Large white butterfly

Identification

A large day-flying butterfly with a wingspan of 50-65 mm; the wings are white, the forewings with black tips, and in the females additionally with two black spots. The yellow eggs are laid in groups on the underside of leaves. The caterpillars are yellow-green with black spots and yellow stripes, and feed gregariously. Symptoms: the caterpillars strip the leaves of brassicas (cabbage, broccoli, cauliflower, oilseed rape, nasturtium) down to the thick veins and foul the heads with droppings. Two, or in warm years three, generations usually develop per year; the insect overwinters as a pupa, pupating in sheltered places - on trunks, walls or fences. Butterflies of the first generation appear from May, but the summer generation, feeding from July to September, is the most numerous and the most damaging. The yellow, barrel-shaped eggs are laid in clusters of several dozen; young caterpillars at first skeletonize the leaf, leaving a network of veins, while older ones disperse and eat the whole blade down to the thick veins - a full-grown caterpillar reaches about 40 mm. Besides brassica vegetables (including Brussels sprouts, kale and kohlrabi), the hosts also include nasturtium, mignonette and wild cruciferous weeds, which form a reservoir for the pest. The dark green droppings that foul the heads lower the market value of the crop and encourage secondary rots; warning signs are white butterflies circling over the crop and yellow egg clusters on the underside of leaves.

Treatment

Products with Bacillus thuringiensis var. kurstaki against young caterpillars, hand-picking caterpillars and crushing egg clusters, and protective netting or fleece that prevents the butterflies from laying eggs. It is worth encouraging parasitic wasps (Cotesia glomerata) and removing brassica weeds that act as reservoir hosts. The basis of effective control is regular inspection of the underside of leaves from May onwards and the destruction of egg clusters and nests of gregariously feeding young caterpillars before they disperse - this is the easiest moment to intervene. Fleece or fine-mesh netting, fitted tightly before the butterflies fly in, is the most effective preventive method in small plots, completely preventing egg-laying. Bacillus thuringiensis works only after being eaten by young caterpillars and breaks down under UV radiation, so it is applied at the first larvae, in the evening, and repeated if needed; it does not control eggs or grown caterpillars. The population is also reduced by natural enemies - the braconid wasp Cotesia glomerata, the wasp Pteromalus puparum, predatory bugs and birds - whose protection requires giving up broad-spectrum insecticides. Only in the event of a mass outbreak are chemical products used: contact pyrethroids or the more selective, newer stomach-acting groups (spinosyns), always observing the pre-harvest interval, rotating modes of action against resistance, and protecting pollinators (spraying in the evening, never on flowering plants).

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.