Thermal performance drifts between the egg parasitoid <i>Telenomus remus</i> and the fall armyworm may threaten the efficacy of biological control under climate change

Abstract The fall armyworm, Spodoptera frugiperda J.E. Smith, is a significant global agricultural pest known for its rapid invasion and devastating impact on crops. While pesticides may be effective for controlling the pest in the short‐term, they cause several socioeconomic and ecological costs that highlight the need for more sustainable management strategies. Telenomus remus (Nixon) is a promising egg parasitoid for its biological control. For the parasitoid to provide effective ecosystem services, it should be able to survive and coexist within the same ecological niches as its host. However, there is limited information regarding the potential responses of T. remus to thermal changes. In the context of the changing climate environments, it is key to understand the parasitoid's overall environmental fitness in relation to its host. We investigated the effects of short‐term (2 h) and long‐term (6 h) acclimation pretreatment of T. remus adults and S. frugiperda eggs to high and low temperatures (18°C and 32°C, respectively) in comparison with the control (28°C). Telenomus remus thermal fitness (critical thermal maxima [CT max ], heat knockdown time and critical thermal minima [CT min ]), parasitism rates, and adult emergence were determined. Pretreated S. frugiperda eggs were assessed for hatchability under the control conditions. Acclimation at low (18°C) and high (32°C) temperatures significantly reduced and increased heat tolerance, respectively. Both temperatures, however, reduced cold tolerance. The parasitoid thermal tolerance polygons following acclimation pretreatment, showed significant heat‐ but not cold‐tolerance gains. Fall armyworm eggs short‐term acclimated to 32°C had significantly higher ( p &lt; 0.05) but comparable hatchability to the control treatment. Similarly, parasitism and adult emergence rates were significantly lower ( p &lt; 0.001) following long‐term acclimation of host eggs and the parasitoid to 32°C. These findings suggest that high temperatures may decouple the T. remus ‐fall armyworm ecological relationship, threatening its success in warming regions. The findings provide valuable insights into the potential environmental resilience and suitability of T. remus as a biological control agent across different climates or geographies.