If your tomato plants look like overnight vandals have cut them at the stem, the actual marauder is more likely to be a cutworm. These are the larvae of several insect species, mainly the Agrotis ipsilon moth, which is greasy looking and gray in its caterpillar stage, or the variegated cutworm, Peridroma saucia, with a yellow dot in the middle of each segment. Your cutworm-damaged plant will begin to wilt and die, but you can prevent similar damage in the future.
Cutworms reach almost 2 inches in length and are gray to almost black in color, according to the University of Florida IFAS Extension Service. These nocturnal creatures remain below the soil surface at day and emerge during the night to feed on leaves and stems of young tomato transplants. Distinguish cutworm larvae from other caterpillar larvae by seeing if they curl into a C-shape when disturbed, notes the Oklahoma Cooperative Extension Service.
Remove grass and till the soil well in advance of planting. Make a small sleeve from a paper cup with the bottom removed and press it into the soil around your tomato transplants to make a mechanical barrier to cutworms, recommends the University of Florida. Remove these collars after two to four weeks so they do not interfere with normal plant growth. Remove weeds in and around the garden, as well as tomato plants at the end of each season and leaf material from any legumes preceding the tomato crop in the garden, to eliminate sources of additional cutworms.
Look for cutworms by digging around the base of plants and sifting the soil for caterpillars at dawn or at night with a flashlight. They may hide in soil, under clods or in debris cluttering the base of the plants.
Spray insecticides such as carbaryl, chlorpyrifos or permethrin on the base of the tomato where it emerges from the soil and wet the soil extending 3 to 5 inches away from the plant.
Although you can use pellet baits such as carbaryl (Sevin) late in the afternoon so they will be fresh when the cutworm emerges, these baits may be toxic to birds feeding in the garden, notes the University of Guelph in Ontario.