The Small White, Pieris rapae
The Small White is one of the really common butterflies around. Size is not a very good guide to its identification – it is roughly the same size as the Green-veined White, and there is an overlap of size with the so-called Large White.
The key to identifying this butterfly correctly is to look firstly at the underside of the rear-wing. It should be an overall yellow like the Large White, not a pattern of green lines along the veins, like the Green-veined White. Now look at the “tip” or outside corner of the front wing, the dark “corner mark” should lie straight along the edge, simply cutting off the corner. In the Large White alone the “corner mark” clearly curves around the corner of the wing, giving a “boomerang” shape.
The Small White has a number of dark spots on the forewing. This is normally one in males and two in males (on each wing of course). This is a summer (second generation ) male:
The Small White wanders freely across the parish and is also found everywhere in Kent.
Its distribution internationally is now almost worldwide because of repeated introductions by humans, deliberately or accidentally. It is now found even in New Zealand, but has not established itself in South America. It is thought to have evolved originally in Europe or Asia.
In the UK the Small White has two adult generations, the first in May-June and the second on July-August. Each individual female can live for about 3 weeks. They lay eggs singly. The greenish caterpillars feed on the underside of the leaves of Cabbage, Kale or Broccoli crops or their “weed” wild relatives such as Charlock or Hedge Mustard. These members of the Brassica family have glucosinolates in their leaves which form isothiocyanates which deter most insect pests. However the White butterfly caterpillars have a resistance to these chemicals.
Wasps in the genus Cotesia parasitise the caterpillars internally, laying egss in the early stages of the caterpillars, so that the larvae eat away, eventually killing the caterpillars before they can reach pupation. The next generation of wasps then hatch out of the dead bodies of the caterpillars. These wasp species may therefore be used as biocontrol agents in countries that the Small White has been introduced to.
Pteromalus puparum is a less important endoparasitoid of the pupal stages of many Lepidoptera, including the Small White.