Biology and Feeding Potential of Galerucella placida Baly (Coleoptera:Chrysomelidae), a Weed Biocontrol Agent for Polygonum hydropiper Linn.

Galerucella placida Baly is a small leaf beetle belonging to the family Chrysomelidae. which feeds on aquatic weed Polygonum hydropiper Linn. The insect was reported from various regions of India during 1910-1936. Investigation on some biological parameters of G. placida and feeding of the P. hydropiper by G. placida was conducted in laboratory. The results indicated the fecundity of G. placida was 710-1210 eggs per female. Eggs were markedly bright yellow, pyriform basally rounded and oval at tip. It measured 0.67 mm in length and 0.46 mm in width. Average incubation period was 3.80 days. Larvae of G. placida underwent three moults. The first instar larva was yellow in colour and measured 1.26 mm in length and 0.40 mm in width. The second instar was yellowish in colour but after an hour of feeding, the colour of the grub changes to blackish brown from yellow. It measured 2.64 mm in length and 0.77 mm in width. The third instar measured 5.59 mm in length and 1.96 mm in width. The average total larval duration of G. placida was 13.30 days. The fully developed pupa looked black in colour and measured 4.58 mm in length and 2.37 mm in width. Adult 4.50 days. The adult female was bigger than male. The adult male measured 5.60 mm in length and 2.35 mm in width and adult female measured 6.25 mm in length and 2.50 mm in width. Longevity of adult male was 35.40 days and while female was 40.40 days. Total life cycle duration of G. placida was completed in 21.60 days from egg to adult emergence. The damage was caused by both adult as well as grubs. At high population densities of adults and grubs, complete defoliation was seen. The average leaf area scrapped by larva and adult was 511.61 sq. cm and 866.35 sq. cm respectively.