Cutworms

Identification

Thick, fat caterpillars of noctuid moths, grey-brown or earth-coloured, with smooth skin, feeding at night. During the day they hide shallowly in the soil, curled into the characteristic shape of the letter C. The host plants are vegetables, transplants, perennials and strawberries. Symptoms: cutworms gnaw young plants right at the soil surface, severing seedlings as if with a knife, and in addition eat roots, tubers and the lower leaves. The name 'cutworms' covers several noctuid species — most often the turnip moth (Agrotis segetum), the heart and dart moth (A. exclamationis) and the black cutworm (A. ipsilon). The adults are inconspicuous nocturnal moths with grey-brown forewings and a wingspan of about 3.5–4.5 cm, active from May; the females lay eggs singly or in groups on the soil, plant debris and weeds. Young caterpillars feed at first on leaves low to the ground, and only the older ones (growing to 4–5 cm) move down into the soil and switch to gnawing plants at the base. Usually one to two generations develop per year, and the caterpillars overwinter in the soil. The greatest damage occurs on light, dried-out soils and in dry, warm summers, especially on freshly planted beds and on fields established just after a ploughed-in fallow or lawn. A characteristic sign is a young plant cut through and toppled just above the ground, and after scraping away the soil at its base — a curled, fat caterpillar.

Treatment

It helps to dig over the soil while hand-collecting the caterpillars in the evening or from under clods of earth, to fit protective collars around transplants, and to remove weeds carefully, which limits egg-laying sites. On a larger scale, entomopathogenic nematodes are used, along with pheromone traps that catch the moths and signal their flight. The foundation is prevention: destroying weeds before planting, deep soil cultivation in autumn (which brings the caterpillars up to the surface, where they die from frost and are eaten by birds) and — where possible — keeping the soil moist, because cutworms prefer dry conditions. Pheromone traps serve mainly for monitoring: they set the timing of treatment against young caterpillars still feeding on leaves, which are a much easier target than the older individuals hidden in the soil. Entomopathogenic nematodes (Steinernema, Heterorhabditis) watered into warm, moist soil in the evening reach the caterpillars in the ground, while collars and sleeves around transplants form a simple physical barrier. Biological products based on Bacillus thuringiensis act only on young caterpillars and only as a supplement, because the pest feeds at night and quickly hides in the soil. Chemical treatment is reserved for high pressure: pyrethroids are applied in the evening, when the caterpillars come out to feed, or in the form of poison baits; spraying older caterpillars hidden in the soil is not very effective, so an early timing based on monitoring is crucial.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.