Diapause Induction, Color Changes, and Supercooling Point of Diapause Larvae of Tetrastichus septentrionalis Yang (Hymenoptera: Eulophidae)

SIMPLE SUMMARY: Hyphantria cunea (Drury) (Lepidoptera: Erebidae), also known as the fall webworm, is a worldwide quarantine pest. In recent years, it has spread and caused serious damage in China. Tetrastichus septentrionalis Yang is a native dominant parasitic natural enemy population found in China that has potential application value for biological control. We selected Tenebrio molitor as an alternative host for artificially rearing T. septentrionalis via screening. We found that diapause in T. septentrionalis is of the long-day type, and can also be used in insect enemy mass production. This study’s findings may allow the preservation period of this species to be prolonged by utilizing the phenomenon of diapause in the future and provide a reference for the biological control of the fall webworm and other leaf-eating pests. ABSTRACT: The chalcid wasp Tetrastichus septentrionalis Yang (Hymenoptera: Eulophidae) is one of the dominant pupal parasitoids of Hyphantria cunea (Drury) (Lepidoptera: Erebidae). In this study, the photoperiod’s effect on diapause induction in T. septentrionalis using the alternative host Tenebrio molitor was measured, revealing that T. septentrionalis is of the long-day type. The critical photoperiods for diapause induction in T. septentrionalis were estimated to be between photoperiods of 13:11 and 14:10 (L:D) h at 18 °C, and between photoperiods of 12:12 and 13:11 (L:D) h at 21 °C and 24 °C. We also found that T. septentrionalis diapausing larvae were grey-brown, while normally developed (non-diapausing) individuals were light yellow. The diapause-sensitive insect state was the larval stage, and the short light exposure treatment had a significant cumulative effect on diapause induction. The least squares method was used to calculate a lower developmental threshold of 13.34 ± 0.50 °C and an effective cumulative temperature of 184.46 ± 11.46 d·°C for post-diapause development. The average supercooling point of diapausing mature larvae was significantly lower than that of non-diapausing ones. Our research on T. septentrionalis provides a reference for the biological control of H. cunea and other leaf-eating pests.