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PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Sirex degrade in logs

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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 98, July 2000.

Recently a sawing study was undertaken on pruned logs obtained from a coastal forest north of Auckland. The sample had been selected to include batches of logs showing varying levels of external resin bleeding in an effort to establish links between these external signs and defects affecting clearwood quality, such as resin pockets. A disc was removed from the small end of each log and all but 6 of the 37 discs showed resinous blemishes that appeared to be Sirex noctilio (sirex wood wasp) damage, which was rather a surprise. Forest Research entomologists confirmed the resinous patches were the result of unsuccessful oviposition attempts by S. noctilio.

A total of 174 resinous blemishes were recorded with a significant number between 1986 and 1991. The blemishes followed a normal distribution peaking in 1988. It is interesting to note that the earliest evidence of attack was 1982 the year in which the stand was high pruned and thinned to a final crop stocking of 350 spha. Records in the Forest Health Database confirm the regular sighting of S. noctilio in suppressed trees in the forest over this period. However there was no correlation between the extent of external resin bleeding and internal resinous blemishes. In terms of clearwood quality these resinous blemishes associated with S. noctilio attack prevent the recovery of top clearwood grades and therefore have the potential to influence log value depending on severity. Sirex noctilio is attracted to suppressed trees but will also attack dominants,  though  this is usually unsuccessful with the oviposition hole becoming flooded with resin and development of the wasp's symbiotic fungus (Amylostereum areolatum) inhibited. Between 1946 and 1951 S. noctilio and A. areolatum were responsible for the death of approximately 30% of the trees throughout 121,500 ha of heavily stocked Pinus radiata stands in the central North Island. The introduction of a number of biocontrol agents along with the implementation of silvicultural treatments to maintain tree vigour have led to outbreaks of S. noctilio becoming very infrequent in New Zealand forests. For more information see Forest and Timber Insects in New Zealand No. 20.

Don McConchie & Margaret Dick, Forest Research

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(Scion is the trading name of the New Zealand Forest Research Institute Limited.)

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