Risk assessment of the host range of Hydrellia lagarosiphon for the biological control of Lagarosiphon major in Ireland

ABSTRACT Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) is a submerged freshwater plant native to South Africa, and a destructive invader of waterways across Europe, Australasia and the U.S.A. Native range exploration identified a leaf mining ephydrid fly, Hydrellia lagarosiphon, as a promising biological control candidate agent. Host specificity was conducted on H. lagarosiphon, using laboratory no-choice and paired choice tests. A number of non-target native Potamogeton species sustained feeding damage under no-choice and paired choice testing. To prevent rejection of a potentially safe agent, multi-generational population persistence trials were conducted on select native Potamogetonaceae. The non-target species could not sustain a viable H. lagarosiphon population beyond two generations. A risk assessment, incorporating the preference and relative survival, indicated that three non-target species presented extremely low (<1.3%) risk of sustaining damage relative to that of L. major. Potamogeton polygonifolius and P. x lanceolatus present a reproductive risk of 5.61% and 11.5% respectively but could not support a viable population beyond the 2nd generation. These results, coupled with damage efficacy and predicted colonisation, demonstrate the potential H. lagarosiphon has as a biological control agent for L. major in Ireland. Highlights Classical biological control could provide a safe, cost-effective method of control of the invasive submerged weed Lagarosiphon major. Multigenerational population persistence tests strengthen host specificity testing to predict feeding and reproductive risks of a biological control agent. Risk assessment data suggest a low level of Hydrellia lagarosiphon larval feeding could occur on three non-target Potamogeton species. While two native Potamogeton species sustain a relatively low reproductive risk, they failed to sustain the fly beyond the 2nd generation. Classical biological control provides an alternative control strategy to costly mechanical or environmentally unsuitable chemical control as the safety of Hydrellia lagarosiphon has been established in this pre-release testing procedure. The risk assessment presented here could form the basis of an application to the regulatory authorities to release H. lagarosiphon on L. major in Ireland.