In the majority of years there are two broods (April to June then July to September) and in good years there is a partial third lasting into October.
The generations overlap so that the butterflies seem to be around continuously through the summer and this is also supplemented by immigrants from Europe. Arrivals from North Sea crossings may be localised and quite densely packed but have often thinned out by the time they get to Yorkshire. There may be some moves back south in the autumn.
Using their antennae, female Large Whites are able to detect the strength of mustard oil in potential larval foodplants and seek to choose plants with the greatest concentration. The oil (a burning irritant) makes larvae taste bad to predators, particularly birds, and so the caterpillars do not try to hide on the food plant, despite being quite obvious and visible.
Eggs are laid in batches of about 50 on a wide range of wild and cultivated cabbage family plants and are pale yellow when laid, eventually becoming a golden orange and hatching after one to two weeks. The larvae eat their eggshells, then the leaf on which they were laid, before moving on to other leaves or plants. They usually only eat the plant's outer leaves (compared to Small White larvae which mainly eat the inner leaves) and they grow to about four to five centimetres long (about two inches) through 5 moults. They are pale green and yellow with black spots and have a distinctive unpleasant mustard smell. They stay together as large group where possible.
When they are ready to pupate they walk away on their own looking for somewhere to pupate. They can go large distances and climb up to the eaves of house or up trees. They put a web of silken threads on the surface they have chosen and attach themselves with silken hooks and a girdle.
Pupae are variable in colour, usually in shades from green to brown, and speckled with black dots. The first spring brood spend about two to three weeks as a pupa but the second brood overwinter in this stage.
The butterfly emerges when the weather is warm enough, usually around the end of April in Yorkshire, although recent warmer years have also seen March sightings.
The species is often attacked by a tiny wasp Apanteles glomeratus. The wasp injects eggs just below the skin of the larvae, enabling the resulting wasp larvae to develop without killing their hosts until just before pupation. As many as 80 wasps may develop on each larva and these wasps may explain the year to year fluctuations in Large White numbers.