Biology

The turnip moth is an innocuous nocturnal moth of various shades of brown, with a wingspan of approximately 4 cm. Its forewings are elongated and feature a grey spot in the middle (kidney-shaped stigma). The caterpillars are grey-brown with a light-brown stripe along their backs. They first have two, later four, ventral pairs of legs. The caterpillars grow up to ca. 4 cm in length. The 0.5 mm small, spherical and finely sculpted eggs are laid individually and are yellow, later brown. The pupa is about 2 cm long, 5 mm in diameter and can only be distinguished from other butterfly pupae deposited on the surface of the soil by experts.

Cutworms are found on almost all vegetable crops. The caterpillars overwinter in the soil before emerging to the surface in spring, once the ground temperature has exceeded 10°C. However, at that point they do not feed anymore, but build a cocoon just below the ground surface where they pupate. The adult owlet moth emerges from these cocoons at the end of May/beginning of June. They fly on dark, windless nights, once temperatures have reached a minimum of 15 °C.
Numerous eggs are deposited on various low-growing plants following mating. The little caterpillars first feed on leaves above the ground surface, leaving holes in them. Young caterpillars in their second instar usually appear around the flowering time of the small-leaved lime. They change their behaviour once they have reached the third larvae phase, remaining in the soil near plants during the daytime, emerging only to search for food during the night. This results in the stems of younger plants being literally chewed through.

However, the caterpillars can also be found on the surface during daytime following rain or due to artificial irrigation. The older larvae have a voracious appetite that causes them to migrate continuously from destroyed to healthy plants. After 3-4 weeks of feeding, the 4 cm long caterpillars pupate in a small hole in the earth lined with spider silk (earth cocoon). The moths emerge in August, following a short pupae period. The caterpillars from this generation of moths overwinter almost fully grown in the soil.

Turnip moths can fly well, being able to cover wide distances in search of a suitable habitat. Mass reproduction can be observed on an annual basis, mainly in dry regions. Experience shows that the most serious damage occurs when the weather is dry and warm at the time the eggs are deposited and during larvae development. A type of ichneumonids (Eutanyacra picta)) -- i.e. parasitoid wasps -- often lay their eggs in cutworms. Moreover, cutworms are often infested with entomopathogenic viruses and microsporidia (sporozoa), reducing  caterpillar vitality.