Egg mortality of rice leaffolders Cnaphalocrocis medinalis and Marasmia patnalis in irrigated rice fields

Egg mortality of rice leaffolders Cnaphalocrocis medinalis and Marasmia patnalis was studied in unsprayed irrigated rice fields in Laguna Province, the Philippines. Mortality was assessed by field exposure of laboratory-laid eggs for two days and by monitoring of field-laid eggs. Egg disappearance, the major mortality factor, was low in the first four weeks after transplanting and then increased. Egg parasitism by Trichogramma japonicum was highest at the start of the crop and decreased to a low level towards crop maturity. Non-hatching of eggs was of minor importance. Over the total duration of the egg stage, the average disappearance of exposed laboratory-laid eggs was 40%, and of field-laid eggs 46%. Egg mortality due to parasitism averaged 15% and 18%, respectively. The potential impact of egg parasitism is probably partly obscured by the disappearance of parasitized eggs. Mortality rates were highly variable between egg cohorts, but with multiple regression analysis several factors were identified that statistically explained a significant part of this variation. The results suggest that the predatory crickets Metioche vittaticollis and Anaxipha longipennis play a major role in egg disappearance, and that egg parasitism is positively dependent on the overall density of host eggs of Trichogramma in the field.