Synthetic Pyrethroid Resistance in Field Strains of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) in Tamil Nadu, South India P. Duraimurugan and A. Regupathy

Resistance to synthetic pyrethroids was diagnosed in the field population of American bollworm, Helicoverpa armigera (Hubner) from Coimbatore, Tamil Nadu, South India uring 20032004 cropping seasons. A Discriminating Dose (DD) b ioassay technique was used to monitor fortnightly changes in resistance at Coimbatore whe re number of crops served as host plants for this pest. The resistance level of various synthetic pyr ethroids to DDs varied from 80.0 to 96.4%. The extent of resistance in terms of percent survival w s 88.1-96.4, 87.2-94.3, 87.0-94.0, 84.3-94.2 and 80.0-91.8% for cypermethrin, fenvalerate, deltameth rin, lambda-cyhalothrin and beta-cyfluthrin, respectively.


INTRODUCTION
The American bollworm Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), an important polyphagous pest, is widely distributed in Europe, Africa, Asia and South Pacific regions [1] . It has a wide host range of over 360 plant species including the crop plants namely cotton, maize, sorghum, sunflower, tomato, okra and legumes [2] . Annual yield losses attributable to this pest in India alone over 1000 crores. In Tamil Nadu, the pest is found very wide spread in summer cotton tract often reaching high populations and during 'Rabi', the insect is found in serious proportions in cotton as well as on pigeon pea, chickpea, groundnut, sunflower, bhendi etc. causing extensive damage. Owing to the favorable environment and farming conditions like crop mosaic resulted in extensive and frequent use of pesticides [3] . The pyrethroids, which were considered most potent insecticides for its control, lost their efficacy [4,5] . Earlier reports indicated that cypermethrin and fenvalerate effectively reduced H. armigera population and damage [6][7][8][9][10] . Continuous use of pesticides against this pest resulted in the development of resistance to these pesticides. The first report of pyrethroid resistance in H. armigera reported from Australia in 1983 [11] was followed by reports from other countries viz., Turkey in 1984, Thailand and Columbia in 1984-85 [12] and India in Andra Pradesh [13] , Tamil Nadu [14] , Delhi, Punjab and Haryana [15] . The present investigations hence were conducted with a view to monitor the insecticide resistance in H. armigera to the generally used synthetic pyrethroids at Insecticide Resistance Management Laboratory of Department of Agricultural Entomology, Tamil Nadu Agricultural University

Insecticides and dilutions:
The insecticide resistance monitoring bioassay was conducted with the most commonly used synthetic pyrethroids i.e., cypermethrin, fenvalerate, deltamethrin, lambdacyhalothrin and beta-cyfluthrin. The insecticide dilutions required for the Discriminating Dose (DD) assays were prepared from technical grade insecticides of known purity diluted with analytical grade acetone. The following DDs were used for resistance monitoring: Chemical DD (µg µL¯1) Reference Cypermethrin 0.1 LD99 calibrated for NRI pyrethroid susceptible strain and for Australian susceptible strain in early 1980's [11] . Fenvalerate 0.2 LD99 calibrated for susceptible strain in Australia [16] . Deltamethrin 0.0125 LD99 calibrated for susceptible strain in Australia [17] . Lambda-cyhalothrin 0.025 LD99 calibrated for susceptible strain in Australia [17] . Beta-cyfluthrin 0.2 LD99 calibrated for susceptible strain in Australia [17] .

Insect and bioassay:
The resistance level in H. armigera to different synthetic pyrethroids was monitored using discriminating dose screen by topical application of the insecticides. H. armigera populations of TNAU farm, Coimbatore District, Tamil Nadu were monitored for insecticide resistance to synthetic pyrethroids at fortnightly intervals for a period of one year (May 2003-April 2004). Third to fourth instar H. armigera larvae (30-40 mg) were culled out from field collection and placed individually on semi-synthetic diet in 12-well trays. One µl aliquot of DDs was placed on the thoracic dorsum of each larva using a 1 µL repeats dispenser (PB 600-01, Hamilton Co. Ltd.) fitted with a 50 µL syringe and "Rheodyne needle". Control larvae were treated with acetone alone. For each treatment not less than 50 larvae were used. The observation on the mortality was made daily up to 144 h (6 days) after dosing. A larva is considered dead if it is unable to move in a co-ordinated manner when prodded. The total number of larvae dosed and total mortality was computed. The per cent survival/per cent resistance was calculated by using following formula [18] : Also, the pooled binomial standard error was calculated using the formula: (100 P) P n 1

− −
Where: P = Percentage of larvae surviving discriminating dose n = Total number of larvae tested that fortnight

RESULTS AND DISCUSSION
The resistance frequencies exhibited by H. The armigera population of Coimbatore for different synthetic pyrethroids viz., cypermethrin, fenvalerate, deltamethrin, lambda-cyhalothrin and beta-cyfluthrin were in the range of 80.4 to 98.3% (Table 1). No variation in resistance due to seasonal changes was observed for the five synthetic pyrethroid compounds tested. The difference among the individual compounds was not much throughout the period of monitoring. Irrespective of the hosts, H. armigera larvae showed a higher level of resistance to all the synthetic pyrethroids and the order of resistance was cypermethrin> fenvalerate > deltamethrin > lambdacyhalothrin > beta-cyfluthrin. Survey on resistance monitoring conducted during the present investigation revealed that the field population of H. armigera from Coimbatore, Tamil Nadu showed a very high level of resistance to synthetic pyrethroids ( Table 1). The resistance levels observed confirmed the high level of resistance to cypermethrin and fenvalerate reported earlier through monitoring studies conducted since 1993 by Gowthaman [19] , Pasupathy and Regupathy [14] , Regupathy [20] and Regupathy et al. [3,21,22] .    [23] reported high level of resistance to fenvalerate and deltamethrin. Tamilselvi [24] , Niranjankumar [25] and Ramasubramanian [26] recently confirmed the high level of resistance to deltamethrin, cypermethrin, lambdacyhalothrin, fenvalerate and beta-cyfluthrin in Tamil Nadu. The resistance development due to exposure of earlier compounds was reported from different parts of India [4,13,15,[27][28][29][30][31][32][33][34][35][36] . Thus, resistance to the pyrethroids seems to be static without showing any decline. Obviously the availability of succession of the host crops (okra, tomato, pigeonpea, chickpea, sunflower, cotton) and intense insecticide applications is responsible for the further maintenance and spread of insecticide resistance in South India. The data presented indicate the need for continuous monitoring of insecticide resistance in H. armigera and for identifying the actual mechanisms of resistance involved and in the development of insecticide resistance management strategy.