Answer: Malathion is a broad spectrum insecticide. You should identify the specific cause of the browning first, once you know that you can determine if a spray of any kind will solve the problem, or not. Different pests require different controls, and often the spray must be applied at the right time of year in order to control the pest. To simply "spray twice a year" without having a specific reason for doing so with that particular product means you could just be wasting your time (and money). I really would not recommend routine spraying "in case it works." And apparently, in this case, it is not working.
Most established landscape plants do not need to be fertilized, especially if they are planted near a lawn area that is regularly fertilized. The surface roots you see might be from the nearby trees -- they could be stealing water and nutrients from your arborvitaes through root competition. If this is what happening there is not a lot you can do to prevent it -- the trees are bigger and stronger and will win the battle. When you try to feed and water the arborvitaes, then the tree roots seem to grow that much faster as a result.
However, it is very difficult to diagnose this type of problem long distance -- there may be several different factors stressing your shrubs. I strongly recommend you work wiht your local county extension and/or with a professionally trained and certified nurseryman to figure out what is happening to your arborvitaes. The sooner you have a firm identification of the problem the sooner you can start working on saving them.
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