Biology and Ecology

Woodwasps, also called horntails, in the Hymenopteran Family Siricidae are robust, thick-waisted wasps that complete larval development in woody plants. Species in the subfamily Siricinae colonize conifers, while species in the subfamily Tremicinae colonize angiosperms. There are over 100 species of Siricidae worldwide. In their native ranges, these species function as forest thinning agents, attacking and reproducing within stressed or moribund trees. Others have been introduced outside their native ranges, where they have become pests. The most economically important species worldwide, Sirex noctilio, is native to Europe, Asia, and northern Africa where it is not a pest. Sirex noctilio has been introduced to many countries in the Southern Hemisphere where it has caused substantial economic losses, mainly in exotic pine plantations. Sirex noctilio has recently become established in North America.
Sirex cyaneus
by Whitney Cranshaw
Sirex nigricornis
by Erich Vallery
Sirex noctilio
by Kevin Dodds
Tremex columba
by Steven Katovich
Cambial staining from Amylostereum areolatum
by Dennis Haugen
Siricids are haplodiploid; unfertilized eggs develop into males, fertilized eggs develop into females. Females penetrate the bark with their ovipositor, laying eggs directly into the wood. Many species also carry a symbiotic fungus (S. noctilio carries Amylostereum areolatum) upon which young larvae feed and develop. The symbiotic fungus is injected into the wood during oviposition, and, in combination with a toxic mucus (some species), also weakens trees to create a more suitable habitat for larval development.
Adults are large insects, 2 – 5 cm long, although size is extremely variable, and may be related to suitability of the larval habitat. Number of larval instars varies from 6 to 12, and may also be related to host suitability. Larvae are creamy white, legless, and possess a posterior spine. To date, there are no keys to identify larvae to species. Keys are available for adults.
Adults create round exit holes, 3 – 9 mm in diameter, upon emergence from their larval host tree. Attack by females may illicit a response from the tree, resulting in resin beads or drips; these are visible on the bark surface, and are indicative of infestation.
Sirex noctilio larva
by Vicky Klasmer
Sirex noctilio pupa
by William Ciesla
Sirex noctilio exit hole
by Laurel Haavik
Resin beads on Pinus sylvestris
by Laurel Haavik


Sirex noctilio Worldwide Distribution
Sirex noctilio distribution - from Wikipedia based on Carnegie et al. 2006
North American Distributions of Some Woodwasp Species
S. abietinus and S. cyaneus North America distributions (both native)
from Schiff et al. 2012
S. noctilio (exotic) and S. californicus (native) North America distributions
from Schiff et al. 2012
S. nitidus North America distribution (native)
from Schiff et al. 2012
S. nigricornis North America distribution (native)
from Schiff et al. 2012


Primarily, management practices for siricids have been developed with S. noctilio in mind. Successful management programs consist of a combination of biological control agents and silvicultural techniques.
Important biological control agents include a parasitic nematode and several parasitoids. The nematode, Deladanus siricidicola (=Beddingia siricidicola; there are several strains, the kamona strain being the most virulent), effectively sterilizes S. noctilio eggs, causing the females to deposit nematode-filled eggs into trees. The nematode also has a free-living form that survives on the S. noctilio fungus, A. areolatum. The nematode is an ideal candidate for biological control programs, because it can be maintained for several generations on cultures of the fungus. Little is known of the biology and ecology of fungi and nematodes associated with other Sirex species. Parasitoids that attack early- (Ibalia leucospoides) and late-stage (including Rhyssa persuasoria and Megarhyssa nortoni) S. noctilio larvae have also been important biological control agents.
Silvicultural tactics are aimed at maintaining vigorous, healthy pine stands. Regular thinning is important to remove suppressed or weakened individuals and reduce competition for resources among remaining trees. Pines weakened by drought may be especially susceptible to S. noctilio.
Injecting nematodes for biological control
by William Ciesla
Rhyssa persuasoria
by Boris Hrasovec
Thinned Pinus sylvestris stand
by Laurel Haavik

Sirex links