Biological control of prickly acacia (Vachellia nilotica subsp. indica) in Australia: prospective agents from Ethiopia and Senegal

Abstract

Prickly acacia (Vachellia nilotica subsp. indica) is a serious weed of grazing areas in western Queensland and has the potential to spread throughout northern Australia. Biological control is the most economically viable management option for prickly acacia. Biological control efforts so far have focused on agents from Pakistan, Kenya, South Africa and India, with limited success to date. Hence, search for new biological control agents was redirected to Ethiopia and Senegal with V. nilotica subspecies with moniliform fruits, similar to subsp. indica. Surveys conducted in Ethiopia (July 2014, December 2015 and November 2016) and in Senegal (March 2017) identified natural populations of V. nilotica in the Oromia, Amhara and Afar regions in Ethiopia (subsp. indica, subsp. tomentosa and subsp. leiocarpa), and in the Kaolack and in the Senegal River Valley regions in Senegal (subsp. tomentosa and subsp. adstringens). In Ethiopia, a gall thrips (Acaciothrips ebneri) inducing shoot-tip rosette galls, a gall midge (Lopesia niloticae) inducing leaf rachis galls, and three morphologically distinct eriophyid gall mites (Aceria sp.): type-1 forming red, spherical leaflet galls; type-2 forming creamy-white fluted leaflet galls; and type-3 deforming leaflets, rachides and shoot-tips were prioritised as prospective biological control agents. In Senegal, in addition to the gall thrips, a morphologically different eriophyid gall mite (Aceria sp.) (type-4 deforming emerging leaflets and rachides) and a yet to be identified tephritid inducing stem galls were identified as prospective biological control agents. Based on damage potential, field host range, geographic range and climate match, the gall thrips from Ethiopia was imported into high-security quarantine in Brisbane, Australia and host specificity tests are in progress. The type-3 eriophyid gall mite from Ethiopia has been imported into quarantine in Pretoria, South Africa for colony establishment and host specificity testing. Future native range surveys will focus on subsp. nilotica with moniliform fruits in climatically suitable areas in Egypt.