Effects of cold storage temperature, host status, and larval stage on the biological characteristics of Cotesia chilonis

Shelf life is an important determinant of the quality of parasitoids used as natural enemies in augmentation biological control. Cotesia chilonis (Munakata) (Hymenoptera: Braconidae), an obligate endoparasitoid of Chilo suppressalis (Walker) (Lepidoptera: Crambidae), has potential for use through augmentation biological control in China. Here, the effects of cold storage temperature, host status, and larval stage on the biological characteristics of C. chilonis were investigated to extend the agent's shelf life. Results showed that the number of healthy C. chilonis adults obtained was reduced by 3.5% and 47.7% when C. suppressalis larvae in either natural or induced diapause were used as hosts compared to nondiapausing control larvae. Both parasitism and cold storage significantly reduced the supercooling point of nondiapausing C. suppressalis larvae, whereas the opposite result was observed in those of natural diapause. In order to improve the long-term storage of C. chilonis, nondiapausing C. suppressalis larvae were parasitized for 1, 3, 5, and 7 days to allow C. chilonis to develop into different larval stages before being transferred into cold storage for 30 days at 4 or 10°C. Compared to the control, the C. chilonis cocoon formation and emergence rates of the 1-day-old egg treatment group held at 10°C were reduced by 15% and 17%, respectively. The results also showed that the duration of the larval stage of C. chilonis was prolonged fourfold after storage at 4 and 10°C. Individuals held at 10°C had a significantly longer cocoon and adult stage compared to those held at 4°C. The biological parameters and life history of the F₁ generation of C. chilonis returned to normal levels under laboratory conditions. In conclusion, C. suppressalis larvae were successfully used to extend the shelf life of C. chilonis for use in augmentative biocontrol programs.