Bark and ambrosia beetles are plant-boring weevils that have evolved a special lifestyle. They spend most of their life in wood, from the egg stage to being a larva, pupa and adult beetle. Most species prefer to dwell in dead trees. Only a few species are able to overwhelm the defences of living trees and inhabit them, but some of these species are serious forest pests, and they have a huge ecological and economic impact.
Usage of the term 'bark beetles'
The term 'bark beetles' is ambiguously used. In a taxonomically strict sense, it refers to the subfamily Scolytinae within the weevils (family Curculionidae). In an ecological sense, it is used for Scolytine species that breed in the inner bark of trees, where they cause the typical patterns of tunnels ('galleries'). This does does not apply to all Scolytinae, as some species inhabit other parts of their host plant.
Beetles from the subfamily Platypodinae (pinhole borers) have evolved very similar lifestyles and morphologies to those from the subfamily Scolytinae. This is a case of convergent evolution, as from a taxonomic point of view, Scolytinae and Platypodinae are two seperate subfamilies of weevils. At current, ca. 6000 Scolytinae species and ca. 1400 Platypodinae species have been described.
Some species of Scolytinae, and almost all species of Platypodinae, bore tunnels in the inner wood of trees. These species live in mutualisms with fungi, which they grow on the walls of their tunnels and on which they feed. Weevils that only feed on fungi (xylomycetophygy) are summarized under the term 'ambrosia beetles'. This is not a taxonomic, but an ecological designation.
All Scolytinae and Platypodinae together are usually referred to as 'bark and ambrosia beetles'. But, confusingly, sometimes the term 'bark beetles' alone is used to incorporate all Scolytinae and Platypodinae beetles. On this website, we use the terminology in an ecological sense, i.e., we refer to weevils that live in the inner bark as 'bark beetles', and to weevils that farm fungi as 'ambrosia beetles'.
Biology of bark beetles
All bark beetles are relatively small, mostly between 2 and 5 mm in size, and the morphology of their cylindrical, brownish-coloured bodies is well adapted to their tunneling lifestyle. Bark beetles feed on the inner bark of trunks and branches (phloephagy). Some species also consume a little bit of inner wood, and some transport spores of fungi to the trees they inhabit, which then nutritionally enrich the bark they are feeding on (phloemycetophagy).
There are three different host preferences in bark beetles. The majority of bark beetles colonize recently dead wood material, for example wind-felled trees or dead branches. About a dozen bark beetles species are aggressive, which means that they are able to kill trees that are alive, overwhelming the (mostly chemical) defences of their hosts. These are the species that cause most economic damage in forests, and, not surprisingly, are the ones best studied. Aggressive bark beetles typically undergo pronounced population dynamics. During an endemic state, which can last for several years, the population consists of very few individuals, and they only colonize unhealthy trees. If for example a storm or drought leads to many stressed or dead trees in a forest, the population of bark beetles quickly builds up, and finally reaches an epidemic state, during which the beetles are able to kill healthy trees. This state is usually followed by an aprupt population crash, but the underlying mechanisms are still not understood. A third group of bark beetles with only very few species are parasites of living trees. They inhabit trees that are alive in small numbers without killing their host.
What does 'ambrosia' have to do with beetles? Well, the food of ambrosia beetles has a sweet smell and a yellowish colour, and this made their discoverer think of the delicacy of greek gods. The Augustinian monk Josef Schmidberger found ambrosia beetles in the trees of the garden of the monestery Sankt Florian near Linz (Austria). The sweet-smelling substance inside the beetles' nest was a fungus, but back in 1836 monk Schmidberger was not aware of that . Today we know that ambrosia beetles not only plant the fungi they feed on themselves, but also actively tend them. They are one of the very few groups of insects that have evolved agriculture.
The term 'ambrosia beetles' is not a taxonomic, but rather an ecological designation. It refers to those 3500 species within the beetle subfamilies Platypodinae and Scolytinae that farm fungi for food. Like bark beetles, ambrosia beetles are only a few millimeters in size, and have brownish coloured and cylindrically shaped bodies that are adapted to their tunneling lifestyle. In contrast to bark beetles (see above), however, ambrosia beetles do not build their tunnels in the inner bark (phloem) of trees. Instead, they inhabit the wood of trunks and branches, which lies under the bark and is much less nutritious. But ambrosia beetles have found a solution to that challenge: Instead of feeding the wood directly, they plant their fungi on the walls of the tunnels and chambers that they dig. The fungus then penetrates into the wood, accumulates nutrients, and serves as the beetles' only food. The fungus, on the other hand, is dependent on the beetles, who transport its spores to new nests in a special spore-carrying organ (mycangium, sometimes also called mycetangium). This is an example of an interspecific mutualism, a close association between two species with mutual benefits.
Within their natural habitat, ambrosia beetles only colonize dead trees. Some species that have been accidentally transported outside their native range by humans, however, have become invasive and are now also attacking and killing living trees.
Establishing and maintaining a healthy fungus garden is very labour intensive. But ambrosia beetles have evolved a high level of intraspecific cooperation: They live in social groups, dividing the different cultivation tasks within the nest between group members. Such a social lifestyle is not known in any other beetles. In contrast to classical eusocial insects like bees, ants or termines, however, ambrosia beetle workers keep their ability to reproduce, i.e., they are not sterile. An exception is presumably the ambrosia beetle species Austroplatypus incompertus that is native to Australia. This beetle is assumed to have an obligate social lifestyle with a reproductive queen and sterile workers, but it has not yet been studied in detail. In all other ambrosia beetles known so far, the female group members 'deliberately' resign from reproducing, at least for several weeks. During this time they stay with their mother and sisters within their natal nest, where they help with fungiculture. Only later they disperse and found their own nests. This makes ambrosia beetles very interesting models for studying the factors that lead to the evolution of a social lifestyle, as one can investigate the factors that lead totipotent females to either stay with their family and help, or to disperse and breed independently. Males usually play a minor role for fungiculture: Their only task is to mate with their sisters before they fly away and start their own families.
The ambrosia beetle X. saxesenii
Within our research group, most studies are conducted with the ambrosia feeding fruittree pinhole borer Xyleborus saxesenii Ratzeburg. Even though this species is ubiquitous in Central Europe, it is hardly known outside the scientific community. This is mainly because like all ambrosia beetles, it spends most of its life hidden in nests in the heartwood of trees, and it is very tiny. Females of X. saxesenii are about 2 mm, and males about 1.5 mm in size. A very remarkable finding about X. saxesenii is that even the larvae of this species engage in nest-maintaining tasks. Such division of labour between larvae and adults is not known for any other holometabolous insect, i.e. insects with maggot-like young that look totally different from the adult insects.
Kirkendall et al (2015). Evolution and diversity of bark and ambrosia beetles. In "Bark Beetles: Biology and Ecology of Native and Invasive Species" (F. E. Vega and R. W. Hofstetter, Eds.), pp. 85-156. Academic Press, San Diego.
The poplar spiral gall aphid (Pemphigus spyrothecae Passerini) is monophagous, which means that it only feeds on a single plant. Its host is black poplar (Populus nigra L.), a Central European tree species.
Poplar spiral gall aphids are social insects and live in family groups. In spring, female aphids hatch from fertilized eggs that have overwintered on the tree. When the first leaves flush, the so-called fundatrices (colony founding mother aphids) induce galls (knot-like hollow structures) in the petioles of leaves. These are then inhabited by the aphids in social groups and are commonly defended. Within the gall, the fundatrix reproduces asexually, or more precicely, parthenogenetically. This means that she has offspring without being fertilized. Aphids are hemimetabolous insects and the larvae resemble adult aphids. Two different morphs of larvae have been observed. In addition to the normal morphs that later become reproductive, there are also soliders, which have thicker legs and a stylet, with which they aggressively defend the nest. This is especially important when the aphids need to form little holes in their gall to allow winged (alate) aphids to leave the nest.
Two factors have likely been relevant for the evolution of sociality in gall aphids. First, due to the parthenogenetic reproduction, all aphids within a colony are highly related - in fact, they are clones, sharing the same genes, which favours cooperation (inclusive fitness theory). Second, a gall is a very precious home. Only during a small time window in spring it is possible for the aphids to induce the galls in the leave petioles, and without common defence, galls are vulnerable to predators.
The geographic range of the poplar spiral gall aphid lies in Europe, North Africa (Tunisia), western Siberia, Pakistan and some locations within Canada.
The hosts of the poplar spiral gall aphid, the back poplar trees, are not only colonized by aphids, but by a range of different insect herbivores. To defend themselves against these antagonists, they use chemical means, for example the emission of volatile organic compounds. Apart from infestations with herbivores, the poplar trees face infections with fungi.
Beetles within the tribe Cassidini are commonly referred to as tortoise beetles. The tribe comprises around 40 genera worldwide and is one of the largest within the subfamily Cassidinae, which itself belongs to the family Chrysomelidae, the leaf beetles. Several of the tropical species of this tribe display parental care, but the European species that we study breed solitarily.
Tortoise beetles owe their name to the flattened shape of the adult beetles. Their wing covers are expanded around the edges, which makes them look a bit like a tortoise. The flattened wings are an adaption to protect the beetles against predators, for example ants, who cannot just simply lift the beetles up and carry them away. In addition, the terminal part of the beetles legs, the tarsi, are equipped with bristles, with which the beetles can strongly adhere themselves to a leave.
Like in all holometabolous insects, beetle larvae look much different from the adult beetles and have no wings. But the larvae of the tortoise beetles have evolved other amazing adaptations to protect themselves from predators who want to eat them, and from parasitoids who want to lay their eggs into them. First, their body is usually equipped with spikes which disrupt their outline and make them less easily visible. Second, they have a pair of little forks at their back with which they can defend themselves. And third, on the end of this fork, they build protective shields from accumulated feces and cast skin. The form, size and structure of these so-called 'faecal shields' are extremely variable between species. The shields seem to help the larvae protect themselves from predators and parasitoids. However, the exact function of the faecal shields is not yet fully understood. For example, in the European thistle tortoise beetle Cassida rubiginosa Müller, the shield is effective against parasitoids and generalist predators, but does not protect the larvae from their main antagonist, the specialist paper wasp Polistes dominulus. It has been assumed that the shields contain repellent substances, for example terpenes, but this has been little investigated.
The species we work with in our lab is Cassida rubiginosa, which is sometimes referred to as the 'green thistle beetle'. It is native to Eurasia, but has been intentionally introduced to the US and New Zealand to control thistles in agricultural areas. The creeping thistle Cirsium arvense (also referred to as the Canada thistle) is its main host plant.
The coffee berry borer (Hypothenemus hampei (Ferrari)) is the most devastating insect pest of coffee worldwide. It is originally endemic to Africa, which means that it used to not occur in any other location. However, today it has invaded most coffee producing countries.
Damage ensues after a colonizing female enters the coffee berry and bores tunnel systems (‘galleries’) throughout the two seeds (coffee beans) inside the berry. She then lays her eggs within the tunnels. Upon hatching, larvae feed on the seeds, further reducing the amount and quality of the yield. Brothers and sisters mate within their natal nest inside the berry. Subsequently, when conditions are favorable, the inseminated adult females leave the berry in search of a new coffee berry to colonize.
Vega, F. E., F. Infante, and A. J. Johnson. 2015. The genus Hypothenemus, with emphasis on H. hampei, the coffee berry borer. In "Bark Beetles: Biology and Ecology of Native and Invasive Species" (F. E. Vega and R. W. Hofstetter, Eds.), pp. 427-494. Academic Press, San Diego.