Ecological Notes on theSirexWood Wasps and their Parasites

Abstract

1. The ecological study ofSirexand its parasites was begun in 1927 and continued while making large-scale collections of the parasites for shipment to New Zealand with the ultimate object of the control ofSirex noctilio, a species of woodwasp which has been established in the Dominion for many years.2. A brief outline of the life-cycle ofSirexand the parasites is given. The difference in the method of attack by the parasites on the host is described, and some points of interest in connection with insect behaviour are discussed.3. The methods of collection are described and particulars given about the number of parasites collected.4. The inter-relationship of the host and parasites is discussed. It is shown that a single generation ofSirexcan support three generations ofRhyssaand one generation ofIbalia, and that the effects of parasitism byRhyssaare cumulative. The percentage of parasitism byRhyssacannot, therefore, be calculated on the results of any one year.Rhyssais intrinsically superior toIbaliaand super-parasitism ofIbaliabyRhyssais of very frequent occurrence. It is shown that the figures for parasitism byIbaliaobtained by the dissection ofSirexlarvae collected from a number of logs, over a period including summer months, are liable to be very misleading.