Wasps: In the wild, it is not uncommon to find larvae that have been infected with wasp eggs. Female wasps fly around looking for prey, and upon finding it, lay multiple eggs on larvae. The wasp's eggs can be large and easily seen with the naked eye, or as small as a pin head, and easily overlooked - depending on the species of wasp.
Cecropia larvae in 4th and 5th instars seem to be extremely susceptible to wasp attacks. Often, if you find Cecropia larvae in this stage, they will have wasps eggs on them. It is, however, possible to remove the eggs and allow the larva to be unaffected by the wasp. If the wasp's eggs can be removed before they have hatched and burrowed into the larva, then the larva can be saved. The wasp's ova can usually be somewhat easily removed with a pin. [I have successfully done this with at least 3 Cecropia caterpillars I found in the wild.] But there are no guarantees with this method - if even one egg is missed, or if you do not remove them before they hatch - you may be surprised, come spring, to find that a wasp hatches from the cocoon instead of a moth!
Wasps of a different type, (such as hornets and yellow-jackets) can also be responsible for the death of numerous larvae. These wasps enjoy feasting on the larvae. They tend to find the larvae if they are in large groups (seemingly because they can smell the droppings). If they do find them, they usually put up a fight and often chew large holes in netting to get to them! The best way to prevent this is to empty droppings from nets frequently, so the wasps cannot smell them. If wasps of this type do show up, clear the droppings, and also try to kill as many of the wasps as you can - before they kill the larvae! They have an amazing ability to take out whole groups of caterpillars in a short time.
Birds: Birds are probably the worst enemies to larvae. Birds have a very keen eye, and usually once they spot larvae, they don't give up easily. The best method of preventing bird attacks is to keep larvae protected by placing a net on the branches that containing them. Even this can fail, as persistent birds may peck through the net, not reaching them, but nonetheless, killing them. However, a net dramatically reduces the chances of a bird attack by camouflaging the larvae.
Promethea larvae, mainly in final instars, seem to be a very good target for birds. The larvae are white and do not stop eating during the day, so, these especially require nets to stop attacks.
Rodents: Be sure to keep cocoons in a protected area, where rodents cannot get to them. If they are stored outside, keep them in a closed container for the winter months. Mice and chipmunks will chew through screened containers, and carry cocoons away for a later meal!
Other Insects: Other insects seem to prey on larva an awful lot, too. Especially if you don't have a net on your branches. But even if you use netting, spiders and other small bugs wait at the opening and get inside when you open the net, or can be overlooked when the net is initially put on, and feast on the larvae until you take it off. The insects will also attack through the net, if the larvae are close to the side - so, you may want to consider that when designing and implementing your net. When the net is put onto the branches, it's important to clear out the net by turning it inside out and shaking it. Also, if possible, shaking the branch and running your hand along the leaves is a good method of removing small spiders that otherwise would go undetected and get locked in with larvae when the net is put on.