Ecological Notes on the<i>Sirex</i>Wood Wasps and their Parasites

Abstract

1. The ecological study of Sirex and 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 of Sirex noctilio , a species of woodwasp which has been established in the Dominion for many years. 2. A brief outline of the life-cycle of Sirex and 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 of Sirex can support three generations of Rhyssa and one generation of Ibalia , and that the effects of parasitism by Rhyssa are cumulative. The percentage of parasitism by Rhyssa cannot, therefore, be calculated on the results of any one year. Rhyssa is intrinsically superior to Ibalia and super-parasitism of Ibalia by Rhyssa is of very frequent occurrence. It is shown that the figures for parasitism by Ibalia obtained by the dissection of Sirex larvae collected from a number of logs, over a period including summer months, are liable to be very misleading.