The great white butterfly lays its tiny yellow eggs in clusters of 30 to 100. Eggs are usually found on the underside of the leaves of host plants – particularly nasturtium, honesty, and brassica vegetables such as broccoli and cabbages.The small white butterfly on the other hand, lays its more cream-coloured eggs singly or in pairs.
The great white butterfly caterpillar's appearance changes as it grows. The caterpillars are clustered together until near maturity when they separate out, moving to other parts of the host plant and to other plants.
Young caterpillars are tiny and yellowish with a shiny black head. As the caterpillars mature, dark spots begin to appear on the body.
The caterpillars become more visible in their later, larger stages when they become speckled black and greyish green with three yellow lines along their bodies and lots of pale hairs. Fully-grown caterpillars are 50 mm long.
In contrast, the small white butterfly caterpillar is uniformly green with a faint yellow line along the top of its body and grows to about 30 mm long.
The great white butterfly's pupae are 25 mm long and either pale green or greyish-white. The pupae are distinctively dotted with yellow and black markings and are ridged with blunt spikes.
Pupae can be found near host plants on vertical structures such as fences, poles, and buildings.
The upperside of the wings of adult great white butterflies are white. The forewing of both sexes has a black boomerang shaped marking on its tip which extends down both sides of the wing edge for a similar distance (approximately 10 mm). In females, the forewing also has two black dots and a black smear. The male forewing has two black dots as well, but these are located on the underside so are not visible when viewed from above.
The underside of the wings is pale yellow, dusted with grey. The adult's wingspan for females is about 63mm and for males about 58mm.
In comparison, the small white butterfly forewing has only a small grey to black marking on its tip which generally only extends down one side of the wing edge (never an equal distance down both sides). The female has the same two black dots and black smear on the forewing as the great white butterfly female.
However, the male small white butterfly forewing differs from that of the great white butterfly in having only one centrally located black dot on the upperside of the wing.
The wingspan for both male and female small white butterflies is about 50 mm.
Similar-looking but different caterpillars and eggs
Some similar-looking caterpillars and eggs of other insects are often mistaken for those of the great white butterfly. Here is a guide to tell the difference between them and great white butterfly caterpillars and eggs.
Clusters of small yellow cocoons of a parasitic wasp (Cotesia glomerata) that attacks the caterpillars of the common small white butterfly are often confused with great white butterfly eggs.
The easiest way to tell the difference is that the yellow parasite cocoons have a very fine silk webbing over them that great white butterfly eggs don't have.
Also, although both are clustered together, usually some parasite cocoons are bundled on top of others, whereas great white butterfly's eggs are all attached to the leaf surface and are not on top of each other.
Green vegetable bug eggs
The eggs of the green vegetable bug (Nezara viridula) are similarly packed together. The bug's eggs though are arranged in perfectly straight rows like a line of tiny barrels and because the eggs are firmly attached to each other the cluster feels rigid to touch when compared to a great white butterfly egg cluster where the eggs are not in completely straight rows and the cluster is flexible.
Kowhai moth caterpillars
Kowhai moth caterpillars (Uresiphita polygonalis maorialis) look superficially similar to mid-size great white butterfly caterpillars but have distinctive differences.
The kowhai moth caterpillar has a red head, whereas the head of the great white butterfly caterpillar is black or black and green. Also the upper side of the kowhai moth caterpillar is greenish and it doesn’t have the yellow stripe along its body that the great white caterpillar has.
Kowhai moth caterpillars are found feeding on kowhai, broom and lupins and not on brassica vegetables or nasturtium which the great white butterfly caterpillars feed on.
When poked, kowhai moth caterpillars tend to drop and curl up into a protective spiral. Great white butterfly caterpillars don’t curl up when threatened.
Kowhai moth caterpillars can occur in large numbers and strip many leaves off kowhai trees.
Caterpillars and eggs on swan plants
Eggs and caterpillars found on swan plants are very unlikely to be those of the great white butterfly. The caterpillars are those of the Monarch butterfly, and the clusters of eggs are usually those of the green vegetable bug or may be yellow aphids.