The huhu beetle (Prionoplus reticularis) is a longhorn beetle endemic to New Zealand. It is a large, visually distinctive species whose adults reach up to in length. It is widespread throughout New Zealand, occurring up to in altitude. Although it is unable to eat as an adult, the larva feeds and burrows in the dead wood of trees, almost entirely feeding on gymnosperms such as rimu and Monterey pine. As a larva, it can grow to sizes of and has a cylindrical appearance with a creamy colour. During this stage, which is high in protein and fat, it is edible and has been described as tasting similar to peanut butter. It has a history of consumption by the Māori, with the larva being consumed either raw or traditionally cooked in a hāngī.

The adult lives for roughly two weeks. It is nocturnal and is strongly attracted to light, such as that produced by human habitation. The female appears to produce olfactory cues to attract the male. During its larval stage, the beetle can potentially take one to two years to reach maturity, although this has been achieved more quickly in laboratory conditions. The intestinal tract of the larva hosts a diverse range of microorganisms that may assist in digestion of wood. Because the larva burrows into logs, New Zealand has to treat logs before exporting them to prevent the beetle spreading to other countries. This is done by fumigating or heating the logs, which is done by government-approved third parties. Many predators such as native birds and introduced mammals are known to prey upon the beetle. Prionaphes depressus, a species of fairy wasp exclusively parasitises the beetle's eggs. It was first described in taxonomic literature in 1843 by Scottish zoologist Adam White from a specimen collected in the Bay of Islands.

Taxonomy

The first specimen of huhu beetle to be used for taxonomic purposes was collected by British botanist Andrew Sinclair during a voyage in which he visited the Bay of Islands and went on expeditions alongside British botanists William Colenso and Joseph Dalton Hooker. The specimens he collected from this trip were donated to the British Museum (now Natural History Museum of London). From the material Sinclair collected, Scottish zoologist Adam White formally described the species in 1843. The type specimen (the specimen on which the species description is based) is stored in the Natural History Museum of London. Broun then produced a revised description of the species in his landmark Manual of the New Zealand Coleoptera publication in the same year.

Phylogeny

A 2023 study examined the phylogeny (evolutionary history) of Australian Rhipidocerini using genomic data produced by shotgun sequencing. The study placed Prionoplus in the Rhipidocerini, a tribe within the longhorn beetles, and positioned it as basal to all other Rhipidocerini species that were sequenced (all of which were Australian species), meaning it split off earlier than the other species that were studied.

Etymology

The Māori name is ultimately Austronesian in origin from the root form . Cognates, words in different languages derived from a common ancestor, in the more closely related Malayo-Polynesian languages (like Tagalog ) refer to weevils that infest wood and rice across tropical Southeast Asia. In Māori, has semantically evolved to also refer to its larval form (also , ). As the larva reaches maturity, it ceases to bore in wood and sheds its skin. This stage is known in Māori as . It then develops wings and legs, and while it is still white, it is known as or . Finally, it emerges and flies off to reproduce in its adult stage and is known as .

Description

Adult

The adult huhu beetle is unmistakable and easily identified by its large size (reaching around in length) and distinctive appearance. Overall, the whole body of the adult is a dark brown colour. The head and thorax have a thick layer of brown hair-like setae giving it a fuzzy appearance. As is typical of longhorn beetles, the antennae are long, extending around three quarters of the beetle's length. The mandibles are large and curved with a sharp end. Spines are present on the femur and tibia, the third and fourth segments of the legs. The elytra (hardened plates that cover the abdomen) are brown and have three pale yellow veins running down the length of the elytra with numerous cross veins forming a reticulated pattern. There is a single record of a female developing an extra pair of ovaries.

Pupa

The pupa is in length. The head and the pronotum (the main segment of the thorax), are covered in scattered setae, with the pronotum also having a small protuberance at its sides. The mesonotum and metanotum, the second and third segments of the thorax respectively, both have protuberances too, with the former having them in the middle of the segment's end, and the latter having them at the beginning of the segment. The first to eighth segments of the abdomen are covered in short hairs on the upper surface that gradually become more sparse in each consecutive segment. At the end of the abdomen there are two cone-shaped horns that are darkened at their tips.

Larva

The larva is very large, reaching up to in length before maturation. The mature larva is creamy white and has a robust cylinder shape. The larvae have a well-developed head capsule without any ocelli (simple light detecting organs) and with visible mandibles which are black and shiny. Attached to the head are the antennae, which have three joints. Each segment of the thorax and abdomen is covered in short setae, with the first to sixth abdominal segments having pleural discs (ray-like disc structures that are common in longhorn beetles). The spiracles, openings in the body used for respiration, of the abdomen are also large and easily visible. The first stage larva is identical to the mature larva except for the absence of triangular projections on a few spots of the head.thumb|Huhu beetle larvae feeding on rotting wood|alt=Two larvae burrowed into and feeding upon wood with chewed up mulch left behind by the larvae

During the first instar (developmental stage), the larva burrows into logs and digs longitudinally along the wood's grain, where it makes a small chamber for its first moult. The larva, at various life stages, burrows into the logs, forming a series of galleries that vary in structure depending on the size and age of the log. In more freshly fallen logs, the galleries formed are fairly straight and do not go very deep. However, as the log rots and more larvae feed on the log, the galleries become deeper and more complex in nature, with the tunnels beginning to intersect with those of other larvae.

In its final instar the larva moves to within of the surface of the wood before constructing a chamber for the pupal stage by enlarging the diameter of the normal gallery over a period of one to three days. This process creates fragments of wood similar to wood shavings about in size which are then packed into the larval gallery to form a plug. Once the plug is completed the larva lines the walls of the chamber with the last frass (excretions) voided from its gut. The larva then undergoes a resting period of around ten to fifteen days when the abdominal segments contract and the body darkens slightly whereupon it moults into a pupa. The pupal phase lasts around 25 days.

Known gymnosperm hosts of huhu beetle larvae: Before Europeans settled in New Zealand, the beetle was mainly associated with podocarp (trees from the Podocarpaceae group) and kauri tree forests, but has subsequently spread more widely due to the introduction of exotic conifers. During daytime, the beetle hides in crevices or under debris, only becoming active after dusk. The female becomes active earlier than the male, but tends not to travel around as much. After coming out, the beetle will raise its elytra and begin wing movements to "warm up" its flight muscles before beginning to fly. One study recorded flight activity being abundant throughout the night until 2am before ceasing between 4–5am. The gut microbiome diversity is dependent on the food source the larva has access to. When fed on a diet of just cotton, a cellulose rich food source, the gut microbiome develops a high abundance of cellulose degraders. Conversely, when given a diet of pine wood, the gut microbiome had a high level of microorganisms such as Scheffersomyces, which are known to digest plant cell walls.

As food

The huhu larva is eaten by humans, with a long history of consumption by Māori. Its flavour has been described as like peanut butter. The fully grown larva (called in Māori) is the most prized because there is no undigested wood pulp inside of it anymore. Huhu grubs may be consumed either raw or traditionally cooked in a . It has been proposed that huhu larva could potentially be farmed as a food source, but challenges such as maintaining genetic diversity and uncovering optimal growing conditions need to be investigated to see if this would be viable.

P. reticularis contains substantial amounts of nutrients. The larva and pupa are relatively high in fat (up to 45% and 58% dry weight in large larvae and pupae respectively). The fat in the larva is mostly oleic acid and palmitic acid.

Predators

thumb|[[Huia, which preyed upon huhu beetles|alt=A painting of a pair of Huia]]The beetle is preyed upon by a wide range of predators. Some such predators are introduced to New Zealand, such as ship rats, little owl and magpie. Native predators include short-tailed bats, moreporks and saddlebacks. Before their extinction, huia were observed to utilise huhu beetle larvae as their principal food source. The larva is preyed upon by the larva of Thoramus wakefieldi, a species of click beetle, despite the relative small size of T. wakefieldi when compared to huhu larva. The larva is also preyed upon by Rhipistena cryptarthra, a species of wedge-shaped beetle.

Parasites

It is the only known host of Prionaphes depressus, a parasitoid species of fairy wasp that lays its offspring in the eggs of the huhu beetle. The beetle may also become infected with Beauveria, a parasitic fungus that infects insects. The infected beetle is generally killed by the fungus during the pre-pupal or pupal stage. The fungi generally infect 1 in 1,000 beetles, but this varies by location. The parasitic protozoan Gregarina has also been recorded infecting the larva.

Forestry pest

Because the larva feeds on logs, it is considered a forestry pest. In New Zealand, logs that are to be exported overseas are legally mandated to first be treated with a fumigant, such as methyl bromide, or heated to prevent the beetles (among other insects) from accidentally being exported and introduced to other countries. Because of this, treatment methods must be able to penetrate deeper into the logs. , methyl bromide is still used to treat logs, but cyanogen is now also used as an alternative fumigant. Heat treatment is also used.

See also

  • Māori cuisine

References

  • Landcare Research - data
  • Huhu beetle discussed on RNZ Critter of the Week, 29 November 2019