Bark & Ambrosia Beetle Academy

An Ambriosiodmus beetle excavated from its tunnel in a dead maple tree. The beetle is very small, only a few millimeters long.

Beetles are hard to avoid. They are the most diverse group of organisms, making up a whopping 25% of all multicellular species described to date. With a grab bag of over 350,000 species you're bound to have many that are interesting biologically or important to human endeavors. Scolytinae and Platypodinae (or Scolytidae and Platypodidae for the old schoolers) are two groups of highly specialized weevils (Curculionidae) that fit both of those profiles. Thus, if any insect groups merit their own week-long course it's these amazing beetles.

Luckily, a group of experts had that in mind when they developed the Bark & Ambrosia Beetle Academy, hosted and organized by Jiri Hulcr and his team at the University of Florida. I was fortunate enough to attend the first ever (and surely not the last) class in early May, and although I could tell many stories I am going to just mention some of the informative tidbits I learned through fun facts and photographs I took.

• "Bark beetles" and "ambrosia beetles" are not taxonomic groups. They are not even completely black and white definitions. Some of these beetles feed only on bark and phloem (former), others grow fungi on which they feed (latter), and still others use different strategies like finding fungi or supplementing their wood diet with fungi. There are even "bark beetles" that bore into cacti or herbaceous plants - truly a diverse group.

Galleries under the bark of a loblolly pine (Pinus taeda) made by the southern pine beetle (Dendroctonus frontalis). Dark streaks of Ophiostoma blue stain fungus can be seen which compete with bark beetles and can cause mortality. Life under bark is a diverse ecosystem.

Bore holes lined with fungus grown by beetles. These ambrosia beetles can gnaw away the wood, but need to feed on the fungus to survive.

• Fungus farming has evolved independently at least 11 times in these two groups (once in Platypodinae and several times in the Scolytinae). The beetles have special pockets (mycangia or mycetangia) on various body parts to carry the spores of their fungi. What was even more surprising to learn was that some ambrosia beetles actually "steal" fungus (called mycocleptae) from other beetles by tunneling close to the true farmers and letting the fungus grow into their tunnel!

A composite image of two different ambrosia beetles tunneling in a dead maple. The one on the left is a female Ambrosiophilus while the one on the right is a male Ambrosiodmus (see female in title image). The two upper right tunnels are in such close proximity that the fungus grows between them, a phenomenon that lead some beetles to evolve a fungus stealing strategy (including some species of the genus Ambrosiophilus).

• Like wasps and some other animals, many of theses beetles have unusual sex systems (namely haplodiploidy) whereby males are produced from unfertilized eggs, while females come from fertilized eggs. What's more, in many of these beetles a foundress mother beetle will create a tunnel and lay many female eggs and one male that will end up mating with his sisters. The males in many cases are much smaller, cannot fly and are mainly used for one task - reproduction. In one extreme case, the species Ozopemon uniseriatus, the male is larviform (paedomorphic) and very different from a typical adult beetle. This sexual system along with living in a domicile with other members of the family is likely the reason one species, Austroplatypus incompertus, has become the only known eusocial (i.e. truly social) beetle, with a reproductive "queen" that has many offspring that do not reproduce and instead take care of their brothers and sisters. Colonies can last as long as 37 years (as does the queen) in a single eucalyptus tree!

The black twig borer (Xylosandrus compactus) is a species that exhibits haplodiploidy. Here a tiny, pale male (center) can be seen with his more massive sisters. He will fertilize them before they go colonize other twigs.

• In the South, pines (Pinus sp.) are a prevalent tree that have their own groups of bark beetles, mostly members of the genus Ips and Dendroctonus. Several species inhabit trees just under the bark and are usually located at specific heights depending on the species. Ips are likely to invade unhealthy/dead trees, stumps and logs but can cause damage to healthy ones in certain situations. They are easily identified by the scooped-out rear (elytral declivity) with several spines surrounding the concavity (see below). Dendroctonus have a head that's visible from above and a gradual, even declivity. Though the black turpentine beetle (Dendroctonus terebrans) is large and infests healthy trees, it is rarely ever of concern. The southern pine beetle (Dendroctonus frontalis), on the other hand, aggressively attacks healthy trees en masse until tree death occurs. There are other Dendroctonus species that attack pines out West, with similar consequences (the mountain pine beetle Dendroctonus ponderosae, for example, is extremely damaging). The main difference between identifying D. frontalis and D. terebrans entrance holes and pitch tubes on pine is that the former's pitch flows are larger and nearer the base of the tree, while the latter's are smaller, more numerous and about breast height (see below).

Resin flows from a loblolly pine (Pinus taeda) under attack by southern pine beetles (Dendroctonus frontalis). Note that the "popcorn" (i.e. resin flows) is found in the crack between the bark, unlike Ips which will often bore right through the bark plates.

Close up of the small, hardened resin flow created by a southern pine beetle (Dendroctonus frontalis). These are made when the beetle enters the tree, which uses the resin to push the beetles out. When they exit, the beetles leave tiny dry holes because the tree is dead and does not produce the protective resin.

An adult southern pine beetle  (Dendroctonus frontalis). The total length of the beetle is ~3 mm. Note the head is visible from above as in all Dendroctonus. 

The resin flow of a black turpentine beetle (Dendroctonus terebrans) is much larger, as is the beetle itself  (about 5-8 mm).

Ips, like this Ips grandicollis, are frequently found under pine bark, but rarely cause problems for healthy trees. Their presence may mean that a tree is under stress or unhealthy in some way. This beetle is about 4 mm long and has a head hidden below the pronotum and spines circling the tip of the elytra. 

• As most people know, some species of these beetles are extremely economically important. The classic example is the European elm bark beetle (Scolytus multistriatus), which along with a few other species transmits the causative agent of Dutch elm disease (Ophiostoma sp.). I learned that it's actually the maturation feeding of adult beetles (i.e. when they feed externally on new trees to gain nutrition) that partly helps to spread the disease. Other beetles are as or more destructive. Closer to home, the redbay ambrosia beetle (Xyleborus glabratus) is an exotic species from Asia that attacks healthy members of the family Lauraceae. It brings with it a fungal pathogen (Raffaelea lauricola) that causes laurel wilt, a disease that leads to mortality in these plants, many of which are important for forests and commercial use (e.g. avocado and cinnamon). Lastly, a commodity close to many people's heart is also under attack around the world: coffee. The coffee berry borer (Hypothenemus hampei) threatens the supply of the world's favorite caffeinated drink by boring into the seeds (the very beans we love) and living out their existence inside. Their small colonies significantly reduce the quality of the product and are difficult to control. All of these beetles are being studied to understand the best ways to manage and prevent their destructive, however unintended, nature.

The redbay ambrosia beetle (Xyleborus glabratus) is a tiny beetle that prefers infesting healthy trees in the family Lauraceae. Though not a pest in its native Asia, outside of its home range it transmits Raffaelea lauricola to trees, causing a wilting disease by interfering with the plant's vascular system.

The effects of laurel wilt (Raffaelea lauricola) can at first be seen in the dying leaves atop redbay trees. Upon further investigation, dark streaks can be seen under the bark of trees with the disease.

• It's an uphill battle: every year we get more species of insect pests. Bark beetles, due to their cryptic nature and small size, are among the most commonly imported species. Over 60 species of bark beetles have been introduced and established in the US and the number will surely climb. Many are not important pests, relying largely on dead trees for their homes and food. Those that attack healthy trees, and especially those that bring with them pathogenic fungi, are the ones we should and do worry about. It should be noted, though, that these beetles in general are important decomposers of dead trees in natural systems and play a key role in forest health.

Xylosandrus amputatus was recently (2010) found in Florida and has now been collected in Georgia; its home range is East Asia. Luckily this species attacks dead or dying trees, so it will likely not become a pest.

• Even though it may seem futile, we are developing new methods for controlling them. One researcher is even using the sounds these beetles produce to communicate with each other as a potential source of control, by confusing or repelling them. We are also not in this alone: many species of insects and other animals are predators or parasites of bark and ambrosia beetles. Knowing the good from the bad is important, as is implementing control strategies that reduce pest beetle populations while encouraging the livelihood of these beneficial insects.
  

Darkling beetles (Tenebrionidae) in the genus Corticeus (like this C. thoracicus) are often found among bark beetle galleries where they feed on many things, including the larvae of bark and ambrosia beetles. They can contribute to the control of some pest beetles.

A cylindrical bark beetle (Zopheridae: Colydiinae: Colydium lineola) is the perfect shape to fit into bark and ambrosia beetle tunnels. They are predators of the wood boring beetles.

Checkered beetles (Cleridae) are predators as larvae and adults. Here the larva of one is found under pine bark, likely feeding on the many Dendroctonus larvae located underneath.

A clown beetle (Histeridae) is yet another predator of bark and ambrosia beetles. Though this one was a moderate size (~4 mm), I have seen tiny Plegaderus (~1.5 mm) that were mistaken for bark beetles because they were found inside  tunnels.

It's not just beetles that feed on bark and ambrosia beetle. These maggots of the long-legged fly Medetera (Dolichopodidae) inhabit wood-boring beetle galleries and feed on their young. 

The course was excellent and I advise anyone interested in the subject to attend the next time it is held. I enjoyed meeting all of the participants, and the presenters were very helpful and had a wealth of knowledge. It's great to see so many people are interested in not just controlling these beetles, but understanding their lifestyles and evolution.

Participants signed a copy of one of Stephen Wood's tome on bark and ambrosia beetles.

For more photos, please visit my Flickr album from the trip (including photos of some other insects and nature I encountered).