Herbivory by introduced insects reduces growth and survival of Melaleuca quinquenervia seedlings

Abstract

We studied the influence of herbivory by two introduced insect herbivores on the survival and performance of seedlings of Melaleuca quinquenervia (Cav.) Blake (Myrtaceae), an invasive tree that threatens the Florida Everglades ecosystem. Boreioglycaspis melaleucae (Moore) (Homoptera: Psyllidae) nymphs and Oxyops vitiosa (Pascoe) (Coleoptera: Curculionidae) larvae were transferred onto Melaleuca seedlings within replicated 0.25-m2 caged plots in Palm Beach County, FL. The treatments included three densities of Boreioglycaspis first instars at 1, 15, and 50 nymphs per seedling, one treatment of a single first Oxyops larval instar per seedling, another treatment of both one Oxyops larva and one Boreioglycaspis nymph per seedling, and caged and uncaged controls. Herbivory by Oxyops did not affect Melaleuca seedling height, leaf number, or survival. Feeding by Boreioglycaspis decreased survival, height, and leaf number, with these measures of plant performance ≈50% lower in the high and medium densities compared with controls. In a field plot adjacent to the experimental area, we measured growth, survival, and naturally occurring insect density and damage on 1,100 seedlings. Although insect densities were on average lower in the field plot than in the experiment, mortality and growth rates of the seedlings were comparable with those in the experiment. The results indicate that, above a threshold density, Boreioglycaspis herbivory may be effective in reducing growth and survival of Melaleuca at the potentially critical seedling life stage. It also seemed that effects of the insects were independent rather than antagonistic or synergistic.