Tobacco caterpillar, Spodoptera litura (Fab.) (Lepidoptera: Noctuidae) is a polyphagous insect pest of national importance causing economic damage to a number of agricultural crops. It is known to damage more than 50 crops viz., tobacco, cole crops, castor, cotton, sunflower, chilli, etc. (Murthy et al., 2007). Several synthetic insecticides were tested and recommended to control this pest. However, indiscriminate and frequent application of these chemicals resulted in problems like build up of resistance, secondary pest resurgence and environmental pollution. In recent years, there are reports of S. litura developing resistance to recommended doses of several insecticides (Navatha and Murthy, 2006). In the southern parts of India, it is reported to have developed resistance virtually to all commonly used insecticides such as cypermethrin, fenvalerate, endosulfan, quinalphos and monocrotophos (Armes et al., 1997). These developments necessitate a relook at the dosages recommended by taking into account the crop on which the pest has to be managed. Variable efficacy of insecticides against S. litura larvae grown on different host plants may be related to the secondary metabolites, enzymes, and pH of host plants. Significant findings have been reported by Kulkarni and Lingappa (2001), Santiago et al. (2005) and Massarat (2007) reported on the influence of host plants on susceptibility of insects to different insecticides. This insect being polyphagous in nature, it was considered desirable to study the influence of some of the important host plants on its susceptibility to some newer insecticide molecules. So, this additional information would make existing IPM programmes more effective and sustainable, while decreasing the adverse effect on the crop and associated environment. The experiment was conducted under laboratory conditions at G. B. Pant University of Agriculture and Technology, Pantnagar. The culture of S. litura was SHORT NOTE Influence of host plants on the susceptibility of tobacco caterpillar, Spodoptera litura (Fab.) to certain insecticides ANJU BASERA and R. P. SRIVASTAVA Department of Entomology G. B. Pant University of Agriculture and Technology, Pantnagar-263145 U. S. Nagar (Uttarakhand), India E-mail: basera_anju@rediffmail.com maintained on the leaves of different host plants viz., castor (Ricinus communis), soybean (Glycine max) and brinjal ( Solanum melongena). The host plants were collected from Crop Research Center (CRC) and Vegetable Research Center (VRC), Pantnagar. Larvae of seven days were exposed to the fixed concentration (LC 50 ) of different insecticides viz., indoxacarb (0.0011%), methomyl (0.0014%), imidacloprid (0.0412%) and thiamethoxam (0.0314%) using larval dip method (Ramangauda, 2008). Each concentration was replicated four times. Observation on larval mortality were recorded at 6, 24, 48 and 72 hours after exposure. The susceptibility of insects was expressed as Time- Mortality responses. For calculating LT 50 values, the data obtained on mortality on different hosts were subjected to probit analysis (Finney, 1971) based on computer programme STPR 718. The data indicated that the mortality was significantly influenced by the host plants on which insect larvae were reared (Table 1). Differential susceptibility was compared on the basis of susceptibility parameters i.e. per cent mortality and LT 50 values. In case of indoxacarb, lowest LT 50 value of 19.57h was observed when S. litura larvae were reared on soybean as compared to larvae reared on the castor (23.08h) and brinjal (31.0h). It indicated that larvae reared on soybean were more susceptible to contact toxicity of indoxacarb as they took lesser time to show the mortality response. In case of imidacloprid also, soybean reared larvae were found to be more susceptible (LT 50 =31.97h) as compared to brinjal (45.01h). The LT 50 on castor (31.18h) was almost equal to soybean (Table 1). The third neo-nicotinoid, thiamethoxam also showed the same trend i.e. the soybean reared larvae showed more susceptibility/contact toxicity in terms of LT 50 value (45.01h) as compared to brinjal (57.92h). The LT 50 value 51 Pest Management in Horticultural Ecosystems, Vol. 17, No. 1 pp 51-53 (2011) Pest Management in Horticultural Ecosystems, Vol. 17, No. 1 pp 51-53 (2011)