Not sure without seeing the larvae but have a dig around the roots and see what you can see. Cutworms can be active at this time of year and slugs can also attack sweetcorn. Slugs are often underground causing damage. If you find slugs apply Nemasys Slug Killer nematodes, for any other larvae you find try applying Grow Your Own nematodes to the area around the sweetcorn. This contains two types of nematodes that kill a wide variety of soil borne pests. I hope this helps.