author(s)
Sevda Dadrass, Shahram Mirfakhraie, Shahram Aramideh and Mohammad Hassan Safaralizadeh

abstract
Because alfalfa forage is a high quality crop in terms of nutrients for livestock, it is important to manage the crop efficiently in the absence, as well as the presence, of serious pests. The alfalfa weevil Hypera postica Gyllenhal is a worldwide variable pest that is established in almost all Iranian alfalfa fields and Alfalfa weevil larvae affects quality by attacking the upper canopy. In this research, three insecticides include Deltamethrin, Cypermethrin and Chlorpyrifos were evaluated for their efficacy in the control of 3^rd larval stage of H. postica, under field conditions. The LC₂₅ and LC₅₀ values of Deltamethrin, Cypermethrin and Chlorpyrifos were estimated on 3^rd larval insect. Furthermore, LC₂₅ and LC₅₀ values for Deltamethrin, Cypermethrin and Chlorpyrifos on 3^rd larval insects were 587.89, 6.14, 186.84 and 1142.64, 110.17, 636.56 ppm for 24 hours and 333.30, 12.03, 135.49 and 772.37, 50.24, 370.47 ppm for 48 hours, respectively. To evaluate effects of three insecticides one experiment with completely randomize design in three replicate by recommendation doses carried out in field conditions. The results show that there were significant differences between treatments (Deltamethrin, Cypermethrin, Chlorpyrifos and control) after 24, 48 and 72 hours. After 24 and 48 hours Cypermethrin was more lethal in comparison to other treatments, and after 72 hours the Chlorpyrifos was lethal. The result showed that for control of larval stage of H. postica in fields conditions Chlorpyrifos recommended.