Comparative Impact of Release Frequency of Trichogramma chilonis Ishii against Chilo sacchariphagus indicus (Kapur) in Sugarcane

The egg parasitoid, Trichogramma chilonis Ishii, is widely used for the management of the internode borer (INB), Chilo sacchariphagus indicus (Kapur), in South India. However, there have been controversies regarding the dosage and frequency of releases required to attain desirable level of INB suppression. In the present study, two on-farm trials were conducted during 2008-09, comparing six releases of T. chilonis with continuous releases (from 5 months and 5½ months, respectively in trial I and II to one month prior to harvest) of T. chilonis at weekly intervals for their relative impact on INB incidence, intensity, dead hearts, number of nodes and yield, besides juice quality attributes. The cost-benefit ratio of the releases was also worked out. Continuous releases of T. chilonis were found to be better than six releases in both the trials. In the first trial, 31.1% and 56.46% reduction in the dead heart incidence was observed with six releases and continuous releases, respectively, while the yields were improved by 10.32% and 25.62%, respectively, with reference to control. In the second trial, continuous releases of T. chilonis resulted in significantly lower incidence, intensity, dead hearts and 12.15% increased yield than six releases and control, which were similar. The per cent dead hearts in control was 20.9% and 19.4% in the trial I and II, respectively. The studies also indicated the special benefits from continuous releases when the INB incidence was high. In both the trials, there was a beneficial impact on cane juice quality in terms of significantly improved % polarity and commercial cane sugar (CCS) in continuous releases in relation to control, while in the second trial percentage purity was also found to be significantly enhanced. In two separate assessments on the effect of different grades of INB infestation on the quality, there was a mixed trend in the parameters tested. The yesteryears' work and the present results on the usage of T. chilonis with reference to multiple releases for the management of borers are compared and discussed. It is recommended that in situ decisions on the frequency of releases are required because even though continuous releases are necessary to maintain the population and damage of INB low, the cost-benefit ratio tilts in favour of six releases in the present study.