Assessing the suitability and safety of a well-known bud-galling wasp, Trichilogaster acaciaelongifoliae, for biological control of Acacia longifolia in Portugal

Acacia longifolia is a widespread invasive plant species in Portugal. In South Africa, it is controlled by a bud-galling wasp, Trichilogaster acaciaelongifoliae, which could also be used in Portugal. Biological control of invasive alien plants has received little consideration anywhere in Europe and has never been attempted in Portugal. The lack of a suitably-large quarantine facility necessitated the use of a novel approach to test non-target species in Portugal. Mature T. acaciaelongifoliae galls were shipped to Portugal from South Africa to obtain adult female wasps which were confined in Petri dishes each with a bud-bearing branch of one of 40 non-target plant species. The time spent by the wasps exploring and probing the buds was measured after which buds were dissected to detect any egg deposition. The results showed that T. acaciaelongifoliae did not respond to the buds of most (23) species. The females spent time on the buds of the other 17 species but only laid eggs in three species besides A. longifolia. Oviposition on Acacia melanoxylon was expected but was not anticipated on Vitis vinifera, vines, (where eggs were deposited externally in the pubescent coat of the buds) or on Cytisus striatus, broom, (where eggs were inserted into the buds as they are on A. longifolia). Subsequent trials on potted plants showed that galls only developed on A. longifolia. Field surveys in South Africa and Australia showed that galls never occur on either vines or broom. The implications of these findings for the use of T. acaciaelongifoliae for biological control of A. longifolia in Portugal are considered in relation to the wealth of experience and knowledge about the specificity of the wasp and the reliability of conducting host-specificity tests under confined conditions of cages.

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► Eggs laid by T. acaciaelongifoliae on ‘non-hosts’ in quarantine.
► Tests on potted plants and field surveys abroad confirmed findings were false positives.
► The extremely slight risk that the wasps might lay some eggs on ‘non-hosts’, and the minimal consequences thereof, are more than offset by the substantial benefits that will accrue if the project succeeds.