Ph.D

The investigation entitled “Characterisation of baseline susceptibility of banana pseudostem weevil, Odoiporus longicollis (Olivier) (Coleoptera: Curculionidae) to insecticides” was undertaken at the Department of Agricultural Entomology, College of Agriculture, Vellayani, Thiruvananthapuram during the period from 2022 to 2025. The objectives of the study were to assess the insecticide resistance in field population of O. longicollis, investigation on mechanisms of resistance and confirmation on the role of enzymes in imparting resistance and molecular characterization of resistant population.
A detailed documentation was conducted among banana farmers in the districts of Thiruvananthapuram, Alappuzha, Thrissur, Palakkad, Kannur in Kerala and two neighboring districts of Tamil Nadu viz., Coimbatore and Kanniyakumari. About 88.57 per cent of farmers identified O. longicollis as the most prevalent insect pest, while
25.71 per cent reported chlorpyriphos as the most commonly used insecticide, followed by quinalphos at 12.86 per cent. Among farmers interviewed, 27.14 per cent experienced failures in controlling O. longicollis, even after the adoption of chemical control measures. Only 11.43 per cent practiced insecticide rotation to manage resistance and nearly 55.71 per cent of the farmers applied insecticides at dosages higher than recommended levels.
Pseudostem sheath piece dip bioassays were performed to assess insecticide resistance in O. longicollis, utilizing third instar larvae from field-collected adults across 14 distinct population. Two population were sampled from each of the seven districts, with sample codes denoting the district and specific sampling site: TVMVEN and TVMSAS from Thiruvananthapuram; ALPVEE and ALPVET from Alappuzha; TSRNAD and TSRMAD from Thrissur; PKDKIZ and PKDALA from Palakkad; KNRNEL and KNRTHA from Kannur; KKRVIL and KKRANJ from Kanniyakumari; and CBEPIC and CBEMAD from Coimbatore. The LC50 values obtained from field population were compared with those of a laboratory-reared susceptible reference strain (Pop-SUS), maintained for approximately six generations, to calculate Resistance Ratios (RR). The classification of RR followed the framework proposed by Mazzarri and Georghiou (1995), wherein the population was categorized as: no resistance (RR ≤ 1), very low resistance (1 < RR ≤ 3), low resistance (3 < RR ≤ 5), moderate resistance (5 < RR ≤ 10), and high resistance (RR > 10). The insecticides were selected for detection of resistance based on data from documentation viz., chlorpyriphos 20% EC, quinalphos 25% EC, monocrotophos 36% SL, cartap hydrochloride 50% EC and cypermethrin 25% EC. All field population exhibited RR ranging from very low to high RR across the tested insecticides. The CBEPIC population exhibited the highest RR to chlorpyriphos 20% EC (26.930), quinalphos 25% EC (17.398), and monocrotophos 36% SL (17.787), while the KKRVIL population showed the greatest resistance to cypermethrin 25% EC (15.104). Additionally, the TVMVEN population recorded the highest RR to cartap hydrochloride 50% EC (10.227).
The insecticides evaluated for detection of cross and multiple resistance included carbosulfan 25% EC, fenvalerate 20% EC, thiamethoxam 25% WG, cyantraniliprole 10.26% OD and spinosad 45% SC which were not used against
O. longicollis in any of the locations studied. Cross-resistance to carbosulfan 25% EC and fenvalerate 20% EC varied from very low to high across all field population, with the highest RR recorded in CBEPIC (13.455) and KKRVIL (10.873), respectively. Resistance to thiamethoxam 25% WG ranged from very low to moderate, with the TVMVEN population exhibiting the highest RR of 5.795. Very low to low resistance was observed against cyantraniliprole 10.26% OD and spinosad 45% SC in all population, with the CBEPIC population showing the highest RR of 4.292 and 3.233, respectively.
The population exhibiting the highest RR from each district, along with the laboratory-reared susceptible population of O. longicollis were subjected to biochemical analysis to evaluate the role of total protein and detoxifying enzymes viz., carboxylesterases (COEs), cytochrome P450s (P450s) and glutathione S-transferases (GSTs) in imparting insecticide resistance. Total protein content, along with the activities of COEs, P450s, and GSTs, were significantly elevated in all the field population of O. longicollis compared to the susceptible strain, with the highly resistant CBEPIC population exhibiting the highest total protein increase (2.274-fold) and the greatest enzyme activity increases of 3.510-fold for COEs, 3.261-fold for P450s, and 2.210-fold for GSTs. Chlorpyriphos, quinalphos, and monocrotophos to which O. longicollis exhibited the highest RR, were selected to confirm the role of detoxifying enzymes in resistance. This was done through a pseudostem sheath piece dip bioassay with three synergists, piperonyl butoxide (PBO), triphenyl phosphate (TPP), and diethyl maleate (DEM) applied at a 1:4 ratio with insecticides in CBEPIC, TVMVEN, TSRNAD population that recorded high RR to these insecticides as well as the highest activity of detoxifying enzymes. Synergism Ratio (SR), the LC50 of insecticide alone divided by the LC50 of insecticide plus synergist, and Synergism Resistance Ratio (SRR), the SR of the resistant population divided by the SR of the susceptible population was worked out. TPP consistently resulted in the highest SRs across all resistant population tested, followed by PBO and then DEM for all the tested insecticides. In the most resistant CBEPIC population, for chlorpyriphos resistance, TPP produced the highest SR (7.195), with PBO (6.269) and DEM (2.515) following. Similarly, for quinalphos and monocrotophos resistance, TPP again yielded the highest SRs (4.874 and 4.002, respectively), followed by PBO (5.781 and 4.137, respectively) and DEM (2.239 and 2.032, respectively). Similar, trend was observed in TVMVEN, TSRNAD and Pop-SUS population. This confirms that multiple detoxification enzymes, particularly the enhanced activity of COEs and P450s, play a major role in organophosphate resistance in O. longicollis, with GST activity contributing to a lesser extent. SRR>1 across all synergists and tested population confirm the role of synergists in effectively reducing resistance in O. longicollis.
The population exhibiting the highest RR and relative detoxifying enzyme activity (CBEPIC), along with a laboratory-reared susceptible population of
O. longicollis, was selected for molecular characterization using 20 RAPD markers. Polymorphism of 62.86 per cent across all primers indicated genetic variability between the highly resistant CBEPIC population and the susceptible population of O. longicollis. The study demonstrated that O. longicollis population has developed differential resistance to commonly used insecticides, viz., chlorpyriphos 20% EC, quinalphos 25% EC, monocrotophos 36% SL, cartap hydrochloride 50% EC, and cypermethrin 25% EC, with resistance ratios (RR) ranging from very low to high. Field population exhibited cross resistance to insecticides viz., carbosulfan 25% EC and fenvalerate 20% EC and multiple resistance to thiamethoxam 25% WG, cyantraniliprole 10.26% OD and spinosad 45% SC. The study confirmed increased activity of the detoxifying enzymes, carboxylesterases and cytochrome P450s against organophosphate insecticides. The presence of genetic polymorphism between resistant and susceptible population, further confirmed the development of resistance.