The potential of Hydrellia egeriae sp. nov. (Diptera: Ephydridae) as a biological control agent for the submerged aquatic weed, Egeria densa Planch. in South Africa

The potential of Hydrellia egeriae sp. nov. (Diptera: Ephydridae) as a biological control agent for the submerged aquatic weed, Egeria densa Planch. in South Africa
Rosali Smith, Julie A. Coetzee, Rosie Mangan
Biological Control Research Group, Department of Zoology & Entomology, Rhodes University, Grahamstown

Egeria densa Planch. (Hydrocharitaceae) is the widest established invasive submerged aquatic weed in South Africa. Native to South America, it forms dense stands in water bodies, prohibiting water usage and threatening indigenous biodiversity. Surveys in its native region yielded a promising biological control agent, a leaf-mining fly from the Ephydridae family, Hydrellia egeriae sp. nov. The damage capacity and host-specificity of the fly was investigated to determine its potential as a biological control agent in South Africa. Larvae consumed on average 14.8 ± 0.6 whole E. densa leaves; feeding from the crown of the shoot downwards. Damaged leaves are susceptible for further pathogenic infection resulting in shoot dieback. Density tests showed that larvae placed at densities of 1, 2 and 5 per shoot had 100% survival rate, while 8 larvae per shoot resulted in a lower survival percentage. No-choice tests showed a small degree of larval feeding and development on non-target species within the Hydrocharitaceae and Potamogetonaceae families. However, during paired-choice tests, H. egeriae showed a strong preference for E. densa. Oviposition site selection ranged predominantly from protruding E. densa leaves to any other surface or plant material available. Larval survival was not affected by oviposition site, as larvae readily moved to E. densa leaves after eclosion. Damage inflicted by H. egeriae is significant, and given it’s specificity for E. densa, should be considered for release in South Africa.