Investigating the Residual Status of Imidacloprid and Thiamethoxam in Lettuce after Seed Treatment

The area under cultivation of lettuce in the country is 14,000 hectares, and the amount of lettuce production in Iran is 300,000 tons. Among the important damaging factors in lettuce production is the tomato spotted wilt virus (TSWV). With its wide host range, this virus is transmitted mechanically through infected plant sap by thrips. In Iran, various insecticides such as spinosad, diazinon, imidacloprid, profenofos, etc., which are from different groups, have been registered to control thrips. New reports show that other insecticides like thiamethoxam and imidacloprid are effective on thrips, too.The use of pesticides should be performed very carefully due not only to the residue on the surface of the products, but also its penetration into the tissue of fruits, vegetables, and even grains. Excessive use of pesticides in the production of agricultural products causes pesticide residue, which is considered a risk factor for human health and the environment. High pesticide residues in all kinds of products, especially lettuce, which is consumed fresh, is of particular importance. Therefore, appropriate measures must be taken to reduce the risks of high residue levels of pesticides.

The insecticides imidacloprid and thiamethoxam were used to treat lettuce seeds to control the thrips, which is the vector of this virus. The first treatment consisted of seed treatment of lettuce with imidacloprid and the second one with thiamethoxam. Twenty transplants from each treatment were evaluated regarding thrips infection and TSWV viral disease. To extract the samples for the measurement of pesticide residues QuEChers method was used. The extracted solution was injected into the LC-MS/MS to identify and measure the pesticide residue in the lettuce samples after eight and eleven weeks of cultivated treated seeds.

The results show that thiamethoxam, with 84.5%, and imidacloprid, with 45% efficacy, can prevent infection with TSWV up to one month after transplantation. The results show that no detectable imidacloprid residue was observed in the lettuce at the two sampling periods, and only 0.33 mg/kg and 0.28 mg/kg of thiamethoxam were detectable after 8 and 11 weeks, respectively. According to the national Maximum Residue Levels (MRL) of thiamethoxam (3 mg/kg), the measured residue levels are lower than the MRL.

Therefore, according to the results obtained, the insecticides used for the lettuce seed treatment to control the thrips with the doses mentioned above are safe for the consumers of the final product as the residue of these insecticides after treatment is well below the national MRLs.