There are many different types of aphids and infestations. Are you applying an insecticide as a preventive? Then you should apply in the early spring prior to insect activity in the tree. If you are applying as a remedial on trees already showing symptoms of infestations, you should apply as needed spring through fall, provided that the bark is pliable enough to accept the chemical injections. Which one to use? Your distributor can help with that decision or you can check the labels of our insecticides. For the list of chemicals registered in your state and more information, click here.