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.