Instar determination and larval distribution in brinjal shoot and fruit borer, Leucinoides orbonalis (Guen)

Abstract

Larval instars of Leucinodes orbonalis Guen. reared on Solanum melingena (L) were determined by measuring the width of the intact as well as the shed larval head capsules. Larvae were separable into five, distinct, non-overlapping instars by cluster analysis (of both intact and shed head capsule measurements) and by frequency distribution (of only the shed head capsule measurements). Larvae were found to shed their head capsules five times during a total larval period of 11-16 days. The mean widths of the shed head capsules corresponding to the five instars were 0.2, 0.36, 0.64, 0.95 and 1.46 mm respectively. The stadial periods of the five instars were 3.5, 1.6, 1.8, 3.2 and 2.0 days respectively. First instar larvae were confined to flower-buds and flowers whilst the second instars larvae were present in all the susceptible parts of the plant. Third to fourth instar larvae were confined to the shoots and fruits while final instar larvae fed exclusively on the fruits. The size of the entrance hole made by a larva entering the plant was found to be a good indicator of its instar status.