EUROPEAN WOOD WASP
Sirex noctilio F. -- Siricidae
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Biological control attempts against the woodwasp are one of the very few large projects directed against a wood-boring insect. Woodwasps usually are considered secondary pests and attack dead or dying trees. Sirex noctillio occurs in Canada and throughout Europe but is most common in the Mediterranean area. It is somewhat specific to Pinus species (Spradbery & Kirk 1978), and is unique among the siricids in Europe in that it is able to kill standing green trees. Under the right circumstances, as occurred in New Zealand and Australia, this insect was able to cause serious losses to Monterey pine, Pinus radiata D. Don., plantations. The pest was first discovered on the North Island of New Zealand about 1900 but it was not until 1927 that it was abundant enough in exotic pine plantations for control to begin (Taylor 1981). High mortality occurred in P. radiata plantations between 1940-49 in New Zealand, and S. noctilio reached Australia in southern Tasmania in 1952 and Victoria in 1961 (Taylor 1976).
There is a special relationship of S. noctilio to a symbiotic fungus, Amylostereum areolatum (Fr.) Boidin, that serves as a kairomone for the parasitoids of the woodwasp. Also the parasitic nematode, Deladenus siricidicola Bedding, is wholly dependent in nature on the woodwasp and the fungus (Bedding 1972). Adults of S. noctilio emerge from midsummer to late fall and mate in the upper foliage of trees. Female wood wasps oviposit by drilling holes through the bark into the sapwood of trees that are usually predisposed or damaged. At the time of oviposition the symbiotic fungus is introduced (Taylor 1981). Adults live only a few days in nature. The eggs hatch when the surrounding area has been invaded by the fungi and this occurs when some drying has taken place to favor the fungi. First and second instar larvae feed exclusively on fungus and third and fourth instars begin to tunnel into the wood. The larvae turn back toward the bark to about 5 cm from the bark surface to enter the prepupal stage. Pupation may not occur until the second or third year after hatching, depending on the weather. After pupation adults emerge in about three weeks, and each generation emerges over a period of two to three years with the proportion of individuals emerging in the first, second and third year varying by site (Taylor 1981).
Biological control was initiated in New Zealand in 1927 (Taylor 1981). During 1929-32 the ichneumonid, Rhyssa persuasoria L. was introduced but the control was not satisfactory (Turnock et al. 1976). Then Ibalia leucospoides (Hochenw.) (Ibalidae) was colonized in 1954-58 which resulted in improved control (Zondag 1959). The two parasitoids were then colonized in Tasmania. A large scale biological control effort did not begin until 1961 following the discovery of S. noctilio in Victoria, Australia. A National Sirex Fund was established, which consisted of a consortium of federal, state and private agencies, and a committee was formed to coordinate research and control in Victoria (Taylor 1981). A world wide search for natural enemies was begun by the Division of Entomology, CSIRO in 1962. The search for parasitoids in the northern hemisphere was completed by 1973, and during the 11-year period 21 species of parasitoids were sent to Tasmania for culture (Taylor 1976). The plan was to obtain all the available parasitoids of siricids in conifers and as many strains as possible from different climatic zones with emphasis on the Mediterranean area. This included collections of siricids in conifers other than Pinus and from genera and species other than Sirex noctilio. Ten different parasitic species were released in Tasmania and Victoria, six having become established and one additional species, the ichneumonid Rhyssa hoferi Rohwer, probably established (Taylor 1981). Of the seven species two are holarctic (R. persuasoria and I. leucospoides), two are palearctic (I. rufipes drewseni Borries and the ichneumonid Odontocolon geniculatus Kreichbaumer) and three are nearctic [the stephanid Schlettererius cinctipes Cresson and the ichneumonids Megarhyssa nortoni (Cresson) and R. hoferi].
These species tend to be complementary, although there might be some competition within the guild attacking larger larvae. The Ibalia species attack first or second instar siricid larvae and the two species have different emergence times so that they do not compete directly. The ichneumonids attack the more developed larvae of their host and there may be differential preference based on tree diameter (Taylor 1981). Schlettererius cinctipes emerges after the peak emergence of the ichneumonids, while the other two are also complementary as O. geniculatus is small, emerges in springtime and attacks late hatching larvae that are still closer to the bark surface. Rhyssa hoferi is adapted to drier areas and could do well in drier climates (Taylor 1981).
A parasitic nematode, Deladenus siricidicola, was found in New Zealand in 1962 (Zondag 1969). It causes female wood wasps to lay infertile eggs. Additional nematodes wee sought during 1965-75 without success (Bedding & Akhurst 1974). Different strains of the nematode have also been released throughout wood wasp infested areas in Tasmania and Victoria and it is well established throughout. This nematode also affects the reproduction of some of the female parasitoids (Bedding 1967), which apparently does not adversely affect biological control (Dahlsten & Mills 1999). The nematode is credited with reductions of wood wasp populations to very low levels in certain areas.
Dahlsten & Mills (1999) consider the Sirex noctilio biological control program significant for several reasons. A large group of organizations cooperated in a well funded, extensive worldwide search for parasitoids as well as a research program that examined many aspects of the host tree/Sirex/fungus/parasitoid relationships (Taylor 1981). As with Gilpinia hercyniae there was a fortuitous introduction (the nematode). Sirex noctilio was introduced from the northern to the southern hemisphere and attacked an exotic host plant Pinus radiata (native to California). The search for parasitoids in the north was made from S. noctilio and its host trees to siricids in other genera and species in Pinus as well as other conifers. The project was well planned with attention given to colonizing strains of parasitoids suited to different climatic zones and developmental stages of the host.
It is believed that this biological control project will eventually be completely successful (Turnock et al. 1976). It has been thought that the combination of parasitoids and nematodes along with sound forest management should hold S. noctilio down to the level where losses are not serious (Taylor 1976). For details of the biologies of host, natural enemies and biological control efforts, please see also the following (Chrystal 1928, 1930; Chrystal & Myers 1928a,b; Myers 1928, Miller & Clark 1935, 1937; Clark 1936, Rawlings 1951, 1952, 1953; Stilwell 1960, Zondag & Nutall 1961, Zondag 1965, 1967; Wilson 1965, Dharmadhikari & Achan 1965, Morgan & Stewart 1966, Taylor 1965, 1967a, 1967b; Dharmadhikari 1967, Hocking 1967, Morgan 1968, Clausen 1978).
REFERENCES: [Additional references may be found at: MELVYL Library ]
Bedding, R. A. 1967. Parasitic and free-living cycles in entomogenous nematodes of the genus Deladenus. Nature 214: 174-75.
Bedding, R. A. 1972. Biology of Deladenus siricidicola (Neotylenchidae) an entomophagous-mycetophagous nematode parasitic in siricid woodwasps. Nematologica 18: 482-93.
Bedding, R. A. & R. J. Akhurst. 1974. Use of the nematode Deladenus siricidicola in the biological control of Sirex noctilio in Australia. J. Aust. Ent. Soc. 13: 129-35.
Chrystal, R. N. 1928. The Sirex wood-wasps and their importance in forestry. Bull. Ent. Res. 19: 219-47.
Chrystal, R. N. 1930. Studies on the Sirex parasites. The biology and postembryonic development of Ibalia leucospoides Hochenw. (Hymenoptera-Cynipoidea). Oxford Univ. Mem. 11. 63 p.
Chrystal, R. N. & J. G. Myers. 1928a. The Sirex wood-wasps and their parasites. Empire Forestry J. 7: 145-54.
Chrystal, R. N. & J. G. Myers. 1928b. Natural enemies of Sirex cyaneus Fabr. in England and their life-history. Bull. Ent. Res. 19: 66-77.
Clark, A. F. 1936. Biological control of forest insect pests. New Zeland J. Sci. & Tech. 18: 585-88.
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