PhD

A study on “Characterization and management of insecticide resistance in Spodoptera litura (Fabricius) (Lepidoptera:Noctuidae)” was undertaken at Thiruvananthapuram, Pathanamthitta and Alappuzha districts of Kerala during 2014-2017. Objectives were to assess the insecticide resistance in field populations of S. litura, investigate the mechanisms of resistance and to evaluate the efficacy of new generation insecticides against the resistant populations.
A preliminary survey conducted among vegetable growers in Palappur, Kovilnada, Nedingil, Amaravila and Venganoor locations of Thiruvananthapuram district revealed control failures in the field population of tobacco caterpillar due to the continuous application of insecticide like chlorpyriphos, quinalphos and lambda cyhalothrin. The survey data served as the benchmark for further studies on the insecticide resistance in S. litura.
Mass rearing was undertaken on artificial and natural diets to satisfy the requirements for conducting various bioassays. Reduction of total larval duration of S .litura to 13.08 days was observed in case of larvae reared on castor based diet which was significantly superior when compared to larvae reared on castor leaves (14.08 days), diet-1 (14.50 days), cowpea leaves (20.83 days) and amaranthus leaves (18.33 days). Hence, castor based diet, diet-1 and castor leaves were selected for mass rearing of S. litura throughout the study period.
Bioassay was carried out in CRD to assess insecticide resistance in populations of S. litura collected from different locations viz., Palappur, Kovilnada, Nedingil, Amaravila, Venganoor, Thiruvalla (Pathanamthitta) and Kanjikuzhi (Alappuzha) using insecticides viz. chlorpyriphos, quinalphos, cypemethrin, fenvalerate, lambda-cyhalothrin, chlorantraniliprole and flubendiamide at different doses. Results revealed that population collected from organic fields of Kanjikuzhi was found to be susceptible to insecticides with resistance ratio-1. Population collected from Kovilnada was found to be resistant with a resistant ratio of 6.14, 10.41, 6.48, 1.79, 8.50 and 3.54 when compared with field check (Kanjikuzhi) and 1965, 605, 2566, 916, 826 and 821 when compared with baseline susceptibility check (NBAIR strain) for chlorpyriphos, quinalphos, cypermethrin, fenvalerate, lambda-cyhalothrin and chlorantraniliprole respectively followed by populations collected from Palappur and Nedingil. Whereas, methomyl (2.92 and 183 fold) and flubenidamide (3.31 and 314) resistance were high in populations collected from Amaravila and Venganoor. Among the test locations, Kovilnada and Palappur were selected whose populations showed comparatively higher levels of resistance. These two populations along with one field check from Kanjikurhi and susceptible strain check from National Bureau of Agricultural Insect Resources (NBAIR), Bangalore were selected for further studies to evaluate the levels and mechanisms of resistance when exposed to chlorpyriphos, quinalphos, cypermethrin and lambda-cyhalothrin.
With a view to elucidating the mechanisms of resistance, S. litura populations collected from four locations were monitored to investigate the levels of detoxification enzymes which might have imparted resistance. Biochemical analysis revealed the specific activities of detoxification enzyme Acetyl choline esterase (AChE) was 16.16 times more in Kovilnada population followed by 10.71 times in Palappur and 4.88 times in Alappuzha populations when compared with NBAIR check. Whereas, Mixed Function Oxidase (MFO) enzyme which showed 19.24 times more specific activity in Kovilnada population followed by Palappur with 17.11 fold and Alappuzha with 6.08 indicating the dominant role of AChE and MFO in imparting resistance. Whereas, Carboxyl esterase (CarE) and Glutathion-S-transferase (GST) with 3.62 and 3.37 fold in Kovilnada followed by 2.89 and 2.98 fold in Palappur, 2.10 and 1.15 fold in populations of Kanjikuzhi indicated their partial role in resistance development.
The results obtained in synergism study revealed that piperonyl butoxide (PBO) was observed to be the most effective synergist capable of breaking the resistance in all the test chemicals at 1:4 combination followed by Diethyl maleate (DEM) and Tri phenyl phosphate (TPP) for chlorpyriphos and TPP and DEM against quinalphos, lambda-cyhalothrin and cypermethrin indicating major role of mixed function oxidases and esterases and partial role by GSTs in resistance development against organophosphates and synthetic pyrethroids. Further, the results associated with synergism assays were confirmed by monitoring the detoxification enzyme activities in synergized populations where, 9.32 fold reduction of AChE in chlorpyriphos+TPP treatment and 4.78 fold reduction of GST in chlorpyriphos+DEM and 5.15 fold reduction of CarE in quinalphos+TPP combinations elucidated the role of multiple detoxification enzymes in imparting resistance to organophosphates. Whereas, 7.33 fold reduced activity of AChE in case of lambda-cyhalothrin+TPP and 5.35 fold reduced activity of MFO in case of lambda-cyhalothrin+PBO and 7.36 fold reduced activity of CarE in case of cypermethrin+TPP and 3.60 fold reduction in MFO in case of cypermethrin+PBO elucidated the role of esterases and mixed function oxidases in imparting resistance against synthetic pyrethroids.
In vitro evaluation to check the efficacy of new generation insecticides against the population of S. litura revealed cyantraniliprole- 60 g a.i.ha - 1, spinosad- 100 g a.i.ha - 1, emamectin benzoate- 10 g a.i.ha - 1, indoxacarb- 75 g a.i.ha - 1, thiacloprid-120 g a.i.ha - 1, chlorantraniliprole-30 g a.i.ha - 1, flubendiamide- 48 g a.i.ha - 1 and spinetoram-60 g a.i.ha – 1 were found to be superior when compared to the conventional check quinalphos with per cent mortality of greater than 85 per cent in all the tested populations and hence all the above promising treatments were selected for field experiment.
The field experiment was conducted in RBD with above new generation insecticides against the resistant population of S. litura resulted cent per cent reduction of field populations in emamectin benzoate, indoxacarb, spinetroam, chlorantraniliprole and flubenidiamide treatments. However, after second spray, cent per cent larval reduction was maintained in emamectin benzoate, indoxacarb and spinetoram treatments only. Similarly, lowest extent of leaf damage was noticed in spinetoram which was on par with cyantraniliprole, spinosad, emamectin benzoate, indoxacarb, chlorantraniliprole and flubendiamide. The residues of promising insecticides viz., emamectin benzoate, indoxacarb, chlorantraniliprole and flubendiamide dissipated with in three and ten days in amaranthus and no harvest time residues could be detected, revealing its safety for consumption. However, considering the eco-toxicological effects and the economic feasibility to the farmers, the insecticides chlorantraniliprole 30 g a.i.ha - 1, and flubendiamide 48 g a.i.ha - 1 at early stages, emamectin benzoate 10 g a.i.ha - 1, indoxacarb 75 g a.i.ha - 1 at later stages of crop near to harvest may be recommended to the farmers on rotational basis, based on mode of action.