Pistachio twig borer, Hylesinus vestitus Rey, is an important pest of pistachio trees that causes injury with feeding in buds and twigs, The final damage being yield loss. To assess the effect of semiochemicals as behavior modifying factors, a study was conducted in a non — commercial infested orchard. P.V.C. cylindrical sticky traps were used. Six treatments with three replicates including empty traps, traps containing healthy branch, injured branch and branches that were artificially infested by six males, six females and three pairs of males and females respectively, were evaluated in a randomized block design. Supplementary studies were designed to evaluate the effect of beetle densities on attraction. The results indicated that primary attraction of males and females to breeding sites is due to volatiles (Kairomone) emanating from pistachio tissues. Attack of beetles to host branches and their boring activity increased volatilization of host attractant. This together with aggregation pheromone produced by virgin females were responsible for the attraction of a large number of males and females to the host.
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