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= Huhu beetle =
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Introduction
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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 at altitude. Although the adult is unable to
eat, 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, during which the insect 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. The beetle was first described in
taxonomic literature in 1843 by Scottish zoologist Adam White from a
specimen collected in the Bay of Islands.
Taxonomy
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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. The huhu beetle larva and pupa, the juvenile and pre-adult
life stages, were later described by New Zealand entomologist Thomas
Broun in 1880 and a brief description of their typical habits were
given from samples collected from Parua Bay near Whangārei. Broun then
produced a revised description of the species in his landmark 'Manual
of the New Zealand Coleoptera' publication in the same year.
Phylogeny
===========
{{cladogram|{{clade
|1={{clade
|1={{clade
|1=
|2='Enneaphyllus aeneipennis'
}}
|2='Eboraphyllus middletoni'
}}
|2='Prionoplus reticularis'
}}|align=right|title=Rhipidocerini 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
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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 .
Adult
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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. The male antennae
are longer than the female's and also possess some sensilla
(microscopic sensory organs) that are not present in female antennae.
There is a single record of a female developing an extra pair of
ovaries.
Pupa
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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
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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.
Egg
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The egg is around in length and wide. It is white and is
oval-shaped. The surface is smooth. It is covered in a clear secretion
that sticks it to other eggs and substrate before hardening.
Life cycle
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The huhu beetle deposits clutches of 10-50 eggs (sometimes up to 100)
in cracks and crevices in or under bark. One study has reported that
it can lay 250-300 eggs. From eggs collected in the wild, 94-98% will
hatch. In laboratory conditions of roughly 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 enlarge 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 (tunnel).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.
The larval stages last one to two years in the wild. Under laboratory
conditions, the duration of this stage has been reduced to 250 days
using an artificial diet and maintaining a temperature of .
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
3 x 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.
Eclosion (when the adult emerges from its pupal stage) 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.
The adults are incapable of feeding and live for roughly two weeks. As
adults, the species has a roughly equal sex ratio.
Hosts
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The larva lives and feeds on the dead wood of gymnosperms. There are
fourteen 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.
Known gymnosperm hosts of huhu beetle larvae:
* Kauri
* Miro
* Mataī
* Kahikatea
* Rimu
* Yellow silver pine
* Larch (exotic to New Zealand)
* Oregon pine (exotic to New Zealand)
* Monterey cypress (exotic to New Zealand)
* 'Pinus' spp (exotic to New Zealand), including
:* Monterey pine
:* Loblolly pine
:* Black pine
:* Maritime pine
Distribution and habitat
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The huhu beetle is endemic to New Zealand where it is widespread
throughout the main islands, occurring from sea level up to altitudes
of . 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. 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.
Behaviour
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The beetle is nocturnal and is strongly attracted to light, such as
that from human dwellings. 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 adult female apparently produces an olfactory cue which attracts
the male, with the male seemingly flying more frequently to detect the
female. 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. The beetles escalate conflicts with each other
into combat by grappling with their forelegs, typically throwing one
opponent onto its back. Any object coming into contact with the
mandibles is seized frequently resulting in the loss of appendages.
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 mandibles stimulating the
female's upper surface, a process which may last up to fifteen
minutes. The pair then begin to copulate, which may take up to twelve
minutes.
Gut microbiome
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In one study, the digestive tract of the huhu beetle 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 likely assists in the
digestion process. 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
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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 they
no longer contain undigested wood pulp. 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. 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. The ash content (representing
minerals) is 1.8% dry weight in the large larva, and 2.2% in pupa. 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.
Parasites
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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
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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. This is
done by third parties approved by the government. The huhu beetle has
been identified as one of the five major quarantine risks posed to the
United States by New Zealand wood. Treatments of huhu beetle
infestations are further complicated due to the larva potentially
burrowing up to 2 cm in depth after five months and pupating at deep.
Because of this, treatment methods must be able to penetrate deeper
into the logs. Due to issues with methyl bromide eroding the ozone
layer, alternative ways to treat infested wood have been proposed,
such as using heat treatment or even gamma irradiation. , 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
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* Māori cuisine
External links
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*
[http://www.landcareresearch.co.nz/research/biodiversity/invertebratesprog/invertid/bug_details.asp?Bu_Id=199
Landcare Research] - data
* 'Huhu beetle' discussed on RNZ 'Critter of the Week',
[https://www.rnz.co.nz/national/programmes/afternoons/audio/2018724704/critter-of-the-week-huhu-beetle-huhu-grub
29 November 2019]
License
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Original Article: http://en.wikipedia.org/wiki/Huhu_beetle
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