Some ground-dwelling caterpillars of different owl butterflies appear as pests and are very similar in appearance and damage pattern: besides the winter seed owl(Agrotis segetum) and the ypsilon owl(Agrotis ipsilon), members of the genera Scotia, Euxoa and Noctua are to be mentioned. One of the economically most important owl butterflies in Austria is the Winter Seed Owl, also called Seed Owl. It is described below as a representative of the other species.
The moths of the Winter Seed Owl are colored in different shades of brown and have a wingspan of about 4 cm. The forewings are elongate and have a gray colored spot (kidney macula) in the center. Typically, they have their forewings folded over the lighter colored hindwings in a roof shape when at rest.
The caterpillars show a gray-brown ground color and have light brown dorsal stripes. They have two pairs of abdominal legs when young, and four later. The caterpillars reach a length of about 4 cm.
The spherical eggs , about 0.5 mm in size, are laid singly and are first yellowish, later brownish in color.
The pupa is about 2 cm long, 5 mm thick and can only be distinguished by experts from other butterfly pupae, which are also in earth cocoons.
After overwintering as a caterpillar in the soil, the earth caterpillars appear on the surface in spring as soon as the soil temperature exceeds 10 °C. The caterpillars do not eat anything anymore, however, but build an earth cocoon. However, they do not feed any more, but build an earth cocoon in low soil depth, in which the metamorphosis to the pupa takes place. The finished owl butterflies hatch from this cocoon at the end of May/beginning of June. These fly on dark, windless nights when the air temperatures at dusk are at least 15 °C.
After mating, numerous eggs are laid on various low-growing plants. The caterpillars first feed above ground on the leaves, leaving feeding holes. Young caterpillars in the second stage occur around the time of flowering of the littleleaf linden. From the third larval stage onwards, they change their habits: during the day they stay in the ground near the plants and go in search of food only during the night hours. In doing so, they often bite off younger plants.
After rain or artificial irrigation, however, the caterpillars can also be found on the soil surface during the day. Older larvae feed a lot, migrating from already destroyed to undamaged plants. After three to four weeks of feeding, the caterpillars, now up to 4 cm long, pupate in a small burrow lined with spider silk (burrow cocoon). After a short pupal rest, the moths hatch in August. The caterpillars developing from this moth generation overwinter in the soil in an almost fully grown state.
While the winter sowing owl mainly occurs on field and vegetable crops grown or planted in late summer, the ypsilon owl mainly attacks vegetable crops in early summer.
Besides many flowering plants (asters, carnations, chrysanthemums, ...), mainly vegetable plants (lettuce, beans, peas, spinach, peppers, cabbage, salsify, celery, radish, onions, asparagus) and field crops (potatoes, corn, winter cereals, winter rape, ...) are infested.
The caterpillars of the house mother(Noctua pronuba), on the other hand, gnaw the young buds of vines in spring.
Propagation and transmission
Winter Seed Owls fly well and can travel long distances to find suitable habitat. Mass reproductions can be observed yearly, especially in dry areas. According to practical experience, severe damage is only to be expected if dry-warm weather prevails at the time of egg laying and larval development.
While the first generation of caterpillars of the winter sowing owl is only weakly expressed in early summer, the second generation appears in late summer on almost all crops and can cause very severe damage. When they occur en masse, ground caterpillars can cause not only significant reductions in quality due to pitting and cavity feeding, but also considerable quantitative yield losses in emerging crops, for example.
Prevention and control
- Protection and promotion of habitats of natural enemies such as birds, hedgehogs, toads, moles, shrews, but also predatory ground beetles, parasitic caterpillar flies and ichneumon wasps(Eutanyacra picta).
- Earthworms are sometimes also attacked by insect pathogenic viruses and microsporidia (spore animals), which reduce the caterpillars' vitality.
- Chickens in the allotment decimate ground caterpillars - but compatibility with the crop is essential to consider.
- Artificial irrigation decimates the young larvae.
- Intensive tillage disrupts the development cycle of the burrowing caterpillars.
- Thorough weed control can reduce egg laying.
- The use of Bacillus thringiensis (Ssp. aizawai) preparations is only useful against young larvae. In most cases, however, the infestation is only noticed when the larvae are already large.
- The use of entomopathogenic nematodes(Steinernema carpocapsae) is possible.
- The use of plant protection products (see list of plant protection products approved in Austria) should best be carried out against the young larvae and in the evening.
Bionet brochure: Owl Caterpillars - Owl Butterflies, Lukas Weninger, Roswitha Six (FiBL Austria).
Last updated: 17.11.2021