Enhancing biological control of sugarcane shoot borer, Chilo infuscatellus (Lepidoptera: Pyralidae), through use of radiation to improve laboratory rearing and field augmentation of egg and larval parasitoids

Abstract

Feasibility studies were conducted to evaluate the use of gamma radiation to improve the production and field performance of the egg parasitoid Trichogramma chilonis Ishii and to improve production of the gregarious larval endoparasitoid Cotesia flavipes Cameron for biological control of the sugarcane shoot borer, Chilo infuscatellus Snellen. Sitotroga cerealella (Olivier) eggs were used as hosts for T. chilonis and the suitability of non-irradiated host eggs decreased as the age of the eggs increased, with no success in parasitization of eggs older than 4 days of age. However, irradiation of host eggs using 20–25 Gray (Gy) decreased the age effect and significantly more 2-, 3-, 4- and 6-day-old irradiated eggs were successfully parasitized than non-irradiated eggs. Radiation doses of 20 and 25 Gy were most effective for economical production of T. chilonis. Irradiation of host eggs did not affect the hatch percentage up to a dose of 15 Gy. Hatchability was significantly reduced at higher doses, with negligible hatching at 50 Gy. Irradiation also skewed the sex ratio of S. cerealella in favor of males at higher doses. Radiation at 60–80 Gy improved the suitability of C. infuscatellus larvae for parasitism by C. flavipes, allowing normally unsuitable fourth and fifth instar larvae of C. infuscatellus to be successfully parasitized. The sex ratio of parasitoids reared on irradiated larvae was skewed in favor of females. Irradiation also slowed immature development of C. flavipes and the combination of irradiation and low temperature (10°C) proved effective for prolonged storage of the parasitoids. Pupae of C. flavipes irradiated at 20 Gy could be stored for 2 months at 10° C without apparent loss of quality and deferred emergence by 29–30 days. Provisioning with irradiated supplemental hosts in the field increased overall pest suppression. The infestation by C. infuscatellus was higher in the control sugarcane block, where it remained above the economic threshold level (10% infestation) from April to October. High temperatures and low relative humidity during May to July reduced increases of the parasitoid populations. These findings were used to facilitate the area-wide application of biocontrol agents in a 40,000 hectare area to suppress sugarcane borers to sub-economic levels (<10% infestation). Sugarcane borer damage was 5.9% in treated blocks vs. 19.2% in untreated control blocks.