A comparison of host range and performance of congeneric leaf-mining flies, Hydrellia pakistanae (Diptera: Ephydridae) and Hydrellia sp., two candidate biological control agents for the South African biotype of Hydrilla verticillata (Hydrocharitaceae)

• Two Hydrellia flies were assessed as biocontrol agents for Hydrilla verticillata.
• Host range, performance and leaf-mining were compared.
• Host range and damage potential were similar but performance differed significantly.
• Hydrellia sp. was selected for biocontrol of H. verticillata in South Africa.

A monoecious Malaysian/Indonesian biotype of the invasive aquatic weed, Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) recently invaded South Africa and has been targeted for biological control. Two ephydrid leaf-mining flies, originating on genetically distinct H. verticillata biotypes were considered as prospective agents. Hydrellia pakistanae Deonier (Diptera: Ephydridae) is a widely established biological control agent of a dioecious Indian/Pakistani H. verticillata biotype in North America, and an unnamed Hydrellia fly, Hydrellia sp. was collected in Singapore on a similar host plant biotype to the one invading South Africa. The suitability of the two fly species as biocontrol agents for H. verticillata in South Africa was assessed by comparing their fundamental host ranges as well as their performance and leaf-mining on the South African biotype of H. verticillata. Additionally, fitness parameters of H. pakistanae reared on South African H. verticillata were compared to the same parameters measured on the U.S. dioecious biotype. The two fly species showed minor differences in non-target host use in no-choice larval development trials. Hydrellia sp. had higher survival, longevity and fecundity and shorter egg to adult development times on South African H. verticillata compared to H. pakistanae. Further, leaf-mining by the two fly species was similar and H. pakistanae’s performance on South African H. verticillata was inferior in comparison to its performance on the U.S. dioecious H. verticillata biotype. These findings guided a decision to reject H. pakistanae as a biocontrol agent for H. verticillata in South Africa in favor of its congener, Hydrellia sp.

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