Telephone-pole beetle

Telephone-pole beetle Temporal range: Miocene–Present PreꞒ Ꞓ O S D C P T J K Pg N
"Ghost adult" stage
Larval stage
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Suborder:	Archostemata
Family:	Micromalthidae Barber, 1913
Genus:	Micromalthus LeConte, 1878
Species:	M. debilis
Binomial name
Micromalthus debilis LeConte, 1878
Other species
†Micromalthus eocenicus Kirejtshuk, Nel & Collomb, 2010 †Micromalthus priabonicus Perkovsky, 2016
Synonyms[1]
Micromalthus anansi Perkovsky, 2008

The telephone-pole beetle (Micromalthus debilis) is a beetle native to the eastern United States and the only living representative of the otherwise extinct family Micromalthidae. Larvae of the beetle live in decaying wood and can be pests to wooden structures, lending them their common name, the 'telephone-pole beetle.'

The

Reports of the species are infrequent and it is unknown whether they are rare, or common and unrecognized. A recent study by Bertone et al. (2016)[2] found telephone-pole beetles in a survey of the indoor arthropod fauna in 50 houses located in and around Raleigh, North Carolina.^[3] A recent survey found that the species had spread to every continent except Australia. With finds in South Africa, Hong Kong, Belize,^[4] Cuba, Brazil, Japan, Hawaii, Italy and Austria, the dispersal is likely connected to the timber trade.^[5]

Taxonomy

Classification of M. debilis was historically controversial and unsettled. The species, first reported by

Female-producing (thelytokous) female larvae

The thelytokous female larva resembles a

Male-producing (arrhenotokous) female larvae

The penultimate instar of the male-producing larva is vaguely described as having a dense white color due to much fat. The body is cylindrically shaped and marked by segmented constrictions.^[8]

Male larvae

Unlike female larvae, which resemble the carabid type when first hatched, male larvae resemble that of a weevil.^[9] The body is short and stubby with stump-like legs.^[8]

Adults

The adult beetle is elongated and a dark brown to blackish color, with brownish-yellow legs and antennae. The head is larger than the thorax, with large eyes protruding from either side.^[8] According to H.S. Barber, the adult female is responsible for

Feeding

The

The life cycle of M. debilis is one of the most complicated life cycles among beetles because it involves viviparity, hypermetamorphosis, parthenogenesis, and paedogenesis.^[8] The life cycle is also highly unusual because the larvae rarely mature into adults in both sexes. Haplodiploidy is another notable feature of this species; haploid males are hatched from eggs within the mother's body via haploid parthenogenesis, and diploid females are birthed live via diploid parthenogenesis.^[10]

Females

Upon being birthed live, the larva emerges as a minute white creature with long, slender legs that resemble the carabid type.

Depending on circumstances, the larvae may either pupate (though this is described as rare) or undergo another molt, ultimately revealing the paedogenetic form, in which they can produce young.[11] Paedogenesis is the process by larvae reproduce by giving birth to more larvae without the production of adults and is a process exclusive to females.^[11] Once the paedogenetic form emerges, it typically takes around two weeks for the new generation to be born. The young larvae are born tail-first and begin the cycle anew, continuing the species' life cycle.^[9]

Males

Male larvae are hatched from a single, large egg that adheres to the male-producing larva's body for 8-10 days. The larvae feed on the mother's body and will grow rapidly.

In naturally occurring paedogenetic larvae, the sex ratio is strongly biased toward females. None of the three canonical explanations for biased sex ratios, local mate competition, local resource competition, and local resource enhancement, are likely explanations for the biased sex ratio in telephone-pole beetle larvae. Local mate competition selects for female-biased sex ratios when male siblings compete to fertilize their female siblings, but this is unlikely in this species which females tend to avoid mating with siblings.[11] Local resource competition selects for biased sex ratios but typically involves competition between females for resources and thus selects for male-biased ratios.^[11] Lastly, local resource enhancement can select for biased sex ratios if the offspring of one sex increases the fitness of parents. ^[11] However, because female offspring feed on the mother, there is more likely competition between female larvae, contradicting this explanation.^[11] As such, the cause of sex ratio deviation remains unclear.^[11]

Adulthood

Adult telephone-pole beetles are unable to copulate, and adult females do not have the physiological mechanisms to reproduce because they are unable to lay eggs or produce live progeny, either sexually or by parthenogenesis.^[11]

Since adults do not have a role in reproduction, they are not a physiological part of the life cycle. Thus, the rarity of adult development in the natural world may be an evolutionary response to the lack of their reproductive role. In laboratory settings, development into adults can be induced by high temperatures, but this also results in high mortality because only one out of hundreds of heat-treated larvae will survive and pupate into an adult.^[11]

The adult females live for about six days and males only live for around 12 hours, with a strongly biased sex ratio towards females. The adults of both sexes are sterile and are vestigial remnants of a time when the life cycle involved sexual reproduction. The loss of sexual reproduction is likely associated with its infection by Wolbachia bacteria.^[11]

Literature

Limited observations and experiments on M. debilis have resulted in conflicting observational conclusions, particularly regarding the beetle's reproduction, in the existing literature. Pollock & Normack reported the existence of reproductive adult males, but this was based on the incorrect conclusions by Barber^[9][11]. However, all existing experimental literature states that adults are fertile.^[11]

Mating behavior

A 2016 experimental study used heating to generate substantial numbers of adults to simulate the now non-functional adult reproductive behavior. Due to parthenogenetic reproduction in telephone-pole beetles, information on the sexual mating system of this beetle was previously lost, but recent research is working to uncover this information. Both adult female and adult male modern telephone-pole beetles are sterile, but they still exhibit mating behavior.^[11]

Sex role reversal

The study revealed sex-role reversal, meaning that females face more competition for mates compared to males. This was demonstrated by greater initiative by females to mate and increased fighting between females in the presence of unrelated males. Female also display more aggressive mating behavior, as they may grasp the male genitalia with their own genitalia.^[11]

Rejection behavior

Adult female beetles exhibit rejection behavior to avoid inbreeding with related males, which are adult male beetles that are located on the same natal log. Right after pupating, males expose their reproductive organs as a mate-seeking gesture. Despite tending to compete for male mates, females will ignore these males because they originate from the same log. The males would need to take a short flight to a neighboring log for female mates. Females also perform a 'kin dance' involving shaking of their abdomens and beating of their wing, which is thought to be a deterring signal to related males. Such rejection behaviors demonstrate that sex roles are not fixed because females can also display choosy behavior. The rejection behavior also supports that the female-biased sex ratio is not due to local mate competition.^[11]

Mounting

Females initiated more interactions by actively mounting males, further supporting that female compete for male mates. In cases where several females pile on top of a male, a female may try to dislodge the others with her mandible. Female-female mounting can also occur, and the frequency of this does not change depending on the presence or absence of males.^[11]

Evolutionary history

Genetic studies have placed Micromalthus as more closely related to

Fossils of Micromalthus are known from the Miocene aged Dominican amber (adults and larvae, which were found to not be distinguishable from the living species^[1]) and Mexican amber (larvae), the late Eocene aged Rovno amber of Ukraine (Micromalthus priabonicus),^[18] and the early Eocene (Ypresian) aged Oise amber of France (Micromalthus eocenicus).^[19] A possible specimen of Micromalthus is known from Burmese amber,^[20] but the poor preservation of the specimen makes the assignment tentantive.^[17]

Wood pests

Telephone-pole beetle larvae infest timber by burrowing into the wood, where they feed and develop. They tunnel through the timber, creating galleries as they consume the wood fibers. This activity weakens the structural integrity of the timber, leading to decay and potential collapse. Additionally, their presence can attract fungi, further contributing to the degradation of the timber.^[21]

In the 1930s, telephone-pole beetle larvae were reported as the perpetrators of a gold mine infestation in the