Biological control of Egeria densa in South Africa
Egeria densa (Hydrocharitaceae: EPPO List of Invasive Alien Plants) is a submergent aquatic plant species native to South America. It is reported as invasive in a number of EPPO countries and is a regulated Invasive Alien Plant in Estonia, Jordan and Spain. In South Africa, E. densa is widespread in waterways and traditional management methods including physical and mechanical control can be counter productive as these methods can facilitate dispersal through fragmentation. In addition, herbicide use in or around water is undesirable due to negative environmental impacts. The leaf mining fly Hydrellia egeriae (Diptera: Ephydridae), has been evaluated as a biological control agent for E. densa in South Africa using host specificity testing (i.e. no-choice and paired choice tests). During no-choice tests, H. egeriae only mined into closely related species within the Hydrocharitaceae (Lagarosiphon major, L. muscoides, L. cordofanus, Hydrilla verticillata and Vallisneria spiralis).Larval damage and survival were greater on the target species compared to the non-target species. During choice tests, H. egeriae showed a preference for the target species compared to non-target species. One non-target plant species native to South Africa, Lagarosiphon major (Hydrocharitaceae), supported larval development to adult during choice tests. Further tests were conducted to evaluate if H. egeriae could sustain a population on the native non-target species. Eggs of H. egeriae were transferred to shoots of L. major and left to feed and develop. However, no viable adults were produced in the first-generation experiment on the non-target. The experiments conclude that the feeding and reproductive risk to L. major is ten times lower compared to the target species E. densa. Based on the proven safety of H. egeriae as a classical biological control agent for E. densa, permission has been granted for the release of the biocontrol agent in South Africa.
Smith R, Mangan R, Coetzee J (2019) Risk assessment to interpret the physiological host range of Hydrellia egeriae, a biocontrol agent for Egeria densa. BioControl 64, 447-456.