Tuesday, 21 May 2013

On the Wing: Green-Veined White

After a break of a few weeks, I have decided to start a new occasional series this year on the various butterflies that I come across on walks or in my garden, and despite the late spring and bad breeding season last year a few have finally started to show up. I will start off with a very pretty little butterfly that is often mistaken for one of its relatives, the Green-Veined White Pieris napi. Although much the same size as the cabbage pest the Small White, it can be distinguished by the heavy green veining on the underside of the hind wings.

Generally speaking, animals which go out of their way to be conspicuous have some form of defence, and the various white butterflies of the Pieridae are almost all distasteful to predators. This is because they feed as caterpillars on various members of the Cruciferae, both wild plants and domesticated ones like cabbages and other brassicas, and these plants contain mustard oils. These compounds are retained in the body of the adult butterfly and ensure that a bird that eats one will quickly learn not to eat another.

Unlike its pest relatives, the Large White P.brassicae and the Small White P.rapae, Green-Veined Whites prefer to feed on wild crucifers, chiefly (in the UK at least) Cuckooflower, Garlic Mustard, Hedge Mustard and Watercress, choosing by preference smaller plants or even seedlings.All these plants prefer fairly damp situations, and Green-Veined Whites are typically found on woodland rides, damp grassland, and similar situations. They avoid higher elevations and exposed situations such as open grassland.


Garlic Mustard

Garlic Mustard - whole plant
In the UK, they have two broods a year, the first brood emerges from overwintering chrysalids from April onwards and are on the wing into June. These produce a summer brood, which is usually much more numerous, in July and August. The two broods can be quite different in appearance, with the summer brood having more black on the upperwings and less on the dark greenish veins on the underside of the hindwing that gives the butterfly its name. There is also a difference between the sexes, with males being less heavily marked and smaller than the females.

Male upperside
As with many butterflies, male Green-Veined Whites use a special “love dust” to shower the female during courtship. This is produced by modified scales on the fore wing called androconia, and the scent is so strong even humans can detect it – apparently it smells of lemon verbena. After mating however, they also cover the female with a deterrent scent to discourage other males from mating with her.

While males try to mate with as many females as possible, females display two different reproductive strategies. Some mate only once before beginning egg laying, and this strategy is mostly followed by females in cooler and wetter climates. In warmer areas females will sometimes go looking for additional males. They do not need this for fertilization of their eggs, but because as well as sperm the males can transfer as much as 15% of their body weight in proteins and other nutrients in the spermatophore. Females with multiple partners can then go on to lay more and larger eggs over a longer period. During their lives they feed heavily on spring flowers, but males also will feed from mud puddles, probably to replenish reserves of minerals expended in mating.

The caterpillars are smooth, green and slightly hairy, with a yellow circle around each spiracle. They feed on the developing leaves of the young plants as they grow rapidly, and form a fairly typical chrysalis which may be either green or brown.

Green-Veined Whites are a very widespread and adaptable butterfly, found across the Old World from the UK north into Scandinavia and south as far as India and North Africa. In North America it is replaced by several similar species, of which the Mustard White P.oleracea is sometimes considered conspecific. With such a large range it is not considered in any special danger as a species, but their may be local subspecies or varieties which are more in need of conservation assistance.

Mustard White

(images from wikipedia)

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