Question
I have 42 Leland in my backyard the previous owner said they come back every year just to go to homedept and pick up some insecticide. I did and it did nothing, what can i buy to use that will either kill them for good or make them go dormant

Answer
Not sure what insecticide you used. Here are the recommended ones.
Chemical Control-Insecticide Sprays Stomach insecticides are very useful for control of bagworms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Again, early sprays against young larvae are more effective than later applications. Products registered for bagworm control are: acephate (Orthene),  bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban), diazinon, dimethoate (Cygon),  malathion, nicotine sulfate, pyrethrum, and rotenone.

If you control them early in their life cycle they are fairly easy to control. If the tree is small hand picking them is best and easy.
To control the bagworms the easiest method is to hand pick or clip off the bags from the plants during the winter or early spring before the eggs hatch. I pick the bagworms off whenever I see them on a plant, no matter what the time of year. The bags should be destroyed by crushing or putting them in soapy water. When infestation is too heavy for hand picking a biological control such as BT spray can be used. BT (Bacillus thuringiensis) is a type of bacteria that only kills certain insects and does not affect humans or animals. It works well only on young bagworms and must be applied between June 15th and July 15th to be effective.

Insecticide sprays are effective against the young larvae but bags that are 3/4 inch long or longer are very difficult to control. The bacterium Bacillus thuringiensis is effective on young caterpillars, but the material must be ingested--so thorough plant coverage is essential. Spinosad very effectively works by contact and ingestion. Cyfluthrin and trichlorfon are recommended for larger larvae but again thorough coverage is essential.

Handpicking of the bags is the only control from fall through midspring so find a really bored child.