Invasion of a West Everglades wetland by Melaleuca quinquenervia countered by classical biological control

The population dynamics of Melaleuca quinquenervia were monitored over a 5-year period in a cypress-pine wetland while subjected to two levels of herbivory. The trees had been recruited during 1998–1999 after a destructive crown fire. Half of 26 experimental plots were sprayed every 4–6 weeks with an insecticide to reduce herbivory by the biological control agents Oxyops vitiosa and Boreioglycaspis melaleucae. After only 1-year melaleuca density increased by 26% in sprayed plots and by 7% in unsprayed plots. However, over the entire 5-year period melaleuca density increased in sprayed plots by 0.1% while decreasing by 47.9% in unsprayed plots when compared to initial densities. Annual mortality of melaleuca never exceeded 6% in any year in sprayed plots but ranged from 11% to 25% in unsprayed plots. There was a significant year by treatment interaction indicating the importance of the environment on tree mortality. Limited seed production occurred on sprayed trees but never on unsprayed trees. Mean tree height increased by 19.6% in sprayed plots while declining by 30.6% in unsprayed plots. Coverage by native vegetation did not increase with decreasing melaleuca density. This is the first study with controls that quantifies the population level regulation of melaleuca by introduced biological control agents and corroborates other correlative studies that documented significant changes in melaleuca communities after the introduction and establishment of biological control agents.