Turnip moth

Agrotis segetum Denis & Schiffmüller

Lepidoptera: Noctuidae

The Turnip moth was first described in 1775 under the name Noctua segetum by Denis and Schiffermüller. Today the preferred name for this species is Agrotis segetum. The turnip moth is worldwide distributed except in the American and its northern geographical limit is determined primarily by temperature.

Host plants and damage

The Turnip moth is polyphagous and attacks plants belonging to more than 20 families. Plants of economic importance are Allium cepa (onion), Zea mays (corn), Lactuca sativa (lettuce), Daucus carota (carrots), and Solanum tuberosum (potatos), Solanum tuberosum (potato), Brassica oleracea (cauliflowers), Asparagus officinalis (asparagus), Brassica oleracea var. capitata (cabbage), Capsicum annuum (bell pepper), Brassica juncea var. juncea (Indian mustard), Raphanus sativus (radish), Glycine max (soyabean), and Ipomoea batatas (sweet potato).

Turnip moth larvae feed at night, gnawing the foliage and cutting the petioles. Young larvae eat often only the upper epidermis but leave the lower epidermis of leaves. This results in tiny round “windowpanes”. The activity of older larvae is easier to recognize because whole leaves may fall off the plant after being cut through at the base of the stalk. Larvae may also hide in the soil and feed voraciously on roots. Whole plants can fall over and deep holes at and above the soil surface become visible. Damage is fare more severe under dry conditions.

Morphology & biology

Adult females deposit eggs singly or in clusters on the undersides of their host plant leaves, on stems, as well as on the near ground. They lay up to 2000 eggs. The eggs are initially white but develop later a reddish pattern. Eggs are circular at the base and slightly compressed vertically. The surface has 42-44 longitudinal ridges which are only visible under a microscope.

First instars hatch after 3-14 days, depending on the temperature. Some authors report that first-instars exhibit a strong positive phototaxis and that they crawl up to the top of the plant. From there they can be spread by the wind. Under magnification larvae are covered with long hairs with apical droplets. First instars have three, second instars between six and ten prolegs. Third instars and older larvae feed during nighttime and hide in the soil or plant debris during daytime. Larvae have a grayish body and a reddish head. On the body are two parallel longitudinal lines in the middle region. On each segment are two small black spots at the front and two at the back. The fully-grown larvae are 40-50 mm in length. Larval period lasts only 25 days at 30°C but much longer at lower temperatures. Overwintering in diapause takes place in sixth-instar larvae.

The pupal stage lasts for 1-4 weeks. Pupae are 20-30 mm long and yellow-brown in color.

Adults have a wingspan of approximate 40 to 45 mm. Fore wings are dark brown with a clear circular spot in the middle. Male adults have white hind wings, whereas they are gray in females. The periphery of the wings bears a thin black boarder. Males appear more slender than the females and the abdominal tip of the males is slightly extended to the side because of the genital valves. Females release a sex pheromone, but the males do not respond fully until they are a few days old. The sex pheromone consists of several components, which may vary geographically.

Monitoring

Pheromone traps can be used for monitoring adult Turnip moth populations. Look for falling leaves, and holes in roots and tubers to detect infestation.

Pest management

Biological control

Many parasitoids of Turnip moth have been recorded, but only few were used for biological control. In the Netherlands the biological control of Turnip moth in lettuce with the nematode Neoaplectana bibionis showed promising results. In Italy, treatment with a suspension of Metarrhizium anisopliae spores in a variety of vegetables resulted in 80% mortality of Turnip moth larvae. In contrast to this was Bacillus thuringiensis less effective.

In Mongolia tachinid fly larvae parasitize up to 35% of Turnip moth larvae and 15-40% of larvae are parasitized by ichneumonid and braconid wasps in warm areas of Russia and India. Important parasitoid species are Aleiodes dimidiatus in Hungary, Cotesia telengai and Ctenichneumon panzeri in Turkmenistan, Macrocentrus collaris in Armenia, and many Trichogramma species as egg parasites in Europe. In many regions, mortality is caused by pathogens such as Beauveria bassiana, Metarhizium anisopliae, and Nuclear polyhedrosis virus (NPV).

Cultural control

Careful weeding on onion fields can control this pest. Young larvae cannot survive on onion plants.

Host plant resistance is unlikely because the insect is so polyphagous.

Some authors have reported that less cutworm damage occurs in the humid areas of a field. Therefore systematic irrigation against small larvae might help reducing damage in small fields.

Chemical Control

Treatments with pyretroids have been reported as effective against Turnip moth.