I am finding large larva sacks on...

Assuming the larva sack (also known as a cocoon) harbors the pupa of a pest that truly needs to be controlled, you could do either one. There are not very many pests that make cocoons that will eat enough leaves to lower the yield on your trees, so before you spend a lot of time on this project, identify the cocoon. Here is one link that might help you identify it: cocoon info. The bottom line on cocoons is that, when you see one, the insect has already eaten all it is going to eat. Now it has entered a non-feeding stage, and until the larva matures, it will not affect your plant. You will get better control if you find out what it is, and then, if you still want to kill it, wait until it has children. Those children will be little, tiny caterpillars that you can kill with low concentrations of whatever pesticide you choose to use.