Question
I have a very large oak tree with aphids.  I found one of your answers about "honeydew" and you said that a systemic insecticide could be injected, but I am seeing mixed info as to the safety and health of the tree by the injection method (drilling small holes at the base).  I had an estimate from an arborist to spray the tree with Orthene to treat, but I'm wondering if the spray on such a large tree is possible, or would be harmful to my pets.

Answer
I would not inject the tree--the options would be spray the foliage or use a soil applied systemic insecticide. Orthene will work as a spray. The new soil applied insecticide is Bayer Advanced.  Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil. It is fairly costly--may want to check the cost against the spray treatment. Spraying a large tree will take a bucket type truck or a power sprayer to get the solution on all the foliage.
The spray will not be harmful to your pets. If you are concerned just keep them inside until the spray has dried. Even if the spray gets on them it will not harm them.