Biological controls: bud-galling wasp assessed

The bud-galling wasp Trichilogaster acaciaelongifoliae could be successfully introduced as a biological control agent of the invasive alien plant Acacia longifolia in coastal areas of Portugal. Its introduction could, however, have an impact on A. longifolia and another acacia species, A. floribunda, where these are cultivated as ornamentals. These are the main findings of EFSA’s assessment of the risks to plant health from the proposed release of the wasp.

Establishment of T. acaciaelongifoliaein Portugal would be likely because: host plants are present; the organism has been successfully established in South Africa; and environmental conditions in the proposed release area are similar to those in its native home of Australia. However, there is some uncertainty as there is no record of the wasp having ever established in the northern hemisphere.

Once established, the wasp would have a “massive” effect on invasive A. longifolia, reducing the growth, reproductive potential and ultimately population density of the plant, EFSA’s plant health experts concluded. It would also significantly reduce the negative effects of A. longifolia on biodiversity in the targeted area.

It is possible that the wasp could spread outside the target area through trade in ornamental A. longifolia and A. floribunda. However, these species are grown in a limited number of nurseries in Europe compared to acacia species which are not hosts of T. acaciaelongifoliae.

There would be little impact on populations of other invasive or ornamental acaciaspecies because of the wasp’s high degree of specificity – it survives and reproduces only in A. longifolia and the closely related A. floribunda – although there may be short-lived spill-overs to other species. One native wild species – the broom Cytisus striatus – and one ornamental species – Acacia retinodes – need to be further investigated because the currently available data on their host status for the wasp are inconclusive.

As with the release of other biological control agents, there would be no way of preventing the spread of T. acaciaelongifoliae to other areas. However, spread would be limited by a number of factors, such as the fragmented nature of host populations outside Portugal, and the fact that the female wasps die unless they find a host plant within three days of hatching.

EFSA was asked to carry out an assessment of the risk of establishment and spread of  T. acaciaelongifoliae in the EUafter Portugal informed the European Commission that it was considering using the organismto control the spread of A. longifolia in coastal sand dune areas. The wasp, which is native to Australia, was released intentionally in South Africa in 1982-1983 to control A. longifolia and has successfully established and spread there. Seed set on affected hosts has subsequently fallen by up to 95%.

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