Summary -Rosaceae longhorned beetle, Osphranteria coerulescens, is an important pest of fruit trees that attacks all fruit trees belonging to the family Rosaceae in cold regions of Iran. The potential efficacy of two species of entomopathogenic nematodes (EPN), Heterorhabditis bacteriophora and Steinernema carpocapsae, against last instar larvae of this pest was tested under laboratory conditions. In plate assays, the larvae were susceptible to both EPN species but were more susceptible to S. carpocapsae (65.0-97.5% mortality) than H. bacteriophora (42.5-87.8%). Both nematode species were able to penetrate and reproduce within O. coerulescens larvae, but reproduction rate for H. bacteriophora was higher than those of S. carpocapsae; however, the penetration rate for S. carpocapsae was greater than H. bacteriophora. In a migration test on agar plate, S. carpocapsae showed negligible attraction to the pest or to Galleria mellonella cues. However, H. bacteriophora was strongly attracted to the sector of Petri dishes containing larvae. In a test using apricot tree branches, both species of EPN passed from mass frass of O. coerulescens larvae in the tip of the branches, penetrated into the larval galleries, and located and killed the larvae of O. coerulescens in their natural habitat deep inside the branches. Our findings provide the first insight into the biocontrol efficacy of EPN against O. coerulescens larvae, and highlight their potential for the control of this pest. Field experiments are needed to evaluate their potential under the environmental conditions in which rosaceae longhorned beetle larvae are found.Keywords -attraction, behaviour, Heterorhabditis bacteriophora, insect pathology, reproduction, Steinernema carpocapsae, virulence.Rosaceae longhorned beetle, Osphranteria coerulescens Redtenbacher (Coleoptera: Cerambycidae), is an economically important pest of fruit trees in cold regions of Iran, Turkey and Syria. This beetle attacks all stone and pome fruit trees belonging to the family Rosaceae, as well as Japanese quince, oleaster, willow and elm (Rajabi, 1967). Adult beetles are active from late May to mid-June. Female insects prefer spring twigs for egg laying. Adult beetles lay a single egg into the upper part of twigs. Each insect lay 30-75 eggs and the average number is 52 (Sharifi et al., 1970). When larvae emerge, they bore through the cambium and hardwood (Behdad, 1974) penetrating into the centre of branches where they feed for a year (Rajabi, 1967). In thick branches and trunks, movement of O. coerulescens larvae and their feeding cause the branch to become fragile, and easily * Corresponding author, e-mail: jkb@ferdowsi.um.ac.ir broken by wind (Esmaeili, 1983). The obvious sign of pest infestation is readily apparent when the tips of infested twigs break off and then larger branches break off as larval boring proceeds. The larva requires 11 months to develop while the pupa completes development during 22-24 days. This pest causes extensive damage to living rosaceous trees in Iran (Sharifi...

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