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Dock's leaf defoliator sawfly, Kokujewia ectrapela Konow (Hym.,Argidae), is a medium sized sawfly native tothe Caspian fauna. The larvae were found living on Rumex spp. (Polygonaceae). In order to determine the biology, host specificity and feeding potential of the species, continuous rearing was conducted in field and laboratory conditions during 2011-2013. The results revealed that K. ectrapela completed six generations within the growing season and hibernated as a fully developed larva inside the protective cocoon in the plant litter surrounding the Rumex plants. The first generation appeared from the early April to the end of May and the last generation in the late September. After emergence, adults copulated and using their saw-like ovipositor fertile females inserted their eggs along the edges of the Rumex leaves. Mean fecundity was showed to be 148.67 ± 37.33 eggs per female. Newly hatched larvae aggregated and fed on the underside of the leaves. However, later instars dispersed on the host plant and continued to feed on the leaves, leaving only the major veins. Larvae developed through three instars and it took 9-23 days depending upon environmental conditions. Pupation occurs within silken whitish cocoons spun among or out of the host plant. Depending upon environmental conditions, the developmental time from eggs to adults lasted 28-43 days. Study on the parasitoids revealed that the larvae of K. Ectrapela were parasitized by Cryptus inquisitor (Hym., Ichneumonidae) and Tetrastichus kokujewiae (Hym., Eulophidae) in Urmia region. Results of no-choice feeding tests with the second instars on 27 plant species belonging to 13 families showed that K. ectrapela completed its life cycle mainly on plants of Rumex and occasionally fed on Polygonum persicaria L. (Polygonaceae). Investigation on feeding activity of the three larval instars of K. ectrapela on Rumex obtusifolius L. under laboratory conditions revealed that a single first-instar larva consumed mean 0.041 ± 0.001 g of R. obtusifolius leaves over its 3 days of development. Second and third instar larvae consumed mean 1.227 ± 0.006 g, and 3.058 ± 0.014 g, over their 4 and 5 days of development, respectively.