Resurgence potential of the rice leaf folder. Cnaphalocrocis.medinalis Guen. as influenced by the soil application of carbofuran granules

Abstract

In investigations on the changes taking place in the extent of feeding, growth and development of the rice leaf folder, Cnaphalocrocis medinalis Guen. on rice plants receiving soil application of carbofuran granules at 0.50, 0.75 and 1.00 kg ai/ha/application at 20, 50 and 20 and 50 days after transplanting, the extent of larval feeding on leaves did not show any variations as a result of carbofuran treatments. In the third, fourth and fifth larval instars and pupae, carbofuran treatments led to their weight gain due to possible enhancement of the digestibility and more effective conversion of ingested food into body matter, under .influence of factors mediated by the metabolites of carbofuran in leaf tissues. In respect of mean larval and pupal mortality, the insecticidal treatments did not show any impact, but the metabolites showed a negative correlation with the mortality levels thereby indicating the favourable influence of the metabolites on the development of C. medinalis. There was improvement in female emergence as a result of treatment with 1.00 kg ai/ha of carbofuran applied at 20 as well as 50 DAT as compared to untreated control, but the sex-ratio variations as a result of treatments were not pronounced. The first instar larval emergence (F1 generation) showed a positive influence by different metabolites of carbofuran. Carbofuran treated plants showed distinctly positive orientational stimulus to C. medinalis adults due to positive olfactory stimuli probably due to emission of vapour-phase admixture of carbofuran metabolites, but there was no improvement in fecundity under such treatments. There was distinct improvement in progeny production from females developing from carbofuran treated rice plants. This is mainly due to the ingress of more female moths and the resultant increase in overall ovipositional output rather than increase in fecundity'. Carbofuran treatments caused increase in the density of leaf hairs in a dose-dependent manner, but such variations were not significant as compared to untreated control. The positive relationship between ovipositional preference on the one hand and leaf hair density of treated plants .on the other brings to focus the role of tactile stimuli provided by leaf hairs in the acceptance of host plants for oviposition. The residues and metabolites of carbofuran in leaves favourably influenced most of the growth and developmental attributes of the rice leaf folder. None of the plant biochemicals showed any changes under influence of carbofuran treatments.