Marchalina hellenica Gennadius is a common pest prevalent in Pinus brutia Ten. forests in Mugla province of Mediterranean Region of Turkey (28 degrees 20'03"E; 37 degrees 12'18"N). The pest is known to suck sap from a number of pine tree species, especially Pinus brutia. The goal of this research is to determine whether the pest has a negative impact on the volume increments of Pinus brutia trees and forests. Two areas with identical site conditions are selected for study: one non-infested and one infested with Marchalina hellenica. The effects of site quality and age on the sampled trees have been eliminated. The diameter growth at breast height, basal area (the cross-sectional area of a tree at breast height) and the increment of tree volume are determined separately for both study areas. A t-test applied to volume increments of both infested and noninfested trees of all ages proves that infestation causes significant negative impacts on trees between the ages of 40 and 80 years. The greatest loss of increment in single trees is 2.0% (of the volume of non-infested tree) which occurs at the age of 70 years old. When considering the entire stand, the greatest loss of increment is 3.4% (of the volume of non-infested stand) which occurs at the age of approximately 45 years.