Environmental factors such as soil moisture and nutrition can affect plant quality. Considering insect pests damage on sugar beet, a study on single and combind effects of water and nitrogen fertilizer amount on pest damage and sugar beet growth was carried out. Three irrigation regimes (100%, 75%, 50% plant water requirement in 2005 and irrigation after 70, 105 and 140 mm cumulative evaporation from class A pan in 2006) and three fertilizer levels (200, 150, 100 kg/ha net nitrogen in both years) were selected in a complete randomized block design at two farms, sprayed and non-sprayed. Pests population density was assessed by weekly sampling. To determine pests damage, agronomic parameters were studied in these two farms. Developmental time and fecundity life table parameters of Spodoptera exigua reared on leaves collected from different plots were studied in laboratory. The results showed that effect of irrigation treatments on Empoasca decipiens population was not significant but the highest and the lowest leafhopper numbers were observed in 200 and 100 kg/ha nitrogen treatments in the first year. In the second year, adults and nymphs of E. decipiens and onion thrips Thrips tabaci were highest in 105 mm irrigation treatment and 200 kg/ha nitrogen treatment. The highest mean population of the black bean aphid Aphis fabae was observed in 100% irrigation treatment in the first year and in 70 mm irrigation treatment in the second year and in 200 kg/ha nitrogen treatment in both years. The effect of irrigation and fertilizer on population of S. exigua was not significant in the first year. The longest larval and pupal developmental time, the lowest pupal weight and shortest adult longevity were related to 70 mm irrigation treatment. Additionally, the longest pupal developmental time, lowest pupal weight and shortest adult longevity was related to 100 kg/ha nitrogen treatment. The highest intrinsic rate of increase (r m ), net reproductive rate (R o ), gross reproductive rate (G. R. R.), finite rate of increase (?), the shortest mean generation time (T) and dubling time (t) were related to 105 mm irrigation and 200 kg/ha nitrogen treatments. The effect of irrigation and fertilizer treatments and their interaction on pests damage, dry weight of foliage and root, root yield, leaf area index, sugar percentage and ?-amino nitrogen were significant. Insect
