Evaluation of the toxic effect of common pesticides cypermethrin, imidacloprid and Neem (Azadirachta indica) fruit extract, on earthworm (Pheretima posthuma)

The toxic effect of common pesticides i.e. cypermethrin and locally used Neem (Azadirachta indica) fruit extract has been investigated on adult earthworm (Pheretima posthuma) and compared with another commonly used pesticide imidacloprid. The earthworms were treated with the test compounds using feeding cum contact method to establish LD50 values. Major organs of the LD50 treated surviving earthworms were subjected to total protein extraction, estimation, SEC FPLC and SDS-PAGE analysis. The total protein contents of cypermethrin (which was found to be the most toxic, LD50 = 0.14 ppm) treated earthworms was 25.2, 37.2 and 38.5 mg/ml in the peristomium, clitellum and abdominal regions, respectively. Whereas Neem fruit extract treatment (which was found to be the less toxic LD50 = 0.48 ppm) demonstrated moderate effect on total protein concentrations (37.8, 54.8 and 66.4 mg/ml). Imidacloprid which is also a commonly used pesticide was found to be more toxic (LD50 = 0.24 ppm) to earthworms as compared to Neem fruit extract and showed protein concentrations of 27.7, 26.1 and 30.3 mg/ml in the peristomium, clitellum and abdominal regions, respectively. The untreated or control animals showed protein concentration of 63.9, 76.8 and 78.7 mg/ml in peristomium, clitellum and abdomen regions, respectively. These results were further confirmed by SEC FPLC profiles which demonstrate drastic differences in terms of the proteins and peptides compared to their respective controls. These results were further complemented by SDS PAGE analysis of extracted proteins of peristomium (head) region which revealed that cypermethrin is selectively more toxic to proteins of the head region as compared to imidacloprid and Neem fruit extract suggesting different mode of action of these pesticides on different organs of the earthworms. In conclusion, our results demonstrate for the first time not only the toxic effect of these commonly used pesticides but also the differential action on the secondary target organisms such as earthworms.