Evolving suitable bioassay technique for base-line data in detecting resistance level in the third instar larvae of Spodoptera litura (Fab.) to endosulfan formulated with different solvents

Ten different bioassay techniques, viz., topical application (TA), direct spray on larvae and fed later on untreated food (DS), direct spray on larvae with leaves (DSLL), leaf residue dipping (LRD), leaf residue spraying (LRS), film residue due to spraying (FRS), film residue due to direct application (FRD), filter paper (FP), larval dipping for 20 seconds (LDBS 20), and larval dipping for 40 seconds (LDBS 40), for measuring tobacco caterpillar, Spodoptera litura (Fab.) susceptibility to endosulfan emulsions formulated in the laboratory with acetone and xylene as also proprietary formulations, were evaluated. Irrespective of different formulations of endosulfan, filter paper method (FP) was found to be the least sensitive and larval dipping bioassay (LDBS 20/LDBS 40), the most sensitive besides topical application (TA). The order of relative efficacy of different techniques was LDBS 40 > LDBS 20> DSLL> DS > LRD > LRS > FRS > FRD > FP. With acetone as solvent, these were respectively 18.39, 11.18, 4.54, 3.00,2.82, 2.68, 1.81, 1.33 times as effective as FP technique. With xylene as solvent, these were 16.11, 12.34, 4.48, 3.25, 2.97, 2.89, 1.94, 1.40 times as effective as FP technique. On the basis of significant differences in the LC₅₀ values, the techniques were categorised into three groups. The first and most effective group comprised two techniques, viz, LDBS 40 and LDBS 20, there being no significant differences between the two techniques. The second group included DSLL, DS, LRD and LRS, equally effective but less effective than the most effective, LDBS 40. The third group comprised three techniques, viz., FRS, FRD and FP, which were found to be at par with each other.