Assessment of Trichogramma japonicum and T. chilonis as Potential Biological Control Agents of Yellow Stem Borer in Rice.

Two species of wasps were assessed for their effectiveness against yellow stem borer . A laboratory cage test with and showed that both species parasitized yellow stem borer egg masses at 60.0% ± 9.13% and 40.7% ± 7.11%, respectively, with egg parasitism rates of 15.8% ± 22.2% for and 2.8% ± 5.0% for . Once the host eggs were parasitized, emergence rates were high for both species (95.7% ± 0.12% for and 100% for ). In paddy field trials, the two species were released at three densities (50,000/ha, 100,000/ha and 200,000/ha) in Southwestern China. Egg mass parasitism was 9% ± 7.7% for and 15% ± 14.1% for , and again only a relatively small fraction of eggs was successfully parasitized. No clear conclusion could be drawn on the most efficient release rate as no significant differences were found among the three release rates. A comparison of field-collected with and mass reared on showed significantly larger body size and ovipositor length in field-collected wasps, suggesting potentially higher effectiveness on yellow stem borer eggs after at least one generation on the target host. Factors contributing to the low field parasitism rates are discussed.