed by S. noctilio to US forests. The USDA Forest Service funded a study of the fundamental biology and ecology of native North American Siricidae in order to obtain baseline knowledge before S. noctilio could significantly impact the environment. The incorporated economic risk assessment (USDA Forest Service, 2006) projected that the woodwasp could spread across the entire southern pine region in 55 years or less, and cause damage ranging from $2 billion to $11 billion. USDA APHIS issued a pest risk analysis in July 2006 (Borchert, 2006) and sponsored two scientific workshops and briefing for stakeholders (in January 2007 and May 2011). These efforts produced no support for regulations. Some scientists questioned the risk because pines native to North America have numerous other associated insects and expected that the management tool used in Southern Hemisphere countries – the parasitic nematode – (see below) could be applied. The parasitic nematode present in North America is yet to be shown as a successful management tool. North Carolina established an external quarantine in 2008, but other states did not follow suit.
For several years APHIS funded surveys using a small appropriation by Congress (e.g., $1.5 million in FY2009). By 2021 there was no longer a systematic survey or monitoring program. The result is considerable uncertainty in baseline knowledge such as where the complex is established. Hajek, Haavik and Stephen (2021) call on forest managers to monitor voluntarily for S. noctilio presence and population growth, especially in pine stands that are near thinning harvest dates, and during and after drought.
Impacts in Northeastern North America
There are no long-term data on Sirex noctilio’s impact on pine forests across the northeastern United States (or, apparently, eastern Canada). Attempting to measure such impacts proves difficult because 1) determining whether a tree’s death was caused by the woodwasp must occur within a short period of time, 2) finding infestations is impeded by the lack of a good lure, and 3) highly fragmented pine stands limit comparative studies. What is clear, however, is that in northeastern North America, the wasp feeds primarily on the native red (Pinus resinosa) and introduced Scots pines (P. sylvestris). In contrast, the native jack pine (P. banksiana) is attacked with low frequency, and the eastern white pine (P. strobus) is rarely attacked. These pines grow primarily in widely scattered stands, most of which were planted 70+ years in the past with limited management today. In all studies, pine mortality has often been restricted to suppressed and intermediate crown classes and damaged or dying trees; however, the small sizes of the studies and difficulty in comparing study results from stands with varied conditions obscures efforts to determine whether impacts are driven primarily by a host species’ vulnerability or by stand conditions.
S. noctilio is now the most abundant siricid colonizing pines in northeast North America and has almost certainly impacted native insect communities. In the absence of data, however, it is unknown whether S. noctilio has negatively affected native siricids with a competing population density, or positively affected them by created an abundance of weakened host trees that native siricids can exploit.
A native congener, Sirex nigricornis, is found across eastern North America and as far west as Alberta. It is the only native woodwasp in eastern North America that exclusively uses pine as its host. It colonizes only severely weakened trees. S. nigricornis and the introduced S. noctilio can attack the same host tree. In almost all instances of co-colonization, S. noctilio greatly outnumbers S. nigricornis. Future interactions between the two woodwasps are difficult to predict. Several native hymenopteran parasitoids parasitize up to 25% of S. noctilio larvae (see Hajek, Haavik and Stephen, 2021 Chapters 6, 7, and 8). All are solitary parasitoids, meaning that one S. noctilio larva yields one parasitoid. These parasitoids do not appear to be a major regulator of the woodwasp’s population dynamics.
Other native and introduced wood-boring insects in North America that attack weakened pines carry their own fungal symbionts (e.g., the bluestain fungus Ophiostoma species.). These fungi often outcompete and suppress the siricid fungus Amylostereum areolatum on which S. noctilio larvae depend for feeding. Researching this issue presents a challenge since the same tree can harbor multiple cohorts of different siricids and different parasitoids, all of them impossible to identify as larvae.
Unclear danger to other parts of North America
S. noctilio’s spread is apparently slower than as initially feared. As noted, to date little impact has been documented. If it continues to spread, the woodwasp will eventually reach the Southeast and Wes, where pines dominate both natural and planted forests. Several of the pine species comprising these forests have been killed by the woodwasp’s attack in plantations in the Southern Hemisphere. In the Southeast, these include loblolly (P. taeda) and slash (P. elliottii) pines; in the West, Monterey pine (P. radiata), lodgepole (P. contorta) ponderosa (P. ponderosa). Mortality of these species has varied among the invaded countries. At the southern and eastern edges of its North American invaded range, S. noctilio has probably already come into contact with Virginia pine (P. virginiana) and pitch pine (P. rigida), but no attacks on these species had been reported as of 2020. Hartshorn (Chapter 9) and Foelker and Haavik (Chapters 10 and 11) discuss possible interactions between S. noctilio and native insect communities of the Southeast and West but are unable to reach firm conclusions. (Several of the western pines are already under attack by the native mountain pine beetle [Dendroctonus ponderosae].)
Climatic suitability is also open to question. One climate model applied in one study found climate suitability for S. noctilio in the Southeast but only marginal suitability in the West. However, the same model wrongly classified the Argentine Patagonia as marginally suitable; in actuality, S. noctilio is causing significant disease there. Foelker and Haavik note that overstocking and drought stress are closely tied to S. noctilio population outbreaks in the Southern Hemisphere. Both conditions are common in the southeast and western regions of North America.
Tetley and Hajek (Chapter 2) conclude that the impacts of the Sirex/Amelosterum complex in the Americam Southeast and West will ultimately depend on the interplay of tree species’ varying susceptibility, management practices, S. noctilio life history traits, natural enemy and competitor communities, and landscape-level or environmental factors that influence S. noctilio dispersal and the availability of suitable hosts. Foelker and Haavik (Chapter 10) say scientists cannot yet specify the quantitative contributions of the three forms of biotic resistance (tree resistance, natural enemies, and competition from other pine-colonizing insects and their associated fungi).
Hajek, Haavik and Stephen (2021) do not discuss the implications for forests intended to be unmanaged, such as in National parks and Wilderness areas.
Management
While the introduced parasitic nematode Deladenus (formerly Beddingia) siricidicola has proved a successful control of the Sirex/Amelosterum complex in some South American countries, the strain introduced inadvertently to North America along with the woodwasp does not sterilize the introduced woodwasp. This “introduced to North America strain”, or INA strain, does parasitize a native North American beetle, Serropalpus substriatus. These factors have impeded efforts to manage S. noctilio in North America. There are, however, five species of Deladenus associated with native siricids and their fungal symbionts. Hajek and Morris (Chapter 7) suggest that North American foresters should explore use of a native nematode, congeneric Deladenus proximus, as a biocontrol. According to Hartshorn (Chapter 9), D. proximus has been inconsistent in regulating S. noctilio.
The INA strain of the Deladenus siricidicola nematode does have a negative impact on the woodwasp because parasitized adults are somewhat smaller, so likely to fly shorter distances and carry fewer eggs.
Sirex invasions in the Southern Hemisphere
Several Southern Hemisphere countries, including Australia, New Zealand, South Africa, and South American countries, have planted huge areas in pines to produce lumber and other products. According to Google, Australia and New Zealand each have ~ 1.6 million hectares of pine plantations. In South America pine plantations occupy 4.6 million hectares (Lantschner and Villacide, 2025). In these countries, separated from pests in their native ranges, pine plantations produced prodigious quantities of fiber. Brazil and Chile became “planted forest powerhouses.” Uruguay and Argentina followed their lead (Payn et al. 2015).
However, international trade has transported some of the pests from regions where pines are native to the plantation realms in the Southern Hemisphere. The intensively managed pine plantations are comprised of closely-spaced trees, all at the same developmental stage, with minimal genetic diversity. These practices led to reduced resilience to pests. Sirex noctilio has become the most significant economic pest of Pinus species in these plantations. Its attacks can cause up to 80% mortality (Villacide and Fuetealba, 2025). Other woodwasps in the family have also been introduced: as noted, four other species—Urocerus gigas, Urocerus flavicornis and Tremex fuscicornis, and Sirex obesus—have been detected in South American pine plantations (Wilcken et al. 2025).
Foresters in several of the Southern Hemisphere countries have reduce damage caused by the woodwasp/Amylosterium complex by thinning trees to reduce density and introducing a parasitic nematode Deladenus (formerly Beddingia) siricidicola. One strain of this nematode, the Kamona strain, sterilizes the female woodwasps, thus reducing woodwasp populations and impacts. Some countries also use classical biocontrol relying on parasitic wasps. Success of these management strategies varies among the invaded countries (see Hajek, Haavik and Stephen, 2021 for details).
Capture Project
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort. The sirex woodwasp is not known to be a threat to any of these 15 most vulnerable species.
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