Population structure and possible origin of Amylostereum areolatum in South Africa

The woodwasp, Sirex noctilio , and its symbiotic fungus, Amylostereum areolatum , cause extensive damage to pine plantations in the Southern Hemisphere. S. noctilio was first reported from South Africa in 1994. In this study, the population diversity of A. areolatum isolates from South Africa, South America, Australasia and Europe was determined by vegetative incompatibility testing. All 108 South African and 26 South American isolates belonged to the same vegetative compatibility group (VCG). This VCG showed a weak incompatibility reaction with the single Tasmanian and single New Zealand isolates tested. This VCG differed from VCGs from Europe. It also differed from isolates associated with the biocontrol nematode, Deladenus siricidicola , which is produced in Australia. It is concluded that the South African and South American populations of A. areolatum share a common origin .     

Host-tree conditions affecting the oviposition activities of the woodwasp,Sirex nitobei Matsumura (hymenoptera: Siricidae)

Experiments were conducted to elucidate how the oviposition activities ofSirex nitobei Matsumura could be affected by different conditions of the host trees, using logs from felled healthy trees and the dead and living trees ofPinus densiflora Sieb. et Zucc. According to days elapsed after tree-felling, the proportions of oviposition defined as the proportion of eggs laid to the potential fecundity of a female during her lifetime, could be divided arbitrarily into three groups. The proportions of oviposition on the logs within 0–3 days after felling,i.e fresh logs, varied greatly from log to log, and about half of female adults showed proportions of oviposition under 50% (mean ±SD: 39±32%). However, all the females that oviposited on the logs 4–24 days after felling,i.e. intermediate logs, invariably showed proportions of oviposition over 50% (88±1.5%). On the other hand, proportions of oviposition in most of the females that oviposited on the logs over 25 days after felling,i.e. old logs, were less than 50% (26±17%). The proportions of oviposition were significantly different between intermediate logs and old logs. The mean proportion of oviposition on living trees and that for dead trees were 45 and 58%, respectively. Thus,S. nitobei in the field would exhibit its preference for weakened trees ofP. densiflora as oviposition sites, as it did for the logs 4–24 days after felling in this study.     

Population variation in traits of Deladenus siricidicola that could influence the biocontrol of Sirex noctilio in South Africa

Deladenus siricidicola is the primary biocontrol agent of the invasive woodwasp, Sirex noctilio. The nematode is mass-reared in culture on the S. noctilio symbiotic fungus, A. areolatum, but can also be induced to convert into its infective form in culture. The aim of this study was to explore the level of variation among a collection of D. siricidicola strains to convert into the infective form in culture. In addition, we measured variation in growth of the nematode on a strain of A. areolatum that is commonly used for laboratory rearing. Both traits could influence laboratory rearing and efficiency in the field. The 22 strains from three countries showed significant variation in the frequency of conversion into the infective form in culture. Even genetically uniform strains from South Africa displayed a range of variation in this trait. There was also significant variation in the growth of D. siricidicola on A. areolatum. These results emphasise the importance of considering variation in traits in D. siricidicola that could influence a biological control program. They also provide tools that could be used to screen larger collections of D. siricidicola for variation in traits linked to virulence and reproduction.