Extended Abstract
1. Introduction: The tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), is one of the most serious pests affecting tomato (Solanum lycopersicum) production worldwide. Originally from South America, it has rapidly spread to many regions, causing severe damage to both open-field and greenhouse tomato crops. Chemical insecticides have long been the primary tool for managing this pest. However, the intensive and often unregulated use of these chemicals has led to widespread resistance in many populations, greatly reducing the effectiveness of several commonly used insecticides. Thiocyclam hydrogen oxalate is among the insecticides recommended for controlling T. absoluta. It acts as a neurotoxin, disrupting nerve function by blocking the ion channels of nicotinic acetylcholine receptors (nAChRs), ultimately leading to paralysis and death in the insect. Despite its effectiveness, the repeated use of thiocyclam may lead to resistance over time, making it essential to monitor susceptibility levels and explore ways to enhance its efficacy. Understanding the resistance status and underlying mechanisms of T. absoluta to thiocyclam in regions like Iran, where tomato cultivation is widespread, is crucial for developing effective resistance management strategies. In this study, we evaluated the susceptibility of T. absoluta populations to thiocyclam hydrogen oxalate and investigated the use of synergists to potentially restore or enhance the insecticide’s potency. These findings aim to support integrated pest management (IPM) approaches and ensure more sustainable control of this highly invasive pest.
2. Materials and Methods: The population of absoluta was collected from different greenhouses in Iran. The population with the lowest LC50 was selected as the sensitive population for the bioassay experiments. The insects were collected in 2020 and tested in the laboratory of Mohaghegh Ardabili University. The insecticidal efficacy of thiocyclam hydrogen oxalate alone and in combination with piperonyl butoxide (PBO) (monooxygenase inhibitor), diethyl maleate (DEM) (inhibitor of glutathione S-transferase), and triphenyl phosphate (TPP) (carboxylesterase inhibitor) was evaluated. For experiments with synergists, concentrations of 1000 ppm for TPP and 200 ppm for DEM and PBO were prepared. The bioassay was performed with second instar larvae, with leaves immersed for ten seconds. Laboratory conditions were 25±1°C, 60% RH, and 16:8 hours of light. A total of eight populations from Iranian greenhouses were studied with five replicates, five concentrations, and ten insects per replicate. Larvae were exposed to the synergists for 2 hours before being used for the bioassay test.
3. Results and Discussion: Based on the bioassay of the insecticide thiocyclam hydrogen oxalate on different populations of absoluta in the second larval instar, different levels of susceptibility can be observed between populations, and the resistance ratio in T. absoluta populations of Ardabil1, Yasouj, Borujen, Yazd, Bushkan, Kerman, Esfahan, and Ardabil2 was 3.65, 2.70, 3.05, 1.71, 1.63, and 1.062 fold, respectively. The population in Ardabil1 was the most resistant population, with a 3.65-fold resistance ratio and an LC₅₀ value of 275.12 mg/L. The population in Yasouj was the most sensitive population, with an LC₅₀ value of 82.75 mg/L. Synergistic effects on resistant and sensitive populations to thiocyclam hydrogen oxalate insecticides showed that the synergist piperonyl butoxide had the greatest reduction in LC₅₀ in different populations of T. absoluta. In addition, the population in Ardabil1 had the highest resistance ratio, with the synergistic effects of piperonyl butoxide, diethyl maleate, and triphenyl phosphate on the tomato leaf miner being 1.95, 1.83, and 1.76-fold, respectively. The highest rate of synergists in the Yazd and Ardabil1 populations was related to the PBO synergist, which reduced the LC₅₀ from 252.27 and 275.12 mg/L (without insecticide) to 102.15 and 141.18 mg/L (larvae were exposed to the synergist). In the Yasouj population, the synergist ratio was slightly reduced for PBO, but for DEM and TPP, the synergist ratio was not reduced or was very low. The results show that the use of the insecticide thiocyclam hydrogen oxalate with the synergists piperonyl butoxide, diethyl maleate, and triphenyl phosphate increased the toxicity and decreased the resistance of thiocyclam hydrogen oxalate.
4. Conclusion: In general, the results seem to indicate that thiocyclam hydrogen oxalate is still effective against tomato leaf miner, and that resistance levels are not critical. Synergists can be used to delay low resistance, reduce spray pressure for this insecticide, and use alternative insecticides. The results of this research will be useful for managing the resistance of tomato leaf miner moths to this insecticide.