Huhu beetle

The first specimen of huhu beetle to be used for taxonomic purposes was collected by Andrew Sinclair during a voyage where he visited the 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 specimens 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 larva 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]

The beetle deposits clutches of 10–50 eggs (sometimes up to 100) in cracks and crevices in or under bark.^[12]: 727 However, another study has reported it may 250 –300 eggs.^[13]: 1135 Around 94–98% of eggs collected in the wild will hatch. In laboratory conditions of roughly 20 °C (68 °F) with a humidity of 75%, eggs hatched in 23 ± 2 days. Before hatching, the larva can be seen moving 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. Setae that are found on the first to sixth abdominal segments assist in providing support as the larva leaves the egg and excavates the initial gallery.^{[12]: 727–728}

The larval stages last two to three years in the wild.^[12]: 728 Under laboratory conditions, the duration of this stage has been reduced to 250 days using an artificial diet and maintaining a temperature of 20 °C (68 °F).^[14] In its final instar (developmental stage) the larva moves to within 7.5–10 cm of the surface of the wood before constructing the pupal chamber. 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 3 cm by 1 cm 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 pupal chamber with the last frass voided from its gut. 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. The pupal phase lasts around 25 days.^[12]: 730

Eclosion (when the adult emerges from its cocoon) occurs with a series of ruptures along the head and thorax. The head, legs and wings are freed during arching movements of the body through the ruptured cuticle. 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]: 731 The adults is incapable of feeding and lives for roughly two weeks.^[15]: 733 As adults, the species has a sex ratio of roughly 1:1.^[15]: 734

The larva lives and feeds on the dead wood of gymnosperms. There are twelve gymnosperm species that are known to act as hosts of the larva. Additionally, there are also two host records of the larva living in tawa and oak, which are angiosperms. However, the latter record is likely a misidentification.^[16]

The huhu beetle is endemic to New Zealand where it is widespread throughout the main islands, occurring from sea level up to altitudes of 1400 m.^[17][15] 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] Before Europeans settled in New Zealand, the beetle was mainly associated with podocarp and kauri forests, but has subsequently spread more widely due to the introduction of exotic conifers.^[12]

The beetle is nocturnal and is strongly attracted to light, such as that from human dwellings.^[19] During daytime, the beetle hides in crevices or under debris. It will become active after dusk, apparently triggered by the decreasing light intensity. The female tends to come out earlier than the male and stay in place. 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 2 am before ceasing between 4-5 am.^[15]: 737

The adult female apparently produces an olfactory cue which attracts the male, with the male seemingly flying more frequently to detect the female.^{[15][9]: 649} The adult of both sexes will exhibit 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. When anything touches the mandibles, it can deliver a bite and will latch onto the object. Between beetles, this can escalate 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.^{[15]: 738–739} Usually, the male copulates with the female between 10 pm and 1 am. This begins when the male approaches the female from behind and mounts it while gripping its thorax with the middle and rear legs. If successful, both beetles become somewhat inactive, with both sexes antennae twitching and with the male using its mandibles to stimulate the female, a process which may last up to fifteen minutes. The pair then begin to copulate, which may take up to twelve minutes.^{[15]: 739–740}

During the first instar, 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 form 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 don't go very deep. However, as the log rots and more larva feed on the log, the galleries become deeper and more complex in nature, with the tunnels beginning to intersect with each other.^{[12]: 728–729}

In one study, the digestive tract of the huhu beetle was found to harbor a diverse range of microorganisms such as Acidobacteriaceae, Burkholderiaceae and Enterobacteriaceae. It was inferred that a large proportion of the microorganisms detected likely assists in the digestion process.^[20] 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.^[21]

The huhu larva is edible to humans, with a long history of consumption by Māori, and its flavour has been described as like peanut butter.^[22] The fully grown larva (called tataka) 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 hāngī.^[23][24] 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.^[13]: 1141

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).^[25]: 4 The fat in the larva is mostly oleic acid and palmitic acid.^[25]: 6 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.^[25]: 4 Protein extracts from huhu larvae and pupae are high in essential amino acids such as isoleucine, lysine, leucine, and valine.^[26] The total essential amino acid content of the larva meets the WHO essential amino acid requirements for human nutrition. The essential amino acid content of huhu is significantly higher than that of mealworms, and is comparable to beef and chickpeas. When reconstituted in water, the protein powders of huhu larvae and pupae are able to form stable foams and emulsions.^[26] The ash content (representing minerals) is 1.8% dry weight in the large larva, and 2.2% in pupa.^[25]: 4 The minerals include manganese, magnesium, phosphorus, iron, copper, and zinc. One study found that the wild caught larva have very low levels of heavy metals and thus are safe for consumption.^[27]

The beetle is 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.^{[15]: 740 [28]} Native predators are known to include short tailed bats, moreporks and saddleback.^{[15]: 740 [29][30]} Before their extinction, huia were observed to utilise huhu beetle larvae as their principle food source.^[31] The larva is known to be 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.^[32] The larva is also known to be preyed upon by of Rhipistena cryptarthra, a species of wedge-shaped beetle.^[33]

Because the larva feeds on logs, it may be considered a forestry pest. In New Zealand, logs that are to be exported overseas must first be treated with methyl bromide to prevent the beetles (among other insects) from accidentally being exported and introduced to other countries. The huhu beetle has been identified as one of the five major quarantine risks posed to the United States by New Zealand wood.^[36] Treatments of huhu beetle infestations are further complicated due to the larva potentially burrowing up to 5 cm in depth after five months.^[37]: 5 Because of this, treatment methods must be able to penetrate deeper into the logs.^[37]: 9 Due to issues with methyl bromide eroding the ozone layer, alternative ways to treat infested wood has been proposed, such as using heat treatment or even gamma irradiation.^[36][38]

• Māori cuisine