Orange-tip Butterflies - More Than Just a Colourful Set of Wings?

This tiny orange rugby ball is the egg of an orange-tip butterfly  Anthocharis cardamines, attached to the flower stalk (pedicel) of a hedge garlic Alliaria petiolata plant in my garden. The caterpillar that hatches will feed on the host plant's developing seed pod, so the caterpillar must hatch soon after the flower is pollinated.

At higher magnification it's possible to see the beautifully patterned egg case, which will be the first thing that the caterpillar will eat when it hatches, before moving on to consumer tender young seed pods.

Female orange-tip butterflies (male seen above with wings open and shut) only lay a single egg per infloresecence, typically on lady's smock Cardamine pratensis but also on hedge garlic A. petiolata or on sweet rocket Hesperis matrionalis, although egg laying has been recorded on 35 members of the cabbage family (Cruciferae). You can see a fine photo of an orange-tip egg on lady's smock over at Stuart Dunlop's Donegal Wildlife blog. If more that one egg is laid per inflorescence the caterpillars resort to cannibalism, but the female butterfly can detect a pheromone signal left by another that has already laid an egg and will avoid that plant, minimising the risk of this gruesome outcome - unless it rains, when the pheromone is washed off. 

Back in 1997 research at Monk’s Wood National Nature Reserve established that the female butterflies that use lady’s smock as a larval food plant are extremely selective in their egg laying habits. They choose large flower heads in open, sunny locations. Choosing a flower head with a large number of buds ensures that there will be enough food (seed pods) for the hungry caterpillar.

Research in Durham University back in 1983 suggested that they also have quite a narrow window of opportunity for egg laying – if the eggs are laid when the inflorescence is too old the developing pods will be too tough for the larvae to eat by the time that they hatch. This can sometimes happen if bad weather delays egg laying, as the butterflies are only active in sunshine.

In my garden this delightful butterfly breeds on all three of the larval food plants - lady's smock, hedge garlic and sweet rocket - which have overlapping flowering periods that collectively span a period of about six weeks. Each plant has different pod development characteristics - hedge garlic, for example, has much larger, tougher, faster developing pods than lady's smock - so I wonder whether any member of the orange-tip population in my local colony can breed on all three - in which case they might need to adjust their egg laying habits to suit the individual host's pod development rates - or whether there are sub-populations that specialise on breeding on each of the different possible hosts. If the latter is the case, that would respresent the first step in subdivision of the population and of one species splitting into three .............. evolution in action...... or maybe they're not that discriminating. Something worth closer study though, I think. With that in mind, I'm planning to breed the butterflies on lady's smock and on sweet rocket, which represent the extremes of the range of flower timing, then see if the resulting butterflies exhibit a preference for laying eggs on either host plant when they hatch.

Sources:

S.P.Courtney and A.E.Duggan (1983) The population biology of the orange tip butterfly Anthocharis cardamines in Britain. Ecological Entomology 8, 271-281.

Dempster, J.P. (1997) The role of larval food resources and adult movement in the population dynamics of the orange-tip butterfly (Anthocharis cardamines). Oecologia 111 (4), 549-556.