Southern Tablelands Farm Forestry Network |
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Pests and diseases of pines - Sirex Wasp [Print
this fact sheet - pdf 76 kb] Regular monitoring to determine the presence of Sirex is essential, as it can rapidly devastate plantations under favourable conditions due to a sudden build up in population levels. Favourable conditions include trees that have been stressed by disease, wind, hail or fire, trees that have had crown or stem damage, and trees that are suffering from moisture deficit due to drought stress. Unthinned older plantations are particularly susceptible to Sirex attack, as they contain a large proportion of suppressed, unhealthy stressed trees. A Sirex wasp is distinguished from other wasps by its lack of the typical narrow waste common to other wasps. The Sirex wasp deposits its eggs in holes that it drills into the wood in summer or autumn. At the same time, it deposits toxic mucus and a symbiotic fungus into the holes with the eggs to weaken the host tree, making for a better environment for the eggs to hatch. As the fungus grows from where the eggs were laid, fungal stains appear in the cambium. After 14 days, the eggs hatch and the larvae begins to tunnel through the wood, usually along the grain, leaving the tunnels packed with frass (chewed wood). These tunnels increase in size as the larvae grow, and then the larvae pupate in hollow chambers in the outer sapwood. Sirex attacked trees begin to show conspicuous symptoms from April onwards, when the crown wilts, turns light green, then yellow, then reddish brown. Beads of resin are seen on the trunk resulting from wounds during the egg laying, and emergence holes that are perfectly round 3mm to 6mm in diameter may be seen. Well managed, healthy plantations support only small populations of the Sirex wasp, and if there are no stressed trees, Sirex outbreaks usually decline naturally. To avoid outbreaks of Sirex wasp, plantations require regular thinning, thus decreasing suppressed and sub dominant stems which are more highly susceptible to Sirex attack. There are two kinds of biological agents used to control Sirex wasp. The parasitic nematode (Deladenus siricidicola) may be injected into infected trees before the wasp emerges, and sterilises adult female wasp by entering her eggs. There are a number of parasitic wasps which lay their eggs down the drills created by the Sirex wasp, therefore killing the Sirex larvae as they pupate. Plantations must be monitored to determine the distribution and level of Sirex wasp, and to determine changes in population levels annually. Results of this monitoring will allow for the planning and implementation of appropriate management responses. However, a healthy, well thinned and well managed plantation is the best defence against the Sirex wasp. If you think you may have a Sirex Wasp problem, contact us and we will come out to your property in the Southern Tablelands for a free evaluation. |