I will speculate on the reasons why the copper is effective. It is the electrical effect of a low voltage shock. This comes from the surface potential of the metal. A wire would be as good as a plate except that a wire needs only to have a small area covered by a leaf or a twig to create a free path. The effectiveness of the metal could be increased if there were an adjacent wire of zinc. The surface potential of zinc is negative while copper is positive and the result is a large potential difference. Both metals need a return path through the ground. There might be some ssituation where this is unhealthy for the plant. Dick