A nematode infection of Sirex noctilio (F.) in New Zealand

Abstract

A nematode, parasitic on the wood wasp Sirex noctilio (Fabricius), was first recorded in 1962 from the North Island; it has not yet been found in the South Island. As high as 95 % of a Sirex population has been parasitised by these nematodes. The nematode, designated by Bedding as a Deladelllls sp., family Neotylenchidae, is bisexual with two female forms, one non-infective and the other infective. The former can repeat its life cycle indefinitely in the wood of a killed tree as long as the symbiotic "sirex" fungus is present; the latter which invades the S. Iloctilio larvae or prepupae, produces progeny viviparously during the host's pupal stage. The juveniles infiltrate the host's gonads causing the testes to hypertrophy and the ovaries to atrophy, thus sterilising the hosts. Sexual and oviposition behaviour of the host is not affected. An infected female Sirex is able to insert her eggs (which contain juvenile nematodes) and oidia of the symbiotic fungus essential for killing trees and/or the development of both the Sirex larvae and the nematodes, into the host trees, infecting the trees with the nematodes. The life history, behaviour and pathology of the nematode within the tree and vis a vis its host and insect parasites are described. The nematodes have not been found in other insects dwelling in bark of pines that have died after successful Sirex attack.