The effect of a braconid ectoparasitoid, Bracon hylobii Ratz., on larval populations of the large pine weevil, Hylobius abietis L.

Abstract

Protection of restock plants from damage by the adult stage of Hylobius abietis in U.K. forestry is presently accomplished solely by the use of pesticides. A natural enemy of H. abietis larvae, the parasitold species Bracon hylobii, was investigated as a potential agent for use as part of an integrated strategy to control weevil populations which aims to reduce or eliminate the need for insecticidal protection.

B. hylobii was found to have a wide distribution within the U.K., occurring on almost all sites where host larvae were available. On average the parasitoid naturally destroyed over 47% of all H. abietis larvae developing in Sitka spruce stumps, but occasionally destroyed over 67%. The rate of increase in parasitoid density after trees have been felled from a site is slow compared to that of the host.

Laboratory studies revealed that in ideal conditions female B. hylobii can each give rise to up to 93 offspring, and attack up to 17 host larvae. Only a small proportion of host larvae are believed to occur in unaccessible regions of tree stumps. Initial parasitoid density is low and many H.abietis larvae escape parasitism by pupating before parasitoid density is high enough to cause a high rate of mortality. For these reasons it should be possible to substantially increase parasitism rate by releasing perhaps 1100 female parasitoids per hectare, in August/September of the first season after felling, when B. hylobii densities are lowest, but hosts are abundant.

With the present culture techniques mass production of the parasitoid as a direct substitute for insecticide use may prove unacceptably expensive. However with further research it should be possible to reduce costs, and maximise the effectiveness of released individuals. B. hylobii may also prove to be useful in conjunction with other natural agents, such as nematodes.

Date of Award	Sept 1995
Original language	English
Supervisor	KEITH DAY (Supervisor)