The thermal physiology of Stenopelmus rufinasus and Neohydronomus affinis (Coleoptera: Curculionidae), two biological control agents for the invasive alien aquatic weeds, Azolla filiculoides and Pistia stratiotes in South Africa

Water lettuce, Pistia stratiotes, and red water fern, Azolla filiculoides, are floating aquatic macrophytes that have become problematic in South Africa. Two weevils, Neohydronomus affinis and Stenopelmus rufinasus, are successful biological control agents of these two species in South Africa. The aim of this study was to investigate the thermal requirements of these two species to explain their establishment patterns in the field. Laboratory results showed that both weevils are widely tolerant to cold and warm temperatures. The critical thermal minima (CTmin) of N. affinis was determined to be 5.58 ± 0.31°C and the critical thermal maxima (CTmax) was 44.52 ± 0.27°C, while the CTmin of S. rufinasus was 5.38 ± 0.33°C and the CTmax 44.0 ± 0.17°C. In addition, the lower lethal temperatures were −9.85 ± 0.06°C for N. affinis and −6.85 ± 0.13°C for S. rufinasus, and the upper lethal temperatures were 42.7 ± 0.85°C for N. affinis and 41.9 ± 2.52°C S. rufinasus. Using the reduced major axis regression method, the development for N. affinis was described using the formula y = 12.976x + 435.24, while the development of S. rufinasus was described by y = 13.6x + 222.45. These results showed that S. rufinasus develops twice as fast as N. affinis. Using these formulae and temperature data obtained from the South African Weather Service, N. affinis was predicted to complete between 4 and 9 generations per year in South Africa, while S. rufinasus was predicted to complete between 5 and 14 generations per year around the country. These results suggest that both species should not be limited by cold winter, nor warm summer temperatures, and should establish throughout the ranges of the weeds in South Africa.