Temperature extremes affect the biocontrol potential of Beauveria bassiana and Metarhizium anisopliae against the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae): towards climate-resilient pest management

Abstract Background Severe climatic events, such as heat and cold waves, can influence the dynamics of pest-pathogen interactions. Fungal efficacy among noctuid pests may be bidirectionally influenced by thermal waves. Therefore, this study aimed to investigate how extreme temperatures influence fungal efficacy against the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) under laboratory conditions. Results After infection with LC 50 of the entomopathogenic fungi, Beauveria bassiana or Metarhizium anisopliae , third instar larvae were exposed to a simulated heat wave (35 °C) or cold wave (15 °C) for five consecutive days, with 25 °C serving as the control. Developmental parameters, adult emergence, reproductive success, and immune responses through phenol oxidase activity and hemolymph protein content were evaluated. Results indicated that concomitant temperature and fungal infection significantly influenced larval weight gain, pupation rate, and time, developmental time, as well as egg hatchability. The weight gain decreased as the temperature increased, whereas the pest developed faster in heat or at normal temperatures. Cold waves led to a notable rise in pupation rates compared to heat waves or normal temperatures. The infection by M. anisopliae adversely affected the pupation rate, emergence, and egg hatchability in comparison to B. bassiana , particularly under thermal stress conditions. Moreover, both thermal waves and fungal infections led to a decrease in phenol oxidase activity in larval hemolymph, indicating impaired immunological responses, with M. anisopliae more than B. bassiana . Conclusion The present findings highlight the link between climatic stressors and biocontrol agents, stressing the necessity of incorporating environmental variability into pest management systems. Timing fungal applications under thermal stress could boost biocontrol and reduce chemical pesticide use.