Huhu beetle

"Huhu" redirects here. For the local-level government area in Papua New Guinea, see Huhu Rural LLG.

The huhu beetle (Prionoplus reticularis) is a longhorn beetle endemic to New Zealand. It is the heaviest beetle found in New Zealand.

Huhu beetle
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Family:	Cerambycidae
Genus:	Prionoplus White, 1843
Species:	P. reticularis
Binomial name
Prionoplus reticularis White, 1843

Taxonomy

The first specimen of huhu beetle to be used for taxonomic purposes was collected by Andrew Sinclair during a voyage where he visited Bay of Islands and went on botanical expeditions alongside 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). ^[1][2] From the specimen Sinclair collected, zoologist Adam White formally described the species in 1843. The type specimen is stored in the Natural History Museum of London.^[2]

The huhu beetle larvae and pupa were later described by Thomas Broun in 1880 and a brief description of their typical habits were given from specimens collected in Whangarei.^[3] Broun then went on to produce a revised description of the species in his landmark “Manual of the New Zealand Coleoptera” publication in the same year.^[4]

Etymology

The Māori name huhu is ultimately Austronesian in origin from root form *bukbuk₃, where cognates in related languages (like Tagalog bukbok) refer to weevils known to infest wood and rice across tropical Southeast Asia.^[5]

In Māori, huhu has semantically evolved to refer explicitly to its larval form (also tunga haere, tunga rākau).^[6] As the huhu larva reaches maturity it ceases to bore in wood and casts its skin. This still edible stage is known in Māori as tataka. It then develops wings and legs, and while it is still white, it is known as pepe or pepe-te-muimui.^[7][6] Finally, it emerges and flies off to reproduce and is known as tunga rere.^[6]

Description

Adult

Larvae

Eggs

Adults

Adult huhu beetles are unmistakable and are easily identified by their large size (reaching around 5cm in length) and distinctive appearance.^[8]

Overall, the whole body of the adult is a dark brown colour. The head and thorax have a thick layer of brown hairs that make it appear somewhat fuzzy. As is typical of long horn 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 legs on the femur and tibia segments. The elytra is coloured brown and has three pale yellow veins running down the length of the elytra with numerous cross veins forming a reticulated pattern.^[4][2]

Larvae

The larvae are very large, reaching up to 50-70mm in length before maturation. They are coloured creamy white and have a robust cylinder shape. The larvae have a well developed head capsule with visible mandibles which are black and shining in colour. ^[9]

Eggs

Eggs are around 3mm in length and 1.2mm wide. They are coloured white and are shaped like cigars. The surface is unsculptured.^[9]

Life cycle

Female adult huhu beetles oviposit their eggs in clutches of 10–50, though up to 100 may be found.^[10][11][12] Eggs are laid in cryptic sites or in cracks in the bark of fallen wood.^[12] In laboratory conditions of 20°C ± 2°C and a relative humidity of c. 75%, eggs hatched in 23 ± 2 days.^[12]

Before hatching, the larva can be seen to move inside the egg and will break free from the egg using its mandibles to pierce the chorion of the egg and then enlarging the opening by chewing, although the chorion itself is not ingested.^[12] Setae that are found on abdominal segments 1-6 assist in providing support as the larva leaves the egg and excavates the initial gallery.^[12]

Huhu beetle larvae feeding on rotting wood

The whitish-coloured larvae normally feed on dead wood of gymnosperms (mainly native and introduced conifers) associated with lowland podocarp forest.^[13] Larval duration of P. reticularis is two to three years in the wild.^[10] Under laboratory conditions, larval duration has been reduced to c. 250 days using an artificial diet and maintaining a temperature of 20°C.^[14] In its final instar the larva moves to within 7.5–10cm of the surface of the wood before constructing the pupal chamber.^[12] The pupal chamber is constructed 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 3cm by 1cm in size which are then packed into the larval gallery to form a plug.^[12] Once the plug is completed the larva lines the walls of the pupal chamber with the last frass voided from its gut.^[12] The larva then undergoes a resting period of around ten to fifteen days where the abdominal segments contract and the body darkens slightly whereupon it moults into a pupa.^[12]

Huhu beetle pupae

The pupal phase lasts around 25 days with gametogenesis being completed during this stage.^[12][15] Eclosion occurs with a rupture along the frontal suture followed by a longitudinal rupture to the posterior border of the mesothorax.^[12] The head, feet and wings are freed during arching movements of the body through the ruptured cuticle.^[12] The emerged adult may then enter an inactive period of three to five days prior to creating an exit tunnel out of the pupal cavity.^[12]

Following pupation and emergence, the adult beetle does not eat and lives for approximately two weeks.^[6]

Hosts

As previously discussed, the larvae of huhu beetles live and feed on the decaying wood of gymnosperms. There are currently twelve gymnosperm species that are known to act as hosts to the larvae. Additionally, there are also two host records of larvae living in angiosperms. They have been reported to occur in tawa and oak, although the latter record is likely a misidentification.^[13]

List of huhu beetle recorded gymnosperm hosts:^[13]

• Kauri
• Miro
• Mataī
• Kahikatea
• Rimu
• Yellow silver pine
• Monterey pine (Exotic to New Zealand)
• Loblolly pine (Exotic to New Zealand)
• Larch (Exotic to New Zealand)
• Oregon pine (Exotic to New Zealand)
• Black pine (Exotic to New Zealand)
• Maritime pine (Exotic to New Zealand)

Distribution

The huhu beetle is endemic to New Zealand. It is widespread throughout the main islands, occurring from sea level up to altitudes of 1400m.^[16][17] There is a single record of a huhu beetle caught in the Kermadecs on Raoul Island. Visiting researchers have searched rotting wood for signs of huhu beetle larvae, but none were found, indicating that it has not established on the island. Because of this, it has been assumed that the specimen was from untreated wood brought to the island and used for construction.^[18]

Behaviour

The beetles are nocturnal and are attracted by the lights of dwellings as noted by Hudson in 1892 "it is greatly attracted to light, and this propensity frequently leads it on summer evenings to invade ladies' drawing-rooms, when its sudden and noisy arrival is apt to cause much needless consternation amongst the inmates".^[19][20] They have powerful mandibles, which can produce a painful bite.

Adult females of P. reticularis produce an olfactory cue which attract adult males to the female.^[21] Adult individuals of both sexes will show a display behaviour if disturbed with the head jutting forward, mandibles opening to their full extent, antennae flailing and the head being raised and lowered.^[21] High intensity displays between individuals may lead to combat with preliminary grappling occurring with fore legs which usually results in an individual being thrown onto its back. Any object coming into contact with the mandibles is seized frequently resulting in the loss of appendages.^[21]

Gut microbiome

In one study, the gut of huhu beetles was found to harbour a diverse range of microorganisms such as Acidobacteriaceae, Burkholderiaceae and Enterobacteriaceae. It was inferred that a large proportion of the microorganisms detected were involved in the digestion process.^[22]

The gut microbiome diversity is also dependent on the food source the larvae 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.^[23]

As food

Advertising cooked huhu grubs at the Hokitika Wildfoods Festival 2021

The larvae of P. reticularis are edible to humans, with a long history of indigenous consumption, and their flavour has been described as like buttery chicken^[24] or peanut butter.^[25] There are different names in Māori for grubs at different stages of development, for example young larvae still actively feeding on timber are called tunga haere or tunga rākau, while full grown grubs which have ceased to feed and are preparing to pupate are called tataka and are the most prized (because there is no undigested wood pulp inside of them at this point). Huhu grubs may be consumed either raw or traditionally cooked in a hāngī, and are an especially rich source of fat in the New Zealand wilderness.^[26]

P. reticularis contains substantial amounts of nutrients. The larvae and pupae are relatively high in fat (up to 45% and 58% dry weight in large larvae and pupae respectively).^[27] The fat in huhu grubs is mostly oleic acid and palmitic acid.^[27] The second most abundant nutrient is protein, which is present at 30% dry weight in the large larvae, and close to 28% dry weight in the pupae.^[27] Protein extracts from huhu larvae and pupae are high in essential amino acids such as isoleucine, lysine, leucine, and valine.^[28] The total essential amino acid content of huhu grubs meets the WHO essential amino acid requirements for human nutrition.^[28] The essential amino acid content of huhu is significantly higher than that of mealworms, and is comparable to beef and chickpeas.^[28] When reconstituted in water, the protein powders of huhu larvae and pupae are able to form stable foams and emulsions. The ash content (representing minerals) of huhu grubs is 1.8% dry weight in large larvae, and 2.2% in pupae.^[27] The minerals include manganese, magnesium, phosphorus, iron, copper, and zinc.^[29]

Predators and parasites

Huhu beetles are known to be preyed upon by a wide range of predators. Some such predators are introduced to New Zealand, such as ship rats,  little owl and magpie.^[30][31] Native predators are known to include short tailed bats, moreporks and saddleback.^[32][30][33] Before their extinction, huia were observed to utilise huhu beetle larvae as their principle food source.^[34]

As larvae, huhu beetles are known to be preyed upon by the larvae of Thoramus wakefieldi, despite the relative small size of T. wakefieldi when compared to huhu beetle larvae.^[35] The larvae are also known to be preyed upon by of Rhipistena cryptarthra.^[36]

Parasites

Huhu beetles are the only known hosts of Prionaphes depressus, which lay their offspring in the eggs of the beetles.^[37]

See also

• Witchetty grub