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

Background/Question/Methods Melaleuca quinquenervia is a serious ecological weed of the Florida Everglades and has been targeted for biological control using coevolved herbivorous arthropods.  To date, two insect species have been established and we evaluated their impact on a field community of M. quinquenervia in the western Everglades area.  The population dynamics of Melaleuca quinquenervia were monitored over a 5-y 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. 

Results/Conclusions After only 1 y melaleuca density increased 26% in sprayed plots and 7% in unsprayed plots.  However, over the entire 5-y period melaleuca density increased in sprayed plots by 0.1% while decreasing 47.9% in unsprayed plots when compared to initial densities.  Annual mortality of melaleuca never exceeded 6% in any year in sprayed plots while ranging 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 19.6% in sprayed plots while declining 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.