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The leafy spurge defoliator moth Simyra dentinosa Freyer is a monophagous insect that eats on herbaceous species of Euphorbia. The biology as well as natural enemies of S. dentinosa were studied in field (during 2002-2003) as well as laboratory conditions in Orumieh. Field and laboratory observations showed that spurge defoliator moth feeds on leaves, buds and flowers of different spurge species in western parts of Orumieh lake. When daily mean temperature reached 10?C, first adults emerged from pupae in early March to mid–May. During 2–4 days after emergence, adults mate and females oviposited in clusters on lower surfaces of the host plants. The eggs were placed in 4–8 regular rows depending upon the width of leaves. Maximum egg number in a cluster was 381 while mean number of eggs per cluster was determined as 195±45. Mean number of eggs laid per female was 520±43. At a mean daily temperature of 14.1?C, the incubation period ranged from 21 to 25 days. The insect has five larval stages, young larvae (instars 1-3) being gregarious while the 4th and 5th instars solitary. At mean daily temperature of 12.1?C, larval development required 26±2 days. Developed larvae left their host plant to pupate and overwinter in silken protective cocoons, made in cell – like spaces of twisted and dry leaves. Thus, in Orumieh weather conditions, leafy spurge defoliator moth has only one generation each year and overwinters as pupa. Study on natural enemies of S. dentinosa indicated that the larvae get parasitized by four species of parasitic wasps as well as two species of parasitic tachinid flies, the parasitoids being as follows:
1- Cotesia ofella Nixon (Hym., Braconidae)
2- Cotesia vanessae Reinhard (Hym., Braconidae)
3- Cotesia plutellae (Kurdjumov) (Hym., Braconidae)
4- Hyposoter didymator (Thunberg) (Hym., Ichneumonidae)
5- Exorista larvarum (Linnaeus) (Dip., Tachinidae)
6- Pales pavida (Meigen) (Dip.,Tachinidae)
With an exclusion of Exorista larvarum other species are new record from Iran.