INTERESTING INSECTS

By N A Martin (2016)

Phyllotreta cyaneum Broun, 1880

This endemic flea beetle is found in both the North and South Islands where its host plant, Glossy karamu, Coprosma robusta (Rubiaceae), grows. Adult beetles feed on new season leaves in summer.

Conservation status: Widespread in native ecosystems, common in the Auckland region.

Adults are found feeding on new season leaves in summer, particularly in January and February. It is presumed that eggs are laid in the soil and that larvae feed on plant roots and that they pupate in the spring.

Adults are typical beetles, each with six legs and hard wing covers (elytra). These black beetles are about 3.5 mm long. The dorsal (upper) side of the thorax (middle body section) and the elytra are covered in punctures. Their wings, which are longer than the wing covers, are kept safely folded up under the wing covers, except when needed for flying. The last pair of legs have enlarged femurs that enable the beetle to jump, hence the name flea beetle.

Nothing is known about the eggs, larvae or pupae of this beetle.

Feeding

Adults have chewing mouth parts. They feed on leaves of Glossy karamu, Coprosma robusta. They feed on young and new season leaves only of this species of shrub. Adult beetles chew leaves from either side, but does not eat right through the leaf, but leave the skin (epidermis) on the far side of the leaf intact, so creating a ‘window’. Beetles probably feed mainly at night, but can be found feeding on leaves during the day.

There are many kinds of beetles in New Zealand. They can only be conclusively identified by an expert. However, black beetles with enlarged hind femurs found on leaves of Glossy karamu, Coprosma robusta in the summer are almost certainly the karamu flea beetle.

The presence of this kind of leaf damage in summer on young and mature new season leaves on Glossy karamu, Coprosma robusta strong indicates the presence of karamu flea beetles. Damage to very young leaves in spring and early summer are most likely caused by other smaller beetles.

No natural enemies of the karamu flea beetles are known. They are probably preyed upon by birds, spiders and predatory insects.

Adult karamu flea beetles only feed on Glossy karamu, Coprosma robusta (Rubiaceae).

Plant damage

Adults only feed on young and new season leaves of this species of shrub. Adult beetles chew the leaf from either side, but do not eat right through the leaf, but leave the skin (epidermis) on the far side of the leaf intact, so creating a ‘window’. Beetles probably feed mainly at night but can be found feeding on leaves during the day.

Research project

The food plants of adults of this and some other chrysomelid beetles are known, but the egg laying site and food of the larvae are unknown. A simple and useful research project would be to collect adults and allow them to mate and lay eggs in pot plants of potential larval host plants. If the larvae can be reared and pupate then it would also be possible to describe the larvae and pupae. The potential host plants tested should include known adult host plants, closely related species and other plants growing in habitats where adults were present.

Samuelson GA 1973. Alticinae of Oceania (Coleoptera: Chrysomelidae). Pacific insects monograph 30: 1-165.

Dr Willy Kuschel, who identified the adult beetles.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.