Effects of Rearing Temperatures on Key Biological Parameters of the Egg Parasitoids Trichogramma cocoeciae and Trichogramma bourarachae (Hymenoptera: Trichogrammatidae): Implications for Biological Control

The field effectiveness of Trichogramma parasitoids (Hymenoptera: Trichogrammatidae) against lepidopteran pests is strongly influenced by temperature, which affects their development, survival, parasitism and reproductive performance. Understanding thermal requirements is therefore essential for optimizing mass rearing and release strategies. The present study evaluated the effects of five constant temperatures (25, 30, 33, 35, and 40 °C) on biological parameters of Trichogramma bourarachae Pintureau & Babault and two strains (S1 and S2) of T. cacoeciae Marchal reared on Ephestia kuehniella Zeller eggs. Emergence rates were higher between 25 °C and 33 °C for all tested parasitoids, decreased markedly at 35 °C for T. cacoeciae, whereas T. bourarachae emergence showed higher tolerance at 35 °C, and no emergence was recorded for all parasitoids at 40 °C. Parasitism capacity was strongly influenced by both temperature and parental thermal history. Trichogramma bourarachae exhibited its highest parasitism rate at 25 °C; however, females originating from the parental generation that developed at 30 °C maintained relatively high parasitism rates at elevated temperatures (30 to 35 °C). For T. cacoeciae S1, parental development at 30 °C enhanced offspring parasitism over a broader temperature range. Conversely, T. cacoeciae S2 achieved maximum parasitism when the parental generation developed at 25 °C, with high parasitism maintained at 25 °C, 30 °C, and 33 °C. At the species level, parasitism was highest between 25 °C and 33 °C, declined at 35 °C, and no parasitism was recorded at 40 °C due to the absence of survival. Within each species, however, strain-specific differences were observed, particularly at 35 °C, indicating variability in thermal tolerance and reproductive performance. Female longevity decreased with increasing temperature in all species and strains. However, individuals originating from parental generation that developed at 30 °C exhibited improved survival when exposed to elevated oviposition temperatures, indicating thermal acclimation. Increasing temperature induced a male-biased sex ratio in T. bourarachae, whereas T. cacoeciae maintained stable thelytokous reproduction across all treatments. These results emphasize the importance of thermal tolerance and parental thermal history for selecting suitable Trichogramma species and strains for mass rearing and field application for biological control under warming climatic conditions.