FIELD-BASED RATES OF POPULATION INCREASE FOR OXYOPS VITIOSA (COLEOPTERA: CURCULIONIDAE), A BIOLOGICAL CONTROL AGENT OF THE INVASIVE TREE MELALEUCA QUINQUENERVIA

Abstract

Melaleuca quinquenervia (Cav.) S. T. Blake (melaleuca) is a large (-30 m tall) broad-leaved tree indigenous to eastern Australia and was introduced into south Florida as an ornamental plant in 1906 (Meskimen 1962). Since introduction, melaleuca has aggressively invaded >200,000 ha of natural areas in southern Florida, including ecologically sensitive habitats such as the Everglades National Park (Turner et al. 1998). Instigated by ecological damage resulting from the melaleuca invasion, a classical weed biological control program was initiated in 1986 (Balciunas et al. 1994). Of the >400 herbivores associated with melaleuca in Australia, the weevil Oxyops vitiosa Pascoe became the first candidate introduced into US quarantine facilities for host specificity screening (Balciunas et al. 1994). After extensive testing the weevil was approved for release into south Florida, where it subsequently established at 9 of 13 initial release sites (Center et al. 2000). Once established, populations of a classical weed biological control agent are expected to increase rapidly in response to the seemingly ubiquitous resource (Grevstad 1999), resulting in eventual suppression of the target plant (Marohasy 1997). Therefore, an important component of post-release evaluations of biological control agents includes an assessment of population increase in the introduced habitat. The intrinsic rate of increase and the finite rate of population increase are commonly used to quantify growth rates of insect populations (Carey 1993, Price 1997). These estimates of population growth are typically calculated from fecundity bioassays conducted under controlled environmental conditions. While estimating population growth rates under laboratory conditions is useful for inter and intraspecific comparisons (Evans 1982), the extrapolation of these data to the field is limited because environments in nature continuously vary. Therefore, laboratory based estimates may not describe the actual rates of increase in heterogeneous (realistic) environments. Herein we quantify population growth rates for 0. vitiosa under environmental conditions experienced in its adventive range and compare these estimates with those of other successful weed biological control agents. Ultimately, these data will be used in the development of mathematical models to describe population dynamics of the weevil in south Florida. For this study, the finite rate of population increase for 0. vitiosa was defined as the multiplication factor that converts one population size to another, one time step later or: