On an unusual gradation of the wood wasp Sirex juvencus on pines and their parasitism by nematodes of the genus Deladenus.
During an outbreak of species of the genus Sirex (mainly S. juvencus) on Pinus sylvestris in Germany in 1991-92, their density on trunks and parasitism by nematodes were studied. An average of 86 larval holes/trunk (12-13 cm in diameter) and infestation at 2-6 m above ground level were observed in a pine forest near Straupitz, Land Brandenburg. Caged trunk segments from a pine forest near Theessen, yielded 179 individuals of S. juvencus and 77 parasitoids of the genus Ibalia. Of the females and males of S. juvencus, 83 and 77%, resp., contained nematodes of the species Deladenus siricidicola. Parasitism of eggs of S. juvencus was low, and no nematodes were found in the parasitic Hymenoptera.