Sirex noctilio (woodwasp)

S. noctilio belongs to the order Hymenoptera, sub-order Symphyta, family Siricidae and sub-family Siricinae. Siricids develop inside the tree trunks of several species and are commonly known as woodwasps or horntails. This group is associated with conifers and angiosperms of northern hemispheric origin (Smith, 1978).

S. noctilio is a minor pest in its native regions of Europe, Asia and North Africa. It has become the main pest in pine plantations where it has been introduced in countries such as New Zealand (1900), Australia (1951), Uruguay (1980), Argentina (1985), Brazil (1988) and more recently, South Africa (1994) (Rawlings and Wilson, 1949; Gilbert and Miller, 1952; Iede et al., 1988; Rebuffo, 1990; Echeverria, 1991; Tribe, 1995). In Chile, S. noctilio was detected in January 2001 in an area with no commercial plantations of radiata pine (Ciesla, 2003). The pest was subsequently eradicated but was later found in Los Lagos and has now spread to other parts of Chile.

According to Smith (1978), S. noctilio is found in Austria, Azores, Belgium, Cyprus, Denmark, Finland, France, Germany, Greece, Hungary, Italy, England, Mongolia, Norway, Poland, Portugal, Romania, former Czechoslovakia and former USSR. It is also found in the Canary Islands (Ortega and Baez, 1986), Estonia (Heidemaa et al., 1998) and the Federal Republic of Yugoslavia (Grujic, 1979).

Siricids are usually intercepted at ports of entry in South America (Chile) and North America (Haack and Cavey, 2000).S. noctilio probably entered New Zealand and Australia from Europe via sea ports, in cargo heavily infested with larvae and adults of siricid species (Rawlings and Wilson, 1949; Neumann et al., 1987). In South Africa and South America, it could have been introduced in solid wood packing material (SWPM) (Tribe, 1995; Iede et al., 2000a). This pest is an A1 quarantine pest for several areas: for example, USA (Kliejunas et al., 2001), Japan and Canada. These countries may require a phytosanitary certificate for pine logs. Infested wood can only be transported outside the infected area once it has been properly treated. The wood used in SWPM must be free from signs of S. noctilio activity, damage or insect specimens.S. noctilio can naturally spread between 30 and 50 km per year. However, the transportation of wood from attacked areas to plantations where the pest remains undetected, increases the possibility of dispersal. This is probably how S. noctilio was introduced from Uruguay to Brazil. Consequently the monitoring of affected areas and the prohibition of wood transportation from attacked to non-attacked areas are strategies created to avoid pest dispersal.

S. noctilio disperses by several short, powerful flights. The natural spread rate is about 20 to 50 km per year (Zondag and Nuttal, 1977; Taylor, 1981; Neumann et al., 1987; Iede et al., 1993).The adults, pupae, larvae and eggs may be carried on solid wood packing material (SWPM), in logs and in saw timber in the internal, regional or international trade (Haugen and Iede, 2001).The shipment of logs is a primary pathway for transporting wood borers. Another important pathway is in green, untreated saw timber, especially if this material has large dimensions and the wood does not rapidly dry out. Untreated SWPM has been identified as a high-risk pathway in international trade. Low-quality wood is frequently used for making crates, pallets and other SWPM (Haugen and Iede, 2001). The Quarantine Pest Inspection Services have intercepted siricids in SWPM in several countries.

S. noctilio is endemic to Eurasia and North Africa, with high density populations in the Mediterranean zone. It shows a preference for species of the genus Pinus, but also attacks Abies, Picea, Larix and Pseudotsuga species (Madden, 1988).

The most important symptom is the progressive and irreversible chlorosis in the crown, which turns a reddish-brown. This is accompanied by a sudden wilting of the foliage, heavy needle fall, and finally death and decay. Initially it is important to inspect the surfaces of the stems for resin drops. Sometimes small resin flows are visible in the stem bark. Narrow bands of brownish fungal stain in the outer sapwood can be noted in infested trees. Other symptoms include larval galls along the wood grain that contain compacted frass, and circular emergence holes that are 5.5 mm in diameter (Neumann et al., 1987; Iede et al., 1993, 1998).

In general, the life cycle of S. noctilio lasts about 1 year, but there is a short period or summer cycle, when the life cycle takes 3 to 4 months. In cooler climates the life cycle can take 2 to 3 years. The woodwasp population is found in the centre and lower half of the upper third of the stem (Penteado et al., 1998). The pattern of adult emergence varies considerably under different climatic conditions. In Australia, the adults emerge between early summer (December) and early autumn (April), with a peak emergence in February (Neumann and Minko, 1981; Neumann et al., 1987); and from mid-spring (October) to early autumn (April) in Brazil. The emergence is followed by attacks on trees and in Brazil the peak emergence occurs between November and December (Carvalho et al., 1993; Iede et al., 1998).The adult longevity varies from 5 to 12 days for the males and from 4 to 5 days for the females in the summer (Neumann et al., 1987; Iede et al., 1988; Carvalho et al., 1993) and 14 days for both sexes in the autumn (Neumann et al., 1987).The different volatiles produced by the phloem and cambium tissues of the stems and large branches of living trees are important in attracting S. noctilio. In its native regions, S. noctilio normally develops in trees that have been damaged or have died due to biotic and abiotic factors, such as fire, wind, other insects, diseases, snowstorms, mechanical operations (pruning), drought or nutritional stress. In a review of major outbreaks in Australasia, Madden (1988) found that most pest outbreaks were characterized by managerial or environmental events (e.g. fire, wind damage, and thinning and harvest operations during the flight season of S. noctilio). The outbreaks are only limited by an absence of suitable host material.After the initial flight period, the females penetrate the tree trunks with their ovipositors and lay their eggs in the sapwood. The eggs that are laid by unmated females hatch to produce males and the eggs from mated females produce females and males. The females lay around 210 to 226 eggs, but the largest females lay 300 to 500 eggs (Morgan and Stewart, 1966; Morgan, 1968; Carvalho et al., 1993). During this process, the females introduce arthrospores of the symbiotic fungus, Amylostereum areolatum, along with a mucous secretion that causes toxicity and subsequent death of the trees. This pathogen, which is a source of nutrients for the pest larvae, dries up the wood and makes it rot. The incubation period of the egg is 14-28 days (Morgan and Stewart, 1966; Morgan, 1968). The larvae that complete their development within 1 year have an average of six to seven instars, whereas those that take 2 years develop in eight instars. In a temperate climate in Tasmania up to twelve instars have been observed, suggesting that larval development is correlated to the ambient temperature (Taylor, 1981). The larvae construct large galleries that are 5 to 26 cm long. The wood quality is also affected by the larvae building galleries and by the entrance of secondary agents that facilitate damage to the wood. This limits its use or decreases its market value. Once the tree is dead, the wood is degraded quickly and must be used within 6 months after the attack (Iede et al., 1993, 1998).The pupal stage takes around 3 to 5 weeks in Australia and 10 to 20 days in Brazil (Morgan and Stewart, 1966; Carvalho et al., 1993).The plantations that are most susceptible to S. noctilio attack are generally 10 to 25 years old and under stress. Those stands that are not subjected to thinning are more susceptible than thinned ones.

Successful experiences in countries where S. noctilio has been introduced have demonstrated that biological control, along with prevention strategies, is the most efficient and economical control method.Biological control was pioneered in Australia: extensive exploration in Europe was carried out in 1962-1967 and the first importations of insect parasitoids ensued. The nematode, Deladenus siricidicola [Beddingia siricidicola], was discovered in New Zealand but had been introduced with the pest from Europe, where other strains have since been obtained. The Australian work has recently been reviewed and summarized by Waterhouse and Sands (2001). New Zealand (reviewed in Cameron et al., 1989) and South Africa (reviewed by Tribe, 2003) soon followed the progress made in Australia. It is only recently that introductions have been made in South America and thus the outcome is not yet fully apparent there. These authors show that, although D. siricidicola is the most effective single natural enemy, it does not spread rapidly once established. This is in contrast to the insect parasitoids that are useful in helping to lower pest densities ahead of the arrival of the nematode.Evidence from different countries showed that D. siricidicola is the most effective biological control agent against S. noctilio (Iede et al., 1998; 2000a). This parasite feeds on the symbiotic fungus, Amylostereum areolatum, in its free-living phase (micetophagous phase) and infects the woodwasp larvae in the parasitic-living phase. The nematode feeds on the fungus, which is inoculated into the trees by S. noctilio. When it finds the S. noctilio larva, it enters into the larval body. The larva then completes its development, but the ensuing adult female S. noctilio is sterile because her eggs are infested with nematodes (Bedding, 1972; Bedding and Akhurst, 1974). Insect parasites were introduced in some countries but they have not been as effective as the nematode, although they are important in maintaining a natural equilibrium. The egg or early stage larval parasite, Ibalia leucospoides, is a small wasp present in all countries where S. noctilio was introduced and causes a parasitism level of up to 39%, with an average close to 25% (Neumann et al., 1987; Carvalho, 1993; Iede et al., 1993, 1998).Megarhyssa nortoni and Rhyssa persuasoria were introduced and released in Brazil in 1996, 1997 and 1998, but at the time of writing this datasheet no specimens had been recovered in the field (Iede et al., 2000b). Certonotus tasmaniensis and Guiglia schausislandi are indigenous parasitoids in Australia and New Zealand, respectively, and occasionally infest S. noctilio larvae but not significantly.The parasitoid complex of Ibalia spp., Rhyssa spp. and Megarhyssa spp. does not cause more than 40% mortality in S. noctilio populations and their activity alone is not sufficient to prevent outbreaks (Haugen and Underdown, 1990b).

In New Zealand between 1946 and 1951, during a severe drought, S. noctilio killed about 30% of radiata pine in 120,000 ha of plantations that had not been thinnned (Rawlings, 1948, 1955). In Tasmania, 40% of the trees in an area of 1092 ha were dead (Madden, 1975).On mainland Australia, the worst outbreak of S. noctilio occurred in Victoria, in a 12- to 15-year-old plantation of 1906 ha between 1932 and 1979. S. noctilio killed 77% of the trees on 25 ha, 63% on 79 ha, 35% on 379 ha and 5% on 701 ha. One area of 388 ha remained undamaged. The total and merchantable volumes of wood were reduced by 50% and 48%, respectively, in the more damaged stands (McKimm and Walls, 1980).In southeastern South Australia and southwestern Victoria between 1987 and 1989, an outbreak of S. noctilio caused the death of five million trees with a value of AUS $10 to 12 million. It was necessary to inoculate 147,000 trees with the parasitic nematode, Deladenus siricidicola. The inoculation programme cost AUS $1.3 million. An intensive project to salvage S. noctilio-infested trees and to release the parasitoid, Megarhyssa nortoni was also implemented (Haugen, 1990; Haugen and Underdown, 1990a, b).In Brazil, the average mortality caused by S. noctilio attacks in 17-year-old stands, which had not undergone thinning, was 9.6% in February, 1988; 30% in August, 1988; and 60% in August, 1989 (Iede et al., 1988, 1998). The tunnelling activity of the larvae degrades the wood and if they are left to tunnel for a half to 1 year, then the wood is unfit for trade.

Most chemical controls are impractical, but when an exotic pest is detected in a new area some farmers and pine plantation owners try to control the pest with chemicals and/or other measures (for example, fumigation or fire). This may produce undesirable environmental effects.Pine trees have become valuable for more reasons than just their economical value. In countries where these trees have become an important part of the landscape, their loss is not acceptable for aesthetic and recreational reasons. In countries where pine trees are indigenous, the environmental impact is considered to be high, because S. noctilio could find enough suitable host material to reach outbreak levels.

The first visible symptom is the progressive and irreversible chlorosis in the crown, which turns reddish-brown. Death and decay of the tree follows. Droplets of resin and sometimes small resin flows are visible on the surface of the bark of infested trees. Circular emergence holes (5.5 mm in diameter) indicate adult emergence (Neumann et al., 1987; Iede et al., 1993, 1998). In the outer sapwood and along the wood grain, narrow bands of brownish fungal stain may be observed. Larval galls containing compacted frass are present in the wood. Imported logs must be inspected in quarantine for these signs. Saw timber and solid wood packing material must be inspected for the presence of galls containing larval frass.Trap-trees are used for monitoring and sampling S. noctilio in the field. Trap-trees are trees that are stressed by injecting them with herbicide. This is the most appropriate and efficient technique for early pest detection, as well as for monitoring dispersion. The stressed trees are attractive to S. noctilio for egg laying (Neumann and Minko, 1981; Haugen and Underdown, 1990a).

S. noctilio is a secondary opportunistic wood-boring insect. Therefore the prevention of severe outbreaks is a forest management problem that can be mitigated through good silvicultural practices, mainly timely thinning and the early removal of damaged and unhealthy or multi-stemmed trees (Neumann et al., 1987; Iede et al., 1993).
An effective strategy is to aim for the early detection and rapid suppression of small S. noctilio populations. The ground surveillance for early detection of the pest includes a trap-tree system. The biocontrol agents used against S. noctilio include the nematode, Deladenus siricidicola and parasitoids, such as Ibalia leucospoides (Haugen, 1990; Haugen and Underdown, 1990a; Iede et al., 1993, 1998). An Integrated Pest Management programme must involve the following activities.

Monitoring for the Early Detection of S. noctilio

Detailed maps of pine plantations should be available and include the number and location of groups of trap-trees. These should also include the areas where S. noctilio is found and the places where nematodes and parasites are released.

For the early detection of S. noctilio, aerial monitoring, with visual observations and estimates of damage in the attacked areas, is imprecise. This is because the trees normally preferred by S. noctilio are not detected in these kinds of surveys (Iede et al., 1998; Ciesla, 2003). For an accurate evaluation of the attacked areas in plantations, infrared photography and photographic interpretation, followed by ground surveys, are more informative than aerial sketch-maps.

The use of trap-trees that are stressed by herbicide injection is the most appropriate and efficient technique for early pest detection and for monitoring dispersion. The detection of S. noctilio during its early developmental stages and colonization helps to define the locations for biological control agent release and allows thinning practices to be carried out before the pest reaches high damaging levels. The maintenance of a trap-tree system may greatly increase the biological control efficiency of S. noctilio (Neumann et al., 1987; Haugen, 1990; Haugen and Underdown, 1990a; Iede et al., 1993, 1998).

The detection method chosen and the intensity with which it should be applied in a region, must be based on a risk analysis of introduction and dispersal of the pest in that region. A general recommendation is that the trap-trees, preferably with diameters at breast height (DBH) of 10 and 20 cm, are installed in groups of five to ten. Also the distance between the groups should vary according to where the pest is established (Neumann et al., 1987; Haugen, 1990; Haugen and Underdown, 1990a; Iede et al., 1993, 1998). The groups of trap-trees should be installed 2 months before the emergence peak of S. noctilio.

Prevention Strategies

The trees that are resistant to S. noctilio are those that remain free of injury and continue to grow vigorously in well-managed blocks and good sites, with favourable climatic and soil conditions. The mortality level of the trees is significantly related to the DBH. The trees with a low DBH show higher levels of mortality than thicker ones in the same stand. Thinning is one of the most important silvicultural practices, conducted in order to accelerate or modify the course of competition (Neumann et al., 1987; Iede et al., 1998).

Most of the thinning practices reduce losses due to damaging agents not only because they work as a prevention strategy but also because the vigour and resistance of the trees are increased. Thinning can only enhance the susceptibility of trees to insect attack under special circumstances, for example when it is carried out during the flight period of the pest (Madden, 1988). During drought periods, thinned plantations are not resistant to S. noctilio attack.

Biological Control

Biological control, along with prevention strategies, is the most efficient and economical method for controlling S. noctilio. D. siricidicola is the most effective biological control agent of S. noctilio. This nematode sterilizes females and has two life cycles: a free-living one during which it feeds on the same symbiotic fungus as S. noctilio associates with and a parasitic-living one inside the larvae, pupae and adults of S. noctilio. As its free-living life cycle is based on the fungus, Amylostereum areolatum, it is easily bred under laboratory conditions and then it can be released into the field by application into trees attacked by S. noctilio. Parasitism levels close to 100% can be achieved, which causes a collapse in the woodwasp population (Zondag, 1969; Bedding, 1972; Haugen, 1990; Haugen and Underdown, 1990a; Iede et al., 1998).

The inoculation of D. siricidicola into trees is described by Bedding (1972), Bedding and Akhurst (1974), Neumann et al. (1987), Haugen (1990), Haugen and Underdown (1990a) and Iede et al. (1993, 1998).

The average level of parasitism achieved with D. siricidicola in Australia was 70% (Bedding and Akhurst, 1974). Although the level of parasitism verified for the nematode in attacked areas of Brazil has been quite variable, it was found to be as high as 70 or 80% in most of the monitored areas (Iede et al., 1998).

I. leucospoides parasitizes the eggs, first-instar and second-instar larvae of S. noctilio. This parasitoid is attracted to the odour of A. areolatum as it begins to spread (Zondag, 1959; Madden, 1968; Taylor, 1981).

The parasites, Rhyssa persuasoria and Megarhyssa nortoni, have long ovipositors and therefore attack larvae that are in the advanced stages of development. The parasites introduce the ovipositor into the wood in search of the host larvae. The larvae are paralyzed when stung by the parasite and the eggs of the parasite are then laid on the body of the host. After they hatch, the larvae of the parasite feed externally and, after consuming the host, they develop into pupae. In this species group, most members of each generation undergo a diapause in the larval stage, once they are completely fed. They pupate in the spring and emerge when the host larvae move towards the tree bark to pupate.

The complex of parasites (Ibalia sp. and Rhyssa sp.) can eliminate about 40% of S. noctilio populations in some places (Neumann et al., 1987). However, it does not exceed this and thus is an insufficient percentage to keep S. noctilio attacks from reaching high levels. Nevertheless the parasitic complex is important for the maintenance of the ecosystem/pest equilibrium (Neumann et al., 1987; Haugen and Underdown, 1990a).