Answer: It is best to have a definite identification of both the pest and the plant before you begin to treat it. (You want your efforts to succeed without harming the plant or your cure may possibly be worse than the ailment!) The reason for this caution is that some treatments will not be tolerated by some plants, and some pests will not respond to some treatments.
Your county extension should be able to help you identify the pest (and the plant) and determine the best way to treat it -- or even determine if it needs to be treated or can be left as is as is the case with some pests that cause unsightliness but do not seriously harm the plant.
It is true that scale should probably be dealt with and it can often be treated with horticultural oil, but this must be timed correctly -- often it is most effective if done in late winter or early spring while the plant is dormant. Some plants also object to the oil or to use of the oil in warm weather.
Your yew may be losing a few needles as a seasonal occurrence, or there could be a problem there as well. I woudl suggest you take samples and consult with your county extension (690-2655) prior to continuing any treatments. Good luck with your plants.
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