ARS | Publication request: Evaluation of mortality factors using life table analysis of Gratiana boliviana, a biological control agent of tropical soda apple in Florida

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 11, 2011
Publication Date: December 1, 2011
Citation: Manrique, V., Diaz, R., Overholt, W.A., Hight, S.D. 2011. Evaluation of mortality factors using life table analysis of Gratiana boliviana, a biological control agent of tropical soda apple in Florida. Biological Control. 59(3):354-360.

Interpretive Summary: Tropical soda apple (TSA) is a native plant from South America and has become a problem in pastures and natural areas throughout Florida. Cattle avoid the spiny leaves and stems of TSA and the weed can take over a pasture. To control TSA, a South American leaf-feeding beetle was introduced into Florida – the only plant on which the beetle survives. Scientists with USDA-ARS Center for Medical, Agricultural and Veterinary Entomology in Tallahassee, FL, along with University of Florida researchers, are studying the mortality factors of the leaf-feeding beetle in Florida, and how these factors influence the survival and population increase of the beetle on TSA. Comparisons were made between beetles reared outside in open cages and closed cages, in both North Florida and Central Florida. In closed cages, beetle survival from egg to adult was 57% in North Florida and 45% in Central Florida. In open cages, beetle survival from egg to adult was 19% in North Florida and 12% in Central Florida. The lower survival in open cages in North and Central Florida was due to mortality from various predators, primarily three species of small mirid insects. These mirids suck juices from the TSA plants but also from eggs and small larvae of the leaf-feeding beetles. Even though this study showed higher survival of the leaf-feeding beetle in North Florida than in Central Florida, the beetle is having problems establishing in North Florida. The beetle population is increasing in Central Florida and reducing TSA growth and densities. Cold winter temperatures in North Florida appear to limit the survival ability of the beetles. Additional studies are being conducted to evaluate the effect of low temperatures on beetle survival in North Florida.

Technical Abstract: Tropical soda apple (TSA), Solanum viarum Dunal (Solanaceae), has invaded many pastures and natural areas in Florida. The biological control agent Gratiana boliviana Spaeth (Coleoptera: Chrysomelidae) is providing adequate control of TSA stands in South and Central Florida. However, poor or no establishment of this agent has occurred in northern Florida. The goal of this study was to examine the mortality factors that influence the population dynamics of G. boliviana in Florida. Horizontal life tables were constructed by following cohorts of individuals in the laboratory and inside closed and open cages at field sites in Central and North Florida. Fertility life table parameters were estimated using laboratory and field data. In addition, TSA plants were sampled every two weeks in pastures in Central Florida, and counts of all G. boliviana and other herbivores and predators were recorded. Survival to adulthood was similar between Central and North Florida (open cages: 12-19%). Intrinsic mortality (laboratory data) and biotic factors (predation) together accounted for 75% of the mortality of immature stages. Survival of beetles in a natural population determined from vertical life tables was of 5%. A complex of three mirid species (Engytatus modesta Distant, Tupiocoris notatus Distant, and Macrolophus sp.) were the most abundant predators found in the field, and are known to feed on G. boliviana eggs and larvae. Positive growth rates (rm = 0.3) during the summer and early fall allow the beetle population to increase and provide suppression of TSA in Central Florida.