Host specificity testing of congeneric species of leaf-mining flies, Hydrellia pakistanae (Diptera: Ephydridae) and Hydrellia sp. – two candidate biocontrol agents for Hydrilla verticillata (Hydrocharitaceae) in South Africa.

Host specificity testing of congeneric species of leaf-mining flies, Hydrellia pakistanae (Diptera: Ephydridae) and Hydrellia sp. – two candidate biocontrol agents for Hydrilla verticillata (Hydrocharitaceae) in South Africa.          Angela Bownes
Agricultural Research Council – Plant Protection Research Institute (ARC-PPRI), Private Bag X6006, Hilton, 3245, South Africa

Hydrilla, Hydrilla verticillata is a submerged aquatic macrophyte, native to Asia and Australia that has invaded South Africa, and which is a serious economic and environmental aquatic weed. Hydrilla is genetically diverse, with a wide distribution in its native range hence biotype matching was an important consideration for the biological control programme. Two leaf-mining flies, Hydrellia pakistanae and Hydrellia sp. of different origins have been under consideration for release against South African hydrilla, a monoecious biotype of Malaysian/Indonesian origin.   Hydrellia sp., was considered the better match for S.A. hydrilla due to its origin on monoecious hydrilla from Singapore and was therefore the focus of host specificity testing. Additionally, comparative studies showed that Hydrellia sp. performs better on S.A. hydrilla compared to H. pakistanae, which originates on Indian dioecious hydrilla. Host range testing focused on ecologically similar and closely related species to hydrilla within the order Alismatales. Hydrellia sp.’s host range was also compared to H. pakistanae, on ‘susceptible’ non-target species to supplement the performance data which led to a decision reject H. pakistanae as a biocontrol agent for hydrilla in South Africa. Test plant species within a closely related genus, Lagarosiphon proved to be the most susceptible to both fly species, supporting complete larval development to varying degrees. However, choice tests and host suitability trials with Hydrellia sp. demonstrated a distinct preference for its host plant as well as limited potential to establish viable populations on the non-target species. Overall, there were minor differences in non-target host use between the two fly species, further supporting the decision to reject H. pakistanae. Hydrellia sp. is considered to pose no risk to native aquatic plant species in South Africa and is thus a promising candidate for biological control of H. verticillata.