Tropical soda apple, Solanum viarum Dunal (Solanaceae) is a perennial shrub, originally from South America that has been spreading throughout Florida at an alarming rate during the last two decades. Currently, the infested area is estimated at more than 400,000 hectares (Medal 2005, Medal et al., 2004). Solanum viarum was first reported in the USA in Glades County, Florida in 1988. This weed is present also in Alabama, Arkansas, Georgia, Louisiana, Mississippi, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas, and Puerto Rico. But infestations in these states have still not reached high levels. This invasive exotic weed was placed on the Florida and Federal Noxious Weed Lists in 1995.
In addition to its invasion of pasture lands and reduction of cattle carrying capacity, S. viarum is a host of at least six viruses that affect vegetable crops including tomato, tobacco, and pepper. Although it is very difficult to estimate the real (direct and indirect) economic losses due to this invasive weed,production loss to Florida ranchers was estimated from $6.5 to $16 million annually (Thomas 2007)
Native to southern Brazil, Paraguay, northeastern Argentina, and Uruguay, S. viarum has spread into other parts of South and Central America. The rapid spread in Florida can be partially attributed to the great reproductive potential and highly effective seed dispersal by cattle and wildlife, such as deer, feral hogs, raccoons, and birds that feed on fruits. One S. viarum plant can produce on average from 100 to 160 fruits/plant, and 41,000 to 50,000 seeds with a germination rate of at least 75%.
Management practices for S. viarum in Florida are based on herbicide applications combined with mechanical (mowing) practices. These control tactics based on use of herbicides and mowing provide temporary weed suppression at an economic cost estimated at US $25 and $19 per acre in chemical and mechanical control methods, respectively (Thomas 2007). However, application of these control methods is not always feasible in rough terrain or inaccessible areas.
A biological control project on this highly invasive non-native weed was started in January 1997 by the University of Florida in collaboration with the Universidade Estadual Paulista, Jaboticabal campus, Brazil, Universidade Federal do Paraná in Curitiba, Brazil, Universidade Regional de Blumenau, Santa Catarina state, Brazil, Universidade Centro-Oeste, in Irati, Paraná state, Brazil, Instituto Nacional de Tecnología Agropecuaria (INTA-Cerro Azul), Misiones province, Argentina, and the USDA-ARS Biological control laboratory in Hurlingham, Buenos Aires province, Argentina. From exploratory surveys conducted in South America, several insects were identified as potential biological control agents of S. viarum including three leaf beetles, Gratiana boliviana Spaeth, Metriona elatior Klug, and Gratiana graminea Klug (Chrysomelidae), and the flower-bud weevil Anthonomus tenebrosus (Boheman) (Curculionidae). These potential agents were initially selected for screening because of the extensive foliage/flower bud plant damage attributed to these beetles in their native range. Two other promising biocontrol candidates that are currently undertaken open-field host specificity tests in Brazil are a leaf beetle Platyphora sp., and a flea beetle (Chrysomelidae).
Gratiana boliviana was approved for field release in the United States by TAG (Technical Advisory Group for Biological Control Agents of Weeds) on April 2002. A high level of specificity and significant defoliation of S. viarum were indicated in host-specificity tests conducted at the Florida Biological Control Laboratory-quarantine in Gainesville, the USDA-ARS quarantine in Stoneville, Mississippi, at the USDA-ARS South American Biological Control Laboratory in Hurlingham, Argentina, and in extensive field surveys and open-field tests conducted in South America. Field releases of G. boliviana in the United States began in May 2003. Requests for field releases of the leaf-feeder beetles Metriona elatior and Gratiana graminea in the USA were submitted to TAG in September, and October 2006, respectively. A petition to release the flowerbud weevil Anthonomus tenebrosus was submited to TAG on October 2007.