Cinara cupressi is an introduced species of insect that can kill forest trees due to a toxic compound that it injects while feeding. The pest is widely distributed in Chile, infesting species of Austrocedrus, Cupressus, Chamaecyparis, Juniperus, Thuja and Cupressocyparis hybrids. Austrocedrus chilensis is affected in all of its geographical distributions. In Africa, Cinara cupressi was identified for attacking the genera Callitris and Widdringtonia, which are phylogenetically related to Fitzroya, Austrocedrus and Pilgerodendron. This aphid can also adapt to F. cupressoides and Pilgerodendron uviferum and therefore threaten both species. The present study aimed to determine the usefulness of an insecticide treatment applied after aphids are detected on A. chilensis, based on the evolution of the damage after the insecticide application. Damage levels were determined based on the various types of symptoms caused by the aphid on the plants. Dimetoate was injected into the trunk, and the evolution of damage was registered every twenty days eight times. Based on the obtained results, it can be concluded that the results of this method of chemical control were statistically significant only for trees showing slight insect damage at the time of the insecticide application. The aphid damage was neither stopped nor retarded significantly when the insecticide was applied onto trees already showing moderate or high levels of insect damage.