I get this question very often. People see the ball moss in the live oaks on the dead limbs and automatically think it is killing the branches and that it will eventually kill the tree if it is not removed. The truth is that ball moss is an epiphyte, or air feeder, and is not parasitic to the tree. Because it is growing inside the canopy in shady areas around here, it looks as though it is killing tree limbs, when in fact the limbs are dying naturally over time due to lack of sunlight penetration. Unfortunately, many less reputable and/or uneducated sales representatives for some local tree care companies often use the interior limb death as a fear factor and a sales tool to sell extensive ball moss removal for their companies.
However, the truth about ball moss with respect to tree health is not always completely cut and dry. I attended a seminar on ball moss about five years ago at Texas A&M by Dr. Todd Watson who is Urban Forest researcher at Texas Agricultural Experiment Station and an expert on mistletoe and ball moss (as well as a great guy). When the class full of professional, certified arborists were asked if ball moss is detrimental to plant health, half of the class raised their hands and the other half did not (I did not). This was very interesting. The reason for the split had to do with where specifically in Texas the arborists were practicing.
In humid areas of Texas, especially around lakes and bodies of water, ball moss infestations can be huge. In the Austin area, however, the air is dryer and even by water, the infestations are generally not as bad. In humid areas where ball moss is severe, it can actually grow over the ends of limbs in the full sun, causing the branches to actually be shaded out at the tips and killing sections of the outer canopy. In these instances, it needs to be pruned off or sprayed.
Here in Central Texas, the way I handle ball moss on live oaks is to prune it out of the trees. The air is generally quite dry here, and the infestations are rarely severe. Trees with larger amounts of dead wood that have formed since the last pruning cycle are more apt to have ball moss. As the canopies thicken back up since the last pruning cycle, the interior limbs die due to lower of sunlight penetration, and more ball moss forms on these limbs, and often on the interior living limbs as well, although generally not to the same extent.
It is important for long-term tree health to remove dead limbs from the interior of oak trees over time, especially large dead limbs. This then allows the trees to heal over and compartmentalize rot, helping to keep the living limbs healthy. As the dead wood is removed, so goes the ball moss. Since the trees generally do not form that much ball moss between pruning cycles, it is not an issue in most cases. Since spraying is costly, does not address the pruning needs of the tree, and is generally not necessary due to mild infestations, you will rarely see ball moss spraying as a preventive measure around here.