Temperature-dependent development and cold tolerance of Gratiana graminea (Chrysomelidae), a potential biological control agent of Solanum viarum in Florida

Tropical soda apple (TSA), Solanum viarumDunal (Solanaceae), is an invasive weed of pastures, rangelands and natural areas in Florida. It is native to South America, and was first observed in Florida in 1988. The biological control agent Gratiana boliviana Spaeth (Chrysomelidae) was released in Florida in 2003. Establishment and successful regulation of TSA have been reported in south and central parts of the state, but in north Florida, G. boliviana does not perform as well. Reasons for inferior performance in north Florida may include poor adaptation to the cooler winter climate and asynchrony between beetle activity and TSA seasonal phenology. A new candidate biocontrol agent, the leaf-feeding beetle Gratiana graminea Klug, is currently under investigation in Florida quarantine. The objective of this study was to evaluate the survival and development of G. graminea at six constant temperatures from 15 to 30°C. In addition, cold tolerance was evaluated by exposing diapausing and reproductive adults to 0°C for different periods of time. Gratiana graminea completed development from egg to adult at 20 to 30°C, while no survival was detected at 15°C. Developmental time from egg to adult was longest at 20°C (34.2 d) and shortest at 30oC (14 d). Using linear models, the lower developmental threshold for immature development was estimated to be 11.68°C, and 312 degree-days were required to complete development. Diapausing adults were more cold tolerant than reproductive adults. The LT50 and LT90 at 0°C for diapausing G. graminea were 19 and 41 days, respectively. Comparisons between the two Gratiana spp. are discussed.