Field enhancement of the parasitoid bracon hylobii (Hymenoptera: Braconidae) to control hylobius abietis (Coleoptera: Curculionidae); the large pine weevil

Abstract

Hylobius abietis (Coleoptera: Curculionidae), the large pine weevil is the most damaging pest of reforestation sites in Ireland. This study examined the possibility of manipulating populations of a species specific parasitoid, Bracon hylobii (Hymenoptera: Braconidae) to reduce weevil populations below damaging levels or to a level where they may be of use in an Integrated Pest Management system. Levels of parasitism were studied in the field on recently felled Sitka spruce and Lodgepole pine sites in an attempt to measure ‘natural’ rates of parasitism. Highest rates of parasitism measured were 42% and 21% on Lodgepole pine and Sitka spruce sites respectively. This was very much lower than required for control. Field studies of parasitoid emergence and laboratory trials indicated that two peaks of adult emergence occurred during the year. Synchronisation of peak emergence with the most vulnerable stage of the weevil is discussed. It may be that an ample portion of the H. abietis population survives to cause economic damage no matter how many parasitoids are on site due to overlapping of generations. Attempts to enhance the populations of the parasitoid in the field failed due to migration of the flying adults. Reasons for this were investigated through field surveys of available plant food (nectar) and laboratory choice chamber experiments. It was concluded that the need for food and possibly, mutual interference resulted in a net migration of adults from the sites in which they emerged from the cocoons. Experimentally it was demonstrated that the presence of food resulted in greater longevity and hence longer periods for searching and oviposition. The presence of flowering plants onsite may result in greater levels of parasitism. However, this is inconclusive. The possibility of a hyperparasitoid being present was investigated using mtDNA techniques. No evidence was found to indicate the presence of a hyperparasitoid. This study indicates that natural levels of parasitism in recently clearfelled Lodgepole pine and Sitka spruce forest sites is insufficient to control Hylobius abietis. The ability to enhance ‘natural’ populations by introducing laboratory reared populations failed because of the tendency of adult parasitoids to disperse. No hyperparasitoid is present.