The painted lady is a migratory species originating from Africa and the Mediterranean. It migrates from North Africa to northern Europe in May and June. The size and shape is similar to other butterflies. The wings are variegated reddish-brown and covered with black and white spots. Light green, oval eggs are laid on leaves. Grown-up caterpillars are 40 mm long, hairy and dark brown in colour, with two yellow lines on the sides. The pupa is 20 mm long and silver-brown in colour or coppery sheen. They are found on the injured leaves.
The whole migration is made by a succession of generations, up to six in a year. The settling of adults in a location depends on weather conditions such as wind direction that affect the migration path and length. The first arriving butterflies can be seen in early spring. After mating, the females lay around 500 eggs on the leaves on a wide range of plants. Various species of thistle are the best-known hosts providing nectar for the adults and leaves for the caterpillars. The wider range of hosts includes soybean. After arriving in May and June, two generations can result in sporadic infestations of soybean. The highest abundance of caterpillars occurs during June and July.
Only the caterpillars are harmful to soybean. They eat the leaf tissue between the leaf veins. Large infestations may cause complete defoliation. Damaged leaves are tied together in web-forming larval nests from which the young butterfly emerges from pupae. Infestation in crops is usually patchy and localised. The soybean is only one of many hosts and it is often the presence of wild hosts in the field that triggers infestation. It is important that other host species, thistles in particular, are removed within soybean crops if infestation is expected from migrating adults.
Control is rarely necessary in practice. The need for control measures can be assessed about one week in advance of an infestation by the presence of adult butterflies that are settling in a location to mate and lay eggs. This provides time to plan treatments which might involve obtaining special permission to use insecticides. An average of two or more recently-hatched caterpillars per plant, or 20 caterpillars per row metre of soybean, or the observation of two nests of infestation within 100 m² is the economic threshold. The condition of the canopy and the stage of development of caterpillars should be considered. Plants with already developed canopy are more tolerant to damage while younger caterpillar instars are more susceptible to insecticides and most of the damage is yet to be made. Sometimes, control can be confined to crop margins or to patches in the crop.
Predicting infestation from the presence of recently arrived and settling adults at the local level is important. Only a few insecticides are approved for control. As this pest occurs only occasionally, no products are registered in several countries for this purpose. In these cases, exceptional use may be admitted on demand (e.g., for Bacillus thuringiensis in Germany). This demand should be organised by a plant protection service or a cooperative in advance, as treatment is worthwhile if the caterpillars are still young. Treatment of large caterpillars is ineffective, as they will stop feeding soon and the damage has already occurred.
Key practice points
- Fields should be scouted regularly and systematically for the presence of adult butterflies, eggs and caterpillars.
- Control measures should only be taken where a caterpillar population approaches an ‘economic’ threshold. Treatment is not justified in the case of most infestations (presence of caterpillars below economic threshold).
- When chemical control is needed, apply the lowest effective amount of the pesticide using equipment that is properly calibrated. Sometimes it is possible to localise treatment to only infested parts of crops.
Bundesanstalt für Landwirtschaft und Ernährung (BLE), Ökolandbau – Distelfalter (Vanessa cardui), website: www.oekolandbau.de/landwirtschaft/pflanze/grundlagen-pflanzenbau/pflanzenschutz/schaderreger/schadorganismen-im-ackerbau/distelfalter-vanessa-cardui/
Butterfly Conservation. Painted Lady, website: www.butterfly-conservation.org/butterflies/painted-lady