Can leaf-mining flies generate damage with significant impact on the submerged weed Lagarosiphon major?

Abstract

Effective biological control of a weed cannot be achieved unless agents can inflict sufficient damage for the plant’s growth and reproduction to be significantly depressed. The leaf-mining fly Hydrellia lagarosiphon is being considered for release in Ireland and New Zealand as a biocontrol agent for the invasive African macrophyte Lagarosiphon major. However no leaf-mining biological control agent has achieved successful control of any submerged macrophyte to date. Pre-release efficacy experiments help predict the likelihood of the biocontrol agents impact provided trials take into account expected agent densities and field conditions. Sustained damage induced by consecutive generations of the leaf-mining fly was capable of halting plant growth and undamaged shoot fragments produced 100 % more shoot biomass in just 70 days when compared with those exposed to larval damage. The number of larvae required to induce this level of damage was less than the maximum number of larvae a plant can sustain under both laboratory and natural field conditions and was between three and four larvae per shoot tip. Larval damage to L. major also had a significant effect on the subsequent establishment of detached apical shoots under field conditions. All control shoots displayed 100 % establishment whereas medium (three larvae per shoot) and high larval density plants (five larvae per shoot) had 85 and 25 % establishment, respectively. Pre-release trials indicate that relatively low larval abundances of H. lagarosiphon can contribute to the suppression of plant growth and reduce the establishment of shoot fragments, making this species a potentially effective biocontrol agent.