My neighbor has the same problem. I dig the plant cuttings back into the soil for composting. Garlic is closely related to onions and prone to the same problems, especially onion maggots. Onion maggots feed on roots, killing seedlings and older plants. They also burrow into the bulbs, making them unfit for use.
Adults are small gray flies that lay eggs early in the spring.
You need to destroy infested plants. The maggots tunnel and feed in the bulbs, causing serious damage. Most injury occurs when the spring seasons are wet. A third generation may attack just prior to harvest and cause them to rot in storage. Effective control has also been provided by applications of Ethan or triton to the soil prior to planting.