Are they harmfull? What can be used to get rid of them? I think they are the reason my Dogwood tree died? HELP!!

the larvae bore into hickory, not dogwood... and you're not going to be able to get rid of them... try to learn to appreciate this pretty little beetle

Longhorn beetles rarely kill trees outright. The tree has to already be severely weakened or dying, or dead already before longhorns take notice. In fact, hickory borers are best known for emerging indoors, from firewood! Further, non-native ornamental trees, and cultivars thereof, are much more susceptible to attack from fungi, insects, and other such problems than native trees. Lastly, trees can rebound even if they 'look' dead. I have seen some trees completely defoliaged by insects one year, and look like nothing happened the next year. So, don't panic:-)