Silver fern flea beetle - Alema paradoxa
By N A Martin (2017)
Biostatus and Distribution
This endemic flea beetle is found in both the North and South Islands. Adults feed on young fronds of silver fern and other Cyathea species. Based on fern frond damage symptoms, this beetle is common in native ecosystems in the Auckland region.
Conservation status: Widespread in native ecosystems, common in the Auckland region.
Life Stages and Annual Cycle
Adults are found feeding on young fern fronds in summer. It is presumed that eggs are laid in the soil and that larvae feed on plant roots and pupate in the spring.
Adults are typical beetles, each with six legs and hard wing covers (elytra). They are small, about 2.5 mm long. They are brown with a pair of darker patches on the wing covers. The body and leg colour varies from pale to medium brown. 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.
Adults have chewing mouth parts. They feed on young frond of Cyathea species such as silver fern. The adult beetle chews the fern frond from either side, but does not eat right through the frond, but leaves the skin (epidermis) on the far side of the frond intact, so creating a ‘window’. Beetles probably feed mainly at night. During the day they hide on the fern or nearby.
There are many kinds of small beetles in New Zealand. They can only be conclusively identified by an expert. However, if small beetles with enlarged hind femurs are found on young fronds of tree ferns belonging to the genus Cyathea, and there are many tiny ‘windows’ in the fronds, the beetles are almost certainly the silver fern flea beetle.
The presence of the distinctive frond damage can be used to determine the presence of the beetles in an area.
No natural enemies of the silver fern flea beetle are known. Birds, spiders and predatory insects probably prey upon them.
Adult silver fern flea beetles feed on young fronds of tree ferns in the genus Cyathea (Cyatheaceae). The beetles and their distinctive damage to fronds have been found on three species, silver fern (Cyathea dealbata), black tree fern (Cyathea medullaris), and soft tree fern (Cyathea smithii).
The adult weevils feed on young fern fronds. They chew the fern frond from either side, but do not eat right through the frond, but leave the skin (epidermis) on the far side of the frond intact, so creating a ‘window’.
|Common Name(s)||Scientific Name||Family||Reliability Index||Biostatus|
|Silver fern, Kaponga, Kātote, Ponga, Punga||Cyathea dealbata (G.Forst.) Sw.||Cyatheaceae||10||endemic|
|Black tree fern, Black mamaku, Korau, Mamaku, Pitau, Katātā||Cyathea medullaris (G.Forst.) Sw.||Cyatheaceae||10||non-endemic|
|Soft tree fern, Smith's tree fern, Ponga, Katote, Neineikura, Whē||Cyathea smithii Hook.f.||Cyatheaceae||10||endemic|
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.