A ring flash can be used in the field very effectively. I photographed this bee on a cone flower with a 50mm macro lens and one extension tube to increase the magnification (this picture is not cropped). The background will go black, though, due to the light falloff from the flash as well the small lens aperture necessary for extensive depth of field. Therefore, you have to go to Plan B -- which is to use a print of out of focus foliage mounted to foam core or taped to any piece of cardboard. When the print is placed close to the subject, say 12 to 18 inches away, the light from the flash will provide enough illumination to make the green background seem entirely natural.