Penny, Odds are your insect is wire worm. Here's the problem, Beneficial nematodes will kill them BUT. Radish is a cool weather plant, Plant out with soil temp in the 40's. Wireworm start working on the radish as soon as it starts to bulb. Beneficial nematodes need the soil temps above 50F. The catch 22 is the Nematodes are too late. Another method is to plant cubes of potatoes in the soil, check cubes daily for worms, Dispose of, if you find a worm. Problem, high daily maintance and cost. Dish soap will kill on contact. Trick is contact. Some folks use radish to catch wireworm. I plant a barrier row of radish around my root crops. Regretfully, diazinon does the best. Byron