Bemisia tabaci Genn.) is due to high resistance development against recommended insecticides

1 Horticulture Research and Development Institute, Department of Agriculture, Peradeniya. 2 Department of Agricultural Biology, Faculty of Agriculture, University of Peradeniya, Peradeniya. 3 Sri Lanka Organization of Agriculture Professionals, Department of Agriculture, Peradeniya. 4 Department of Zoology, Faculty of Science, University of Peradeniya, Peradeniya. 5 National Institute of Fundamental Studies, Hantana Road, Kandy.


INTRODUCTION
Bemisia tabaci Genn. (Aleyrodidae: Hemiptera) is a serious pest that is used to be controlled by insecticides in many horticultural crops including beans, brinjal, tomato, cucurbits and ornamental plants in Sri Lanka.  2005). B. tabaci has been ranked as the 5 th top resistant insect species among agricultural insect pests, being resistant to 54 insecticide compounds with 555 recorded cases throughout the world (Sparks & Nauen, 2015). Acetamiprid, thiamethoxam, profenofos, carbosulfan and etofenprox are among the compounds to which reduced susceptibility levels have been detected (Li et al., 2000;Farghaly, 2010;Houndete et al., 2010).
Insecticide resistance in a population is commonly measured by calculating the resistance factor (RF), which is the ratio of LC 50 between a test population and a known susceptible population. A RF of 10 or greater has been used as a threshold to declare a resistance development et al., 2008). Discriminating dosages or resistance diagnostic dosages, which are supposed to kill all the susceptible individuals leaving the resistant ones, have also been used to determine the resistance et al., 2008). This diagnostic dosage insecticide.
Knowledge on the resistance status of a particular insect pest is vital to evaluate the validity of the insecticides in use and their recommended dosages, in order to achieve effective management of the pest. Insecticide resistance management (IRM) is a group of strategies that helps to maintain the susceptibility status of insects to available insecticides, and maintain their effectiveness by preventing or delaying the development of resistant strains (Tejeda et al., 2009). Thus, the data obtained from resistance studies are essential for integrated IRM programmes for an effective pest management with a minimum risk to health and environment.
In this study, we report the resistance status of three and Intermediate (annual rainfall 1,750 -2,500 mm) Zones of Sri Lanka to thiamethoxam, acetamiprid, etofenprox, carbosulfan and profenofos. Compounds from each of the major classes of insecticides were populations have developed broader resistance spectrum to different insecticide classes.

METHODOLOGY
B. tabaci were the Wet Zone, Kahatagasdigiliya in the Dry Zone and Bandarawela in the Intermediate Zone of Sri Lanka during plastic vials using aspirators, from brinjal, tomato and bean crops. They were chilled and transported to the Entomology Laboratory of the Horticulture Research and Development Institute at Gannoruwa and reared

Insecticide bioassays
(profenofos 500 g/L EC), a carbamate (carbosulfan 200 g/L SC) and a pyrethroid (etofenprox 100 g/L EC) were used for insecticide bioassays. The leaf dip Committee (IRAC, 2009), was adopted as it is closely related to the exposure pattern of the recommended foliar spraying.
Immature brinjal leaves were cut into square shapes of 3 × 3 cm 2 with petioles dipped in different concentrations of commercial formulations of the insecticides for 5 s and left to air dry approximately for 30 mins. Insecticide dilutions were prepared in distilled water and the dosages giving mortality between served as the controls. The insects were exposed to the insecticide treated leaves inside transparent yoghurt cups. Prior to the assay a portion of the centre of the lid (4 cm diameter) and a portion of one side (2 × 4 cm 2 ) of proper ventilation. The petiole of a leaf was inserted through a small tight hole made at the base of the cup, and the tip of the petiole coming out of the cup was covered with a swab of moistened cotton wool. About each cup through the side opening for each assay. Each treatment was replicated three times and the mortalities were recorded after 24 hrs.
Data were taken only if the control mortalities experiments were corrected with control mortalities using the Abbott's formula (Abbott, 1987). Active ingredient concentration present in each dilution was calculated using the information given in the label of each insecticide formulation. Percentage mortalities (probability axis) were plotted against the concentration of the active ingredient (log axis). Lethal concentration, 50 of the population (LC 90 ), slope of the mortality curve, 2 ) were obtained by probit analysis using Sigma Plot (Version 10.0) software. from Gannoruwa (n = Kahatagasdigiliya (n = and Bandarawela (n = 701) exposed to thiamethoxam using bioassays by leaf-dip method. Dosage recommended 50 for a susceptible population (Vassiliou, 2011) and a resistance discrimination dosage (Huang et al., 2007) are also shown for comparison. from Gannoruwa (n = 1028), Kahatagasdigiliya (n = and Bandarawela (n = 951) exposed to acetamiprid using bioassays by leaf-dip method. Dosage recommended by 50 for a susceptible population (Vassiliou, 2011) and a resistance discrimination dosage (Huang et al., 2007) are also shown for comparison. Monitoring programmes have revealed that the patterns of insecticide usage (i.e. continuous, mosaic or from Gannoruwa (n = Kahatagasdigiliya (n = and Bandarawela (n = 701) exposed to profenofos using bioassays by leaf-dip method. Dosage recommended by 50 for a susceptible population (Huang et al., 2007) are also shown for comparison.

LC 50
is recommended for brinjal shoot and pod borer control are exposed to this insecticide regularly, developing the resistance. However, the Bandarawela B. tabaci population, which was collected from bean cultivations where etofenprox is not routinely applied, has also shown a high resistance level against etofenprox. Therefore, it is important to further examine the mechanism of cross resistance development to etofenprox in future reported from other countries (Cahill et al., 1995). Both carbosulfan and profenofos are recommended for the and the farmers tend to apply these on other crops also due to the broad spectrum pest control ability of the insecticides. High effectiveness of profenofos against resistance management (WFRM) programmes.
The underlying mechanisms for the observed carbamate and organophosphate classes need to be studied in detail as both groups share the same target site, acetylcholinesterase. Insecticide resistance of insects can be developed due to qualitative and/or quantitative due to mutated target sites (Karunaratne, 1998). The resistance to neonicotinoids in B. tabaci has been cytochrome P450 mono-oxygenases (Rauch & Nauen, 2003; Jones et al esterases and monooxygenases, and target site mutations in acetylcholineesterases have been associated with insecticide resistance in B. tabaci (Alon et al., 2008;Roditakis et al., 2009). Further studies to investigate the underlying molecular mechanisms for the high insecticide future agricultural pest control programmes of Si Lanka.
According to our results, all the tested insecticides level, which is not economically or environmentally acceptable. Repeated application of insecticides for which pests have already developed resistance, has led populations establishing resistance genes in the populations giving high reproductive dynamics (Heong et al., 2011). An integrated approach including physical and biological control strategies together with effective insecticides followed by regular resistance monitoring WFRM programmes of the country.

CONCLUSION
Zones of Sri Lanka show high resistance to thiamethoxam, acetamiprid, carbosulfan and etofenprox demanding or environmentally viable option, these insecticides with immediate effect. Organophosphates can be used as alternatives. An integrated approach with effective insecticides followed by regular resistance monitoring populations in Sri Lanka.