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Acacia cyclops is one of the 13 species of Australian acacias (Mimosaceae) that have become naturalized in South Africa and are now declared invasive weeds. Dasineura dielsi, a multivoltine midge that induces galls on the ovary of its host, has been introduced for biological control of A. cyclops in South Africa where it now colonizes 100% of the mature trees and substantially reduces their reproductive capabilities. Although A. cyclops is the primary host, galls are sometimes induced by on several other Australian Acacia species, including Acacia longifolia, Acacia melanoxylon and Acacia saligna in South Africa. The mass of D. dielsi galls and adult midges and potential fecundity of the females were about the same on A. cyclops and on A. melanoxylon but were lower on A. longifolia and even lower on A. saligna. Cross-over experiments demonstrated that adults emerging from each host-plant species were capable of mating and producing viable progeny, confirming that these plants are within the fundamental host range of D. dielsi and that they have become incorporated into the realized host range of the midge in South Africa. In contrast to A. cyclops, these three alternate hosts each have relatively brief flowering periods which precede that of A. cyclops. This, in combination with the emergence pattern displayed by D. dielsi, seems to play a role in the extent to which the alternate hosts are utilized by D. dielsi. All indications are that D. dielsi will be of no significance on any species other than A. cyclops.