Impact of summer temperatures on Agasicles Hygrophila, a key biocontrol agent of the invasive weed Alternanthera philoxeroides in Hunan Province, China.
Agasicles hygrophila, a leaf-feeding Chrysomelid beetle, was introduced into China in 1986 as a biological control for Alternanthera philoxeroides (alligatorweed), a noxious perennial invasive weed. However, Hunan's Ag. hygrophila populations decrease suddenly during summers (at high temperatures), leading to the inefficient control of alligatorweed. To investigate this phenomenon, herein, the effects of four different temperatures (26, 30, 36, or 39°C) on the reproductive capacity and egg-hatch, pupation, eclosion, and adult survival rates of Ag. hygrophila were explored. The effects of four different durations of each temperature treatment (4, 8, 12, or 16 days) were investigated. All the studied parameters were significantly affected following each of the four temperature and time treatments. At 30°C, the egg-hatch, pupation, and eclosion rates were considerably lower than those in the control group at 26°C. However, there were no significant differences in the number of eggs laid and the adult survival rates. Heat-shock at 36°C and 39°C led to significant decreases in the number of eggs laid, and the egg-hatch, pupation, eclosion, and adult survival rates compared to those at 26°C. Furthermore, high temperature had the greatest effect on Ag. hygrophila pupae, and the eclosion rate was close to zero; this may have caused the sharp decrease in the Ag. hygrophila population. The results will help better understanding the impact of biological control on Al. philoxeroides during summer temperatures and will also support optimization of biocontrol strategies, notably for the release of biocontrol agents across different geographical regions with different climates.