Suitability of a leaf-mining fly, Hydrellia sp., for biological control of the invasive aquatic weed, Hydrilla verticillata in South Africa

Abstract

South Africa developed its first biological control programme for a submerged aquatic weed following the discovery of Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) in a major tourism dam in 2006. A leaf-mining fly, Hydrellia sp. (Diptera: Ephydridae) from Singapore, originating on a closely related biotype of the weed invading South Africa, was considered a priority candidate agent. Host range was investigated through no-choice and paired-choice larval development trials. ''Host suitability'' trials (multiple generations and performance) were used to further evaluate the potential for non-target host use by the fly. Hydrellia sp. developed on a few closely related species in the Hydrocharitaceae and one species in the Potamogetonaceae. However, pairedchoice tests demonstrated a strong preference for its host plant and the host suitability trials indicated that Hydrellia sp. has very limited potential to establish permanent populations on native aquatic plant species. Hydrellia sp. should therefore be considered safe for release in South Africa.

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)

“…was subjected to intensive host range testing because of its origins on a more suitable biotype of H. verticillata. Comparing these results (Bownes, 2014) with those of Buckingham et al (1989) for H. pakistanae, Hydrellia sp. would initially appear to have the broader host range.…”

“…Both Hydrellia sp. (Bownes, 2014) and H. pakistanae (Buckingham et al, 1989) were able to develop on Najas species native to the relevant recipient region and both developed on P. crispus. Representatives of the African endemic plant genus, Lagarosiphon (Hydrocharitaceae), which was equally susceptible to both fly species were, for obvious reasons, not tested in the laboratory host range trials conducted with H. pakistanae in North America.…”

“…Host range studies on Hydrellia sp. in South Africa indicated the fly has a strong preference for its host plant, H. verticillata and limited potential to establish permanent populations on native plant species (Bownes, 2014) Both fly cultures were transferred to another ARC-PPRI quarantine facility at Cedara (Provincial Department of Agriculture and Environmental Affairs headquarters) near Pietermaritzburg, KZN in July 2008. Cultures of the two fly species were maintained in separate facilities to prevent accidental mixing, however all trials comparing their host range and performance on South African H. verticillata were conducted simultaneously in the same facility, under the same environmental conditions.…”

“…in the trials conducted to evaluate its safety for release in South Africa (Bownes, 2014). H. verticillata was treated as a test plant and not as a control and a total of 11 plant species (Table 1) were tested, 10 of which are in the Hydrocharitaceae.…”

“…Eggs for the trials were obtained by exposing sprigs or portions of the test plants to approximately 100 adult flies per fly species in separate isolation boxes (43 Â 31 Â 55 cm). Both fly species demonstrate indiscriminate oviposition (Bownes, 2014;Buckingham et al, 1989), however eggs for the trial with N. marina had to be transferred on minute pieces of H. verticillata due to limited oviposition by females.…”