A circumboreal species widespread across Europe and Asia, including the Indian subcontinent, Japan, the Maghreb and North America.

It is found in meadows, hedgerows and woodland glades but not as often in gardens and parks like its close relatives the large and small whites, for which it is often mistaken.

The underside hindwings are pale yellow with the veins highlighted by black scales giving a greenish tint, hence green-veined white.

Unlike the large and small whites, it rarely chooses garden cabbages to lay its eggs on, preferring wild crucifers.

Males emit a sex pheromone that is perceptible to humans, citral,[1] the basic flavor-imparting component of lemon peel oil.

Females will lay eggs on it, mistaking this non-native species for a compatible native mustard, resulting in the death of the offspring.

The eggs are laid singly on a wide range of food plants including hedge mustard (Sisymbrium officinale), garlic mustard (Alliaria petiolata), cuckooflower (Cardamine pratense), water-cress (Rorippa nastutium-aquaticum), charlock (Sinapis arvensis), large bitter-cress (Cardamine amara), wild cabbage (Brassica oleracea), and wild radish (Raphanus raphanistrum), and so it is rarely a pest in gardens or field crops.

P. napi is found in damp, grassy places with some shade, forest edges, hedgerows, meadows and wooded river valleys.

In Great Britain, April, May and June specimens have the veins tinged with grey and rather distinct, but are not so strongly marked with black as those belonging to the second flight, which occurs in late July and throughout August.

In the May and June butterfly (plate 13, left side) the veins below are greenish grey, and those of the hindwings are broadly bordered also with this colour.

The ground colour varies from white to cream, sulphur-yellow, chrome yellow and light hues of buff or brown.

Variants, many named, are described by Röber (Europe), Langham (Ireland) and Anon (Britain) [5] [6] [7] Recent research has shown that when males mate with a female, they inject methyl salicylate along with their sperm.

However, a virgin female displaying a very similar posture will release a different chemical that will prolong the courtship ritual.

In P. napi, however, mating is unusually costly to males as the ejaculate matter produced contains not only sperm but accessory substances as well.

These substances average 15% of male body mass and are incorporated into female soma and reproductive tissues during the mating process.

[12] This system is unlike other types of butterflies such as Pararge aegeria, where female reproductive effort is independent of male ejaculate.

This increases the mating costs for females because they are spending more time copulating and receiving fewer nutrients from the ejaculate.

When genetically polyandrous females are forced into monandry, due to suboptimal mating conditions, they experience reduced life spans.

[17] It is hypothesized that this life history difference is why monandry is more common in the most northern parts of the species' range,[17] as early investment in reproduction can be more beneficial with shorter mating seasons.

The maintenance of the two mating systems had been hypothesized to be due to the availability of male nutrients, which can vary within the male-biased operational sex ratio.

Given the size of the nuptial gift provided by males, means that polyandrous females can substantially increase their lifetime fecundity.