Common Imperial Blue Butterfly

Jalmenus evagoras

We are constantly amazed and delighted by the range of arthropods (insects, spiders etc) that both live in and visit our cold climate garden.

One rare visitor is the Common Imperial Blue Butterfly (Jalmenus evagoras).

This butterfly and larvae were observed and photographed in February 2006. This has been the only sighting of this interesting insect in our garden.

The Common Imperial Blue Butterfly is found in all eastern mainland states.

Adult butterflies have a pale, metallic, greenish-blue central area in the wings. The hindwing has two orange-red spots and fine, black tail. When the butterfly is resting the tail, at the base of each wing, moves in the wind rather like an antenna. It is thought that this movement may deceive predators into attacking the tail rather than the butterfly’s head. “Tis better to lose a tail than to lose one’s life”. Lizards adopt the same survival strategy.

The Common Imperial Blue feeds on many species of Acacias. They usually feed on plants between one and two metres in height. In the photo, the larva is feeding on Acacia parvipinnula. The larvae are pale and speckled at first and become reddish brown or green with outgrowths on the body known as tubercles. Each body segment has a pair of fine white diagonal lines.

The pupa is shining black with some orange-brown markings. Pupae cluster together along the stems of food plants. The most noticeable feature of this butterfly is the presence of large numbers of small black ants. These belong to species of the Iridomyrmex.

The ants protect the larvae and pupae from predators. In return the ants are rewarded by nectar that is secreted from the bodies of larvae and pupae. Guardian ants are shown clustered around a larva.

Newly emerged males are rather eager and fly around the unopened pupae waiting for emerging females. Mating often takes place before the wings of emerging females have had time to expand.

Acacia parvipinnula is a suckering Wattle and at Yallaroo many small plants surround a grove of mature specimens. Only one small plant played host to the larvae. Our attention was drawn to the plant by an adult butterfly fluttering around. We only observed one butterfly.

The pupae seem to develop rapidly. Two or three weeks after pupation the pupae cases were empty.

In the future we will be paying close attention to our Wattles and hope that these interesting insects return.

The only other time we have observed the Common Imperial Blue and their hordes of attendant ants was in the Blue Mountains, west of Sydney, over 40 years ago.

Since writing this article we have come across more information about the behaviour of Jalmenus evagoras. The pupae use vibration to attract ants. The pupae rub together a series of teeth-like projections on their bodies. This vibrates the branch to which pupae are attached. Ants detect the vibrations and swarm over the pupae to feast on the sweet-tasting fluid secreted by them. The ants in turn protect the pupae and close-by larvae from predators.

This is probably why the butterflies lay their eggs on small wattle plants. Their stems are smaller and will respond more efficiently to vibration.

We find that the behaviour of so called “lower animals” is a constant wonder.

By Warren and Gloria Sheather