Cutworm

Cutworms are the caterpillars of several species of night-flying moths. Most hide during the day in soil, under plant debris or close to the base of plants and emerge at night to feed. Young plants are particularly vulnerable. Some species climb plants and chew foliage, but the classic damage occurs when a caterpillar wraps around a tender stem and chews through it at or just below soil level, leaving a perfectly healthy-looking seedling lying dead on the ground. Cutworms overwinter as eggs or larvae depending on the species, and weedy or grassy areas can provide both food and egg-laying sites.

How to Fix It
Go hunting: Check around the base of a freshly severed plant and dig through the top few centimetres of soil. The culprit is often still there, curled into a C-shape.
Hand-pick: Remove and destroy caterpillars when you find them. Checking after dark with a flashlight can be surprisingly effective.
Use collars: Protect vulnerable transplants with cardboard, metal or plastic collars extending a few centimetres above and below the soil surface.
Control weeds: Remove weeds and grasses before planting, since they can attract egg-laying moths and support young larvae.
Clean up debris: Remove heavy plant debris and weeds that provide daytime hiding places.
Cultivate before planting: Light cultivation can expose overwintering larvae and pupae to predators and weather.
Beneficial nematodes: Appropriate insect-parasitic nematodes can help suppress soil-dwelling cutworm larvae when soil conditions are suitable.