Cumulative generational effects of periodic heat events on two geographic populations of Agasicles hygrophila (Chrysomelidae), a classical biocontrol agent of invasive weed Alternanthera philoxeroides

Agasicles hygrophila (Chrysomelidae) is a specialized biological control agent for the invasive alligatorweed, Alternanthera philoxeroides. In mid-summer in Changsha City, Hunan Province, rising temperatures caused a substantial decline in the local A. hygrophila population, resulting in ineffective weed control. However, the beetle continued to feed on A. philoxeroides during the same period in Wanning City, Hainan Province. We hypothesized that heat events are the key factors driving the differing fluctuations between the two geographic populations. In this study, we investigated the development, survival, and fecundity of these two geographic populations across multiple consecutive generations under periodic heat exposure. There were no significant differences in fecundity, developmental duration, or survival across three consecutive generations for both populations under periodic heatwave 33 °C (4 h) compared with the control 30 °C (4 h). Population development has remained robust in both cases. However, periodic heat stress at 36 °C (4 h) significantly impacted the development and fecundity of the Changsha population, including reduced female fecundity, prolonged developmental periods across all stages, and decreased survival rates, all adversely affecting population development. The Wanning population experienced a slight decline in female fecundity during the treatment. At 39 °C (4 h), all the F2 generation eggs of the Changsha population failed to hatch. The sex ratio was significantly skewed toward females. While the Wanning population produced F2 generation larvae, none of them developed to the pupal stage. Neither population completed F2 generation development. These findings elucidate the effect of heat stress on cumulative generational effects in both populations of A. hygrophila. They also suggested that the Wanning population might possess greater heat tolerance than the Changsha population, providing valuable insights into the adaptive strategies for insects and other organisms facing climate change.