Garlic eating grubs
Hand picking the larvae off the plant is the best safe & organic method for removal. Use a flashlight to locate at night. Scratch the soil surface around the plant to locate them in the daytime. You also need to control weeds in the area around the garlic to remove hiding places.
Here is some information from the Pacific Northwest Insect Management Handbook - https://pnwhandbooks.org/insect/vegetable/vegetable-pests/hosts-pests/garlic-armyworm-cutworm
You can also try spray application of Bt (Bacillis thuringiensis var. kurstaki). This is a natural bacterial treatment that is effective on caterpillars that are actively eating. Examples include Safer or Monterey brand caterpillar killers.