Bark Beetles and their Hidden Harems

Galleries dug in a grand fir Abies grandis by fir bark beetles Pityophthorus pityographus, photographed by Louis-Michel Nageleisen.

For producers of commercial timber, the above picture would not be a pretty sight. Bark beetles are named after what they feed on: they chew galleries under the bark of trees. In some species that attack otherwise healthy trees, these borings may result in stunted growth or death. The beetles may spread fungal diseases as they move from one tree to another (Dutch elm disease is one example of a well-known disease spread by bark beetles). But on the other hand, many bark beetles play a vital row in nutrient recycling, feeding on already dead and dying trees and breaking down the wood.

The fir bark beetle Pityophthorus pityographus itself, from PaDIL.

The bark beetles belong to a group called the Scolytinae. The scolytines include over 6000 species worldwide (only a relatively small percentage of which, it should be noted, are recognised as significant pests). Oddly enough, they are actually a kind of weevil. The most characteristic feature of most weevils is their elongate snouts, but in scolytines these snouts have been lost (they would probably not be ideal for burrowing through wood). The fir bark beetle belongs to a subgroup of the scolytines called the Corthylini, distinguished from other scolytines by their elytra, which lock down so that a panel on the side of the body called the metepisternum is hidden when the elytra is closed (in other scolytines, it remains at least partially visible), and by the flattened round clubs on their antennae (Wood 1986). The Corthylini are themselves divided into two subgroups, the Corthylina and the Pityophthorina. The two groups are not that easily separated by their morphological appearance, but they are very different in their ecology. The Corthylina don't live directly under the bark, but deeper in the tree amongst the xylem (the central water-conducting tissue). Corthylinans and ecologically similar beetles, known as ambrosia beetles, live in association with a fungus that grows on the xylem. The beetles, which cannot directly digest the xylem themselves, feed instead on the fungus.

The Pityophthorina, on the other hand, are true bark beetles, with most species feeding directly on the tree's phloem (the sugar-conducting tissue around the outer part of the tree). Other species in this group burrow in the tree's seeds, or feed on the pith inside slender stems. The main diversity of Pityophthorina (and of Corthylini in general) is in the Americas, particularly in cooler temperate or tropical highland environments, with over 500 known species in North and South America. Two species, Pityodendron madagascarensis and Sauroptilius sauropterus, are found in Madagascar, while the genus Mimiocurus includes ten species found in Africa and Asia. The largest genus in the Pityophthorina, Pityophthorus, includes about sixty species in Africa and Eurasia in addition to over 300 in the Americas. Wood (1986) suggested that the Eurasian species of Pityopthorus were probably descended from relatively recent migrations from North America, but African and Madagascan species of Pityophthorina may represent more basal lineages.

Female Dendroterus decipiens, photographed by T. H. Atkinson.

As well as their economic and ecological significance, scolytines have attracted attention for the range of breeding behaviours they exhibit (Kirkendall 1983). Bark beetle galleries are not just feeding structures, they are also breeding structures. Females mate and lay their eggs within the galleries, and their larvae hatch and continue to feed there. The Pityophthorina are described as including both monogynous and polygynous species, but these terms refer to the number of females in a gallery, not necessarily the mating habits of the males. In monogynous species, a gallery will be home to only a single female. Construction of this gallery may have been started by the female herself, or it may have been started by a male who was then joined by the female. In most monogynous Pityophthorina, the latter is the case (and the mating system is monogamous as well as monogynous), but the female is the one to start the gallery in the genus Conophthorus (Conophthorus species feed in pine cones, and may be restricted to monogyny by the small spaces available for gallery construction). Conopthorus males that mate with the female may not remain in the gallery, but may leave directly after mating. Males that don't stay in the gallery can mate with more females, of course, but males who do stay will be able to prevent their mate from mating with another male herself before laying her eggs. Also, by helping to maintain the gallery (or by constructing the gallery himself to begin with), the male may encourage the female to oviposit faster, or improve conditions for the larvae when they hatch.

In polygynous species, such as most Pityophthorus species, a single gallery will be home to multiple females. In most polygynous Pityophthorina, a single male will co-habit with a harem of females. A few pityophthorinans of the genus Araptus are inbreeding polygynes: males do not leave their parent gallery, but instead mate with their sisters before the latter leave the gallery. Inbreeding species seem to show remarkable control over sex ratios in the population, with many more female larvae produced than males. Interestingly, monogynous and polygynous galleries tend to differ in physical structure: monogynous galleries tend to be simple and direct, with only one or two arms extending along or across the host plant from the central nuptial chamber. Polygynous galleries, on the other hand, may have several arms radiating from the nuptial chamber, with each arm probably being built by a separate female.

REFERENCES

Kirkendall, L. R. 1983. The evolution of mating systems in bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae). Zoological Journal of the Linnean Society 77: 293-352.

Wood, S. L. 1986. A reclassification of the genera of Scolytidae (Coleoptera). Great Basin Naturalist Memoirs 10: 1-126.

Thrips Wars!

Two males of Elaphrothrips tuberculatus fight it out on the left, while the object of their desire guards her egg-mass on the right. Figure from Crespi (1986).

All around, little dramas are taking place every day, conflicts as intense as the plot of any daytime soap opera. And like most daytime soap operas, the main focus of these dramas often comes down to who is shagging whom. Most people only known thrips as small annoying insects that damage garden plants and crops, but some thrips may engage in remarkable behaviours.

Elaphrothrips is a genus of thrips found almost throughout the tropics (though it is absent from Australasia). They are found on dead leaves, where they feed on fungal spores. Well over a hundred species have been named in Elaphrothrips, though Mound & Palmer (1983) pointed out that many of these may be turn out to be synonymous as individual species can vary significantly in appearance. Males may have thick forelegs with strong tubercles on the femora, while the forelegs of females are usually slender and lack tubercles. Indeed, the sexes are different enough that at one point they have been mistaken for separate genera. The males themselves may vary significantly in size, with larger males having correspondingly larger legs and spines.

A lot of these differences are related to the Elaphrothrips' mating behaviour. The best-studied of the Elaphrothrips species is E. tuberculatus, a widespread species in eastern North America and the largest North American thrips species. Elaphrothrips tuberculatus prefer dead oak leaves that are still hanging in clusters from the tree, where females lay eggs in clusters on the leaves and then stand guard over them. The females are themselves guarded by males, but the males may be challenged by others who want to take the female for themselves. Battles between male Elaphrothrips most commonly take the form of the two males lining up alongside each other, as in the drawing at the top of this post, and then one or each begins batting at the other with his elongate abdomen. Alternatively, one male may attempt to reach under his opponent's abdomen with his own, and then try to flip his opponent over. Crespi (1986) noted that challenging males were more likely to try to flip their opponent than defending males, perhaps because the success rate of flipping attempts was very low, making this tactic more of a gamble. Flipping could also act as a defense against a third attack strategy, in which one male would climb up onto the back of his opponent and use the tubercles on his forelegs to stab at his opponent's thorax. Larger males were more likely to stab their opponents than smaller males, which of course have less developed leg spines. However, a smaller male may also get around larger male through sneaking behaviour, mating with the female before her guarding male realises he is there.

Whichever male mates with the female, one thing is certain: he will only have daughters. Thrips have a haplodiploid sex determination system like that of ants and bees, with males developing from unfertilised ova and females from fertilised ones. Elaphrothrips tuberculatus adds another wrinkle to the system that only females hatch from eggs. Male offspring, on the other hand, develop inside their mother and are born live (Crespi 1989). Nevertheless, an individual female may have both male and female offspring, as she may change her reproductive mode between broods to be a live-bearer or an egg-layer!

REFERENCES

Crespi, B. J. 1986. Size assessment and alternative fighting tactics in Elaphrothrips tuberculatus (Insecta: Thysanoptera). Animal Behaviour 34: 1324-1335.

Crespi, B. J. 1989. Facultative viviparity in a thrips. Nature 337: 357-358.

Mound, L. A., & J. M. Palmer. 1983. The generic and tribal classification of spore-feeding Thysanoptera (Phlaeothripidae: Idolothripinae). Bulletin of the British Museum (Natural History): Entomology 46 (1): 1-174.

The Eupodoidea: Earth Mites and Silk-weaving Mites

Two cocceupodids, possibly Linopodes motatorius, photographed by Christophe Quintin.

For today's post, I'd like to introduce you to the Eupodoidea. These are a group of mites in the Prostigmata (you can find a whirlwind tour of the Prostigmata here) that can be found in soil or vegetation. Various species of eupodoids are predators or plant-eaters; as far as is known, the group doesn't include members symbiotic with other animals. They are mostly soft-bodied (except for members of the family Penthalodidae) and generally small, though members of the Australian genus Eriorhynchus can grow to nearly two millimetres in length (yes, that's big for a mite). Eupodoids are particularly diverse in cooler climates, though once again there are notable exceptions: the Hawaiian species Hawaiieupodes thermophilus inhabits volcanic steam vents with temperatures of up to 41°C (Walter et al. 2009). The first pair of legs are often used as sensory organs, being held in front of the body while the mite walks on the remaining three pairs.

The eupodoids are currently divided between nine families, some of which have only been established recently. The Cocceupodidae was establised by K. Jesionowska in 2010 for the genera Cocceupodes, Linopodes and Filieupodes, and the South African species Dendrodus acarus was described as its own family by P. A. S. Olivier in 2008 (unfortunately, he named this family the 'Dendrochaetidae', which is an invalid name because it was not based on an included genus). The genera Pilorhagidia (known from Hawaii and Europe) and Eriorhynchus are also placed in separate small families, as is the South African species Pentapalpus unguempodius. The habits of these various small families are little-known, but Pilorhagidia and Pentapalpus are probably predatory, and the large hairy Eriorhynchus may be herbivorous. Similarly uncertain in habits are the sclerotised mites of the Penthalodidae, whose two genera Penthalodes and Stereotydeus may be quite colourful and intricately ornamented.

Winter grain mite Penthaleus major, photographed by Scott Justis.

The Penthaleidae is a family of plant-feeding mites that is most notorious for including some significant agricultural pests. These include the winter grain mite Penthaleus major and the red-legged earth mite Halotydeus destructor (it's all in the name, really). Both these species feed on a range of crops. They both have dark central bodies with bright red legs, but the winter grain mite also has a reddish patch on its back marking the dorsal position of the anus (where the drop of liquid is being extruded in the photo above). Red-legged earth mites lack such a patch, and their anus is terminal in position.

The remaining two families are the most diverse. The Eupodidae often have the femora of the hind legs larger than the other legs, allowing them to rapidly jump backwards when threatened (such enlarged femora are also present in the Cocceupodidae). A number of Eupodes species feed on micro-algae. One eupodid, Benoinyssus najae was originally thought to be an animal symbiont as it was collected from the nasal fossae of a cobra. This species has since been found in soil and leaf litter, and it is thought that its original collection site was accidental.

Rhagidiid mite, photographed by Amir Weinstein.

The final family is the Rhagidiidae, a cosmopolitan group of carnivorous mites. Rhagidiids feed on other small arthropods, and are found in a wide range of habitats, including some that are restricted to caves. Members of the Rhagidiidae spin silk from glands near the chelicerae that they use for protection. Nymphs of Rhagidia species spin a web around themselves before they moult. In at least one species, Rhagidia longisensilla, the web is also used to capture prey: springtails that run into the web become entangled in it, and the mite is thus able to capture springtails twice its own size (Ehrnsberger 1979). I am not aware of any other animal outside spiders that use silk in this way, and I'd be interested if anyone else is.

REFERENCES

Ehrnsberger, R. 1979. Spinnvermögen bei Rhagidiidae (Acari, Prostigmata). Osnabrücker naturw. Mitt. 6: 45-72.

Jesionowska, K. 2010. Cocceupodidae, a new family of eupodoid mites, with description of a new genus and two new species from Poland. Part I. (Acari: Prostigmata: Eupodoidea). Genus 21 (4): 637-658.

Olivier, P. A. S. 2008. Dendrochaetidae, a new family of mites (Acari: Prostigmata), with descriptions of a new genus and species from South Africa. African Zoology 43 (1): 16-24.

Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 233-420. Texas Tech University Press.

The Naked Ascus

Hyphae with poorly-differentiated fruiting bodies of Gymnascella marismortui, from Buchalo et al. (1998).

Three years ago, Christopher and I visited my aunt in Jordan. We spent a week visiting various parts of the country (Petra is amazing), including that most touristy of all activities, swimming in the Dead Sea. Except 'swimming' is not really the right word for what you do in the Dead Sea: with the salt content of the water and hence your own buoyancy so high, normal swimming movements are practically impossible. You can't do much more than bob along on your back*. Standing back up again is equally disconcerting: the extra force required to push your legs back down through the water is such that it is quite startling to discover that the bottom is only about two feet below you. The water has a not-particularly-pleasant greasy feel to it, and at one point I had a drop of it splash onto my lower lip. To my surprise, I could not really describe the taste of that drop as salty. Rather, I would say that what the Dead Sea tastes of is Pain.

*But not too far. One bather had a congregation of guards on the shore suddenly begin yelling at her, evidently because she was swimming too far out and was about to inadvertently invade Israel.

It's hard to believe that anything could live in such conditions, but there is life there. Salt-loving prokaryotes and unicellular dense algae can sometimes form dense blooms, and in 1998 Buchalo et al. described the filamentous fungus Gymnascella marismortui, grown from spores collected in Dead Sea water. Fungal hyphae where observed growing on wood in the Dead Sea in spots where its saltiness had been diluted, such as by the inflow of freshwater springs or rain. Gymnascella marismortui may play a significant role in breaking down wood or other plant material that has been washed into the Sea.

Reproductive structures of Kraurogymnocarpa lenticulospora, from Udagawa & Uchiyama (1999).

Gymnascella marismortui is just one species in a family of microscopic fungi known as the Gymnoascaceae. The prefix gymno- in the name means 'naked', and refers to the fact that these fungi do not have the asci (spore-packets) surrounded by a strongly differentiated fruiting-body wall. Instead, the asci are surrounded by a cluster of hyphae that are little differentiated from the remainder of the fungus, or that form a net-like arrangement called a reticuloperidium (an example of the latter can be seen in the lower left of the figure above). Greif & Currah (2003) suggested that the reticuloperidium may be an adaptation to dispersal by insects, as they became caught on the hairs of flies and were then split open to release the spores when the fly was grooming itself. The ascospores themselves are oblate in shape, with polar depressions or equatorial thickenings. Gymnoascaceae are found in such habitats as soil, rotting vegetation or dung, where they break down substances such as cellulose and keratin (Stchigel & Guarro 2007).

Apart from their largely unsung role as decomposers, Gymnoascaceae have little economic impact on humans. They are relatives of the fungi that cause ringworm and tinea (indeed, these fungi have been included in the Gymnoascaceae in the past), and there have been occasional reports of Gymnoascaceae causing similar infections. However, these infections were probably just incidental: after all, to a fungus, keratin is keratin.

REFERENCES

Buchalo, A. S., E. Nevo, S. P. Wasser, A. Oren & H. P. Molitoris. 1998. Fungal life in the extremely hypersaline water of the Dead Sea: first records. Proceedings of the Royal Society of London Series B 265: 1461-1465.

Greif, M. D., & R. S. Currah. 2003. A functional interpretation of the role of the reticuloperidium in whole-ascoma dispersal by arthropods. Mycological Research 107 (1): 77-81.

Stchigel, A. M., & J. Guarro. 2007. A reassessment of cleistothecia as a taxonomic character. Mycological Research 111 (9): 1100-1115.

Udagawa, S., & S. Uchiyama. 1999. Taxonomic studies on new or critical fungi of non-pathogenic Onygenales 1. Mycoscience 40: 277-290.