by Rodrigo Diaz, William A. Overholt and Veronica Manrique, Biological Control Research and Containment Laboratory, University of Florida, Fort Pierce, FL 34945
TSA invasion is a major problem for ranchers due to the loss of pasture area and subsequent lower stocking rates. TSA is a perennial shrub, approximately 1 to 1.4 m tall, and covered with prickles and tiny hairs that contain a sticky substance. TSA fruits are 2 -3 cm in diameter and green when immature with a watermelon color pattern, and turn yellow when ripe. Cattle and small mammals are known to feed on TSA fruits, thus, facilitating the spread of seeds to new habitats. The dominance of TSA is most noticeable during the summer and fall when plants are actively growing due to high temperatures and adequate moisture. Freezing winter temperatures kills TSA above the ground, but new shoots protected by leaf litter and roots often survive to resprout when warmer weather arrives.
Because of the importance of TSA as an invasive weed, a search for biological control agents was initiated in South America in 1994. Several insect herbivores were found causing damage to TSA in the native range. One agent identified and later introduced from Paraguay and northern Argentina was the TSA leaf feeding beetle, Gratiana boliviana.
Gratiana boliviana has four developmental stages (egg, larva, pupa, adult): Eggs are brown and enclosed in a membraneous envelope, larvae are spiny and are pale green, pupae are spiny, flattened and immobile; and adults, are about ¼” long and nearly as wide, and are a deep green color. Adults and larvae usually feed on the upper side TSA leaves while pupae and eggs can be found on the underside of leaves.
Tropical soda apple-infested grasslands and wooded areas.
Scientists from University of Florida and US Department of Agriculture exposed 123 plant species in 35 plant families to G. boliviana and evaluated beetle feeding and survival. Gratiana boliviana completed development only on TSA. On eggplant, very minor feeding damage was seen in laboratory tests, but none in the field. Biological control agents released to control invasive weeds are studied for several years in order to confirm that they will only feed on the target weed, in this case TSA.
The life cycle of the beetle begins when a female lays an egg. Eggs hatch in 5-6 days, larval development takes about 16-18 days, and the pupal stage lasts 6-7 days. Therefore, from egg to adult the beetle takes about 29-31 days to complete development at 77o F. Like all insects, development occurs faster when it’s warm, and slower when it’s cool. The beetles actively feed and reproduce in the field from around March/April until October/November, which is probably sufficient time to complete 7-8 generations. However, for 4-5 months during the winter, the beetles enter an adult resting state called ‘diapause’. During diapause, the beetles feed very little, and do not reproduce. They are difficult to find at this time of the year, as they hide in leaf litter beneath plants.
Larvae and adults of the beetles feed on TSA leaves, with a preference for feeding on younger leaves near the top of plants. The leaf feeding damage can be found from April to November and it is recognized by the characteristic “shot-gun” holes. This damage debilitates TSA and creates wounds that may facilitate attack by plant diseases. The cumulative stress produced by the beetles and diseases reduces the plant size and fruit production. Once TSA becomes less competitive, pasture grasses and native vegetation recover.
Studies conducted by scientists at the University of Florida have demonstrated that G. boliviana can reduce the density of TSA in as little as one year from the time of release, although in some cases it may take longer. Gratiana boliviana will not completely eliminate the weed from an infested area. Once the density of TSA is reduced, the beetle population will decrease due to a lack of food. Eventually, TSA and beetles will reach an equilibrium where the density of beetles regulates the density of plants, and the density of plants regulates the number of beetles.
Beetles are in diapause from about November to April. The use of herbicides or mowing during this period will have minimal effect on beetles which are in leaf litter under plants.
Gratiana boliviana is not commercially available. Florida residents can contact their local county extension agent.
Diaz, R., W. A. Overholt, A. Samayoa, F. Sosa, D. Cordeau, and J. Medal. 2008. Temperature-dependent development, cold tolerance and potential distribution of Gratiana boliviana (Coleoptera: Chrysomelidae), a biological control agent of Tropical Soda Apple, Solanum viarum (Solanaceae). Biocontrol Sci. Techn. 18: 193-207.
Mullahey, J. J., M. Nee, R. P.Wunderlin and K. R. Delaney. 1993. Tropical Soda Apple (Solanum viarum): a new weed threat in subtropical regions. Weed Technol. 7: 783-786.
Overholt, W. A., R. Diaz, K. Hibbard, A. Roda, D. Amalin, A. Fox, S. D. Hight, J. Medal, P. Stansly, B. Carlisle, J. Walter, P. Hogue, L. Gary, L. Wiggins, C. Kirby, and S. Crawford. 2009. Releases, distribution and abundance of Gratiana boliviana (Coleoptera: Chrysomelidae), a biological control agent of Tropical Soda Apple (Solanum viarum, Solanaceae), in Florida. Fla. Entomol. 92: 450-457.
Overholt, W. A., R. Diaz, L. Markle, and J. C. Medal. 2010. The effect of Gratiana boliviana (Coleoptera: Chrysomelidae) herbivory on growth and population density of tropical soda apple (Solanum viarum) in Florida. Biocontrol Sci. Techn. 20: 791-807.