I have found the solution :
panLeft.setBackground(new Color(1, 0, 0, 0));
panLeft.setLayout(null);
panLeft.setPreferredSize(null);
panLeft.addMouseWheelListener(new MouseWheelListenerPanLeft());
scrollPaneLeft.setBounds(10, 35, 250, 525);
scrollPaneLeft.setBackground(new Color(1, 0, 0, 0));
scrollPaneLeft.getVerticalScrollBar().setPreferredSize(new Dimension(0, 0));
scrollPaneLeft.getViewport().setBorder(null);
scrollPaneLeft.setViewportBorder(null);
scrollPaneLeft.setBorder(null);