I have a critter in my hydroponic garden that I haven't been able to identify or irradicate. My growbeds have expanded shale as the medium and it has been invaded by a small (less than a millimeter in length) insect. They are gray to cream colored, oval to elongated in shape. They live below the surface and are not seen unless the gravel is distrubed. They hop (two or three inches in a leap) and quickly scurry back out of sight. I have tried using an inscecticidal on them, but it doesn't seem to phase them. I have had trouble getting seeds to germinate in my beds and I suspect these little guys may be the reason. In a 3' x 7' bed, I woulld suspect there are millions of them. Any ideas would be greatly appreciated.