The Weevil Beetle Empire, or: Lessons from the Euthanists, Secondary Mortality Agents and Climate Change
Figure 1. Head of a Dendroctus pondersa that feasts of pine trees.
Here we take a look at the symbiotic relationship between Bark Beetles and Ambrosia Fungi, whereby a piggyback ride service with a concession stand either on/in the beetle’s back/mouth has turned them into a formidable tree-killing parasitic duo.
POST BY: Alexandra Parvaz
Note: This is a rather late submission meant for the last week of January 2016. Due to some seriously restrictive governmentally-inhibited internet service I faced whilst traveling abroad in Iran (still! a most fascinating country of immense beauty!) I had to delay the post until now. And so, here we shall go:
For over 60 million years, Scolytidae beetles or Bark Beetles have been perhaps the world’s oldest farmers and taxi cab drivers. Of the 3-6,000 different Scolytid species, several have evolved specialized structures in their exoskeletons called Mycangia that collect and support a diverse garden of fungal spores. Mycangia are pits or micro-hair brushes near the beetles’ mandibles and/or prothorax (an insect’s body segment closest to their heads) that serve as mini cabins for transporting the fungi within and between tree hosts that both beetles and fungi symbiotically work together to feed on.
Figure 2. A photo series of Increasingly magnified images of a bark beetle’s prothotax where the mycangia is located.
The Bark beetle family features a range of bark diets and tree hosts, with most specializing on the bark of weakened or dying trees among pine, fir, elm tree, oak, birch, as well as fruit orchard trees including olives and pears. Many beetles and their fungal symbionts are tree species-specific, like the Jeffrey Pine Beetle that attacks only Jeffrey pine. Others, like the Fir Engraver attack several species of trees but they are all within the fir tree family.
Figure 3. Token Bark Beetle, fungi and tree victim, larger than life size
In addition to wood, though, the beetles, no bigger than a grain of rice, dine on their microscopic fungal spore passengers for crucial nutrients, building a complex relationship with deeply interwoven life cycles. All bark beetles undergo 4 stages of metamorphosis; from an egg, to plump larvae, to pupae, and finally a fully-fledged adult.
Figure 4. A graphic demonstrating the lifecycle of a Mountain Pine Beetle.
As an adult, a female’s sensitive sense of smell will pick up on an intoxicating aroma, an ‘eau de distress” pheromone that sounds an alarm that a tree is diseased or suffers from drought or a nutrition deficiency. This chemical fragrance wittingly or not allures the prospective female to the stricken tree’s whereabouts. When she lands on her target, the beetle picks up fungal spores, thousands of which pepper the surface of the tree and are coated in a sticky material that can conveniently latch onto a passerby beetle’s exoskeleton.
Figure 5. Picture of a spore mass that sticks to the beetle's mycangia and along the tree's cambium.
The spores then enter the mycangia, which may or may not be lined with glands that secrete a suite of fatty acids, phospholipids, and sterols that nourish the fungi as it develops. With pampered fungi in tow, the female proceeds to tunnel her way through the cork and bark, munching on the phloem tissues and life support system that transports sugars and metabolites up, down and throughout the tree.
Figure 6. A cross section of a woody stem. The Bark beetles infest only the uppermost cambial regions that in this picture includes the vascular cambium and the phloem, except for the periderm.
Most of the beetle’s lifecycle from larvae up until it metamorphoses into an adult is spent nestled within the phloem.
Figure 7.
While inside the tree stem, the beetle helps inoculate and spread the spores around like a salt-shaker, planting them into deeper depths of the tree where water-conducting tissues called the xylem busily pump the tree’s lifeblood. There the fungi breaks down the tough woody tissues into easily digestible compounds, feeding itself and beetle in keen symbiotic fashion.
One way to tell whether, for example, Ophiostoma and Ceratocystis fungi have been spread about successfully by their own beetle partner, the Mountain Pine Beetles is evidence a glorious greenish-blue stain dying the innermost xylem layers of the tree that the beetles themselves can’t reach on their own. Once a blue-stain invasion happens, though, the tree is already dying as the infection hastens it’s death.
Figure 8. Blue stain fungi radiating throughout the sapwood
Fashion industries like the company Bad Beetle have already been trying to make a killing off of recycling dead stained pine trees to manufacture some interesting wood based products from iPod cases to coffins.
Figure 9. Bad Beetle's iPad cases
While many of these beetle species can chomp their way through the bark by themselves, it’s an energy intensive and draining process and much of the wood’s nutrients remain locked up in cellulose totally inaccessible to the beetle. Symbiosis with the fungi, however, unleashes benefits for both hairy parties, ensuring larger broods, and plumper descendants. In return for the beetle offering food, UV protection, and a taxi service to various parts of the tree, the fungi feed the beetle with concentrated nitrogen, pre-digested cellulose, and protection from other harmful fungal pathogens.
A male soon follows a female down the tunnel’s she’s made and mates with her, whereby a pregnant female will lay several dozen eggs along the sides of its tunnels, called a parental gallery.
Figure 10. Gallery of Pine Engraver Beetle with larvae chomping about.
As the larvae hatch, they will feed both on the fungi the parents ‘farmed’ as well as on the bark itself. The larvae burrow their own tunnels that radiate away from the parental base, spreading all along like a rapidly building subway system throughout the tree’s circumference. By effectively cutting off all the phloem, they are girdling the tree and starve it from its own food. The fungi only speeds up the tree’s starvation as it eats away at its water-pumping xylem tissues for a systematic tree-kill. As the larvae pupate and turn into adults, they’ll bore out of the tree and follow the scent trail to another distressed tree and continue the cycle, often already fungal-coated.
Below is a startling and mesmerizing array of different beetle gallery formations.
Figure 11. Carving in Elm tree by a Scolytus Beetle with 'Ophiostoma ulmi' Fungi in tow. The central tunnel that looks like a spine is the parental gallery
Figure 12. Egg gallery of an Ips Bark Beetle just underneath the bark of a conifer.
A swift and coordinated killing occurs right after the first or pioneer beetle bites into the bark, triggering the cooking up of aggregation pheromones. These chemical signals then call legions of more beetles to descend on the tree for what is called a Mass Attack.
Figure 13. Massive Attack- The eponymous name of the Bristolian trip-hop band. By ridiculously ironic design or serendipity, their Mezzanine album cover features a Stag Beetle, which…don’t actually practice mass attacks, but behold! the beatific body.
http://www.vevo.com/watch/massive-attack/angel/GB1209800350
'Angel' music video that may be an anthropomorphized story of a bark beetle/fungal attack and response.
The congregation and mass gathering of beetles may spill over to neighboring trees, resulting in a “GROUP KILL”. Once a tree’s crowded-out and engorged with beetles, these highly chemically sensitive beetles send out an anti-aggregation pheromone to stop the free-for-all party and move on to fresher targets.
But! The tree is not a passive creature. Depending on the health of the tree and wealthy access to water (somewhat of a rarity), trees wage an in-your-face defense such as flooding the beetle tunnels with resin to drown the bloody buggers in their sap and flush them outside of their woody body chambers.
Figure 14. A smattering of pitch tubes on a pine tree infected with Southern Pine Beetle
Pitch tubes signify the expelled resin from the beetle holes that are often filled with sawdust called “frass” that when oxidized and exposed to air, the yellowish translucence turns a rusty red.
A successful flooding looks like this:
Figure 15.
Without copious amounts of resources or enough tree vigor, however, the tree’s doomed. Other signs of doomedness is crown fading, where the leaves change from green to yellow, brown and then a rusted blood red and ultimately gray, whereby the pine needles fall off the tree.
Figure 16. A picture of dead and dying lodgepole pine trees and a forest completely infested.
Although these may seem like indiscriminate parasites, they’ve an ecological niche, or role as the first-responders to an already dying tree and are known as Secondary Mortality Agents. Although these may seem like indiscriminate parasites, they have an ecological niche, or role as the first-responders to an already dying tree and are known as Secondary Mortality Agents. They are among the first decomposers to recycle the tree’s nutrients back into a forest or ecosystem. As mentioned before, the beetles mostly target trees that are unhealthy and have been weakened by any number of problems, like too high temperatures or limiting resources like water, making them ever more susceptible to an attack.
Today, a combination of risky forces has triggered an explosive epidemic of hungry beetles and fungi that are decimating thousands of acres of heat-stressed forests around the world, killing billions of trees on a scale ten times larger than ever recorded.
Figure17. A snapshot of a mass of dead whitebark pine trees by the hand of Mountain pine beetles in Bridger-Teton National Forest, Wyoming
Increasing global temperatures due to increased in greenhouse gases in the atmosphere from human activities has shortened winters and expanded beetles’ reproductive season, resulting in more beetle bern (wee-ones) every year. Hotter conditions have prolonged droughts and dried up water reserves, drying out forests and, as a result weakening trees’ ability to defend themselves from the ever growing attack. In the US, the Rocky Mountains have faced more warming than any other region in the country, where tens of tree species including Whitebark Pines, Aspens and Pinon pines have faced catastrophic declines.
Figure 18. An illustration of a growing bark beetle attack
Whitebark mortality has hit 90% in recent years, and may be thrown onto the endangered species list. The introduction of invasive bark beetle species from the global timber trade has also strained our nation’s forests, however the majority of beetle pressures are coming from our very own native varieties, including Mountain Bark, Jeffrey Pine beetle, Fir Engraver Beetle, and Western Pine Beetle.
Figure 19. A Dendroctonus species; so small, roughly 5-8 mm, and yet behemothic in number
To combat the beetle threat, the US forest Service has waged a deforestation campaign to weed out the diseased and overly crowded forests that, due to a combination of poor forests management over the past 100 years have become extremely susceptible to infestation. Scientists like Diana Six, however, have complained this measure is wiping out trees that may reveal a robust resistance. Other concerns are that we might be ignoring an important perspective of the beetles as mobilizers of an irrevocably changing forest ecosystem.
Check this out. Diana Six giving one of those mind-blowing TedTalks on her cool research and ways to save forests and build healthier relationships with beetles.
Along with better understanding the effects of climate change, protecting our forests and supporting healthier relations with the beetle and the fungi, it might not be all that bad to give in to the consumerist hook and buy some new blue-stain pine iPhone cases and posh coffins, just for the future.
References:
Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 , 15, 16, 17, 18, 19
Arizona Forest Health: Bark Beetle FAQ." Arizona Cooperative Extension. University of Arizona. http://ag.arizona.edu/extension/fh/bb_faq.html Last Reviewed and Updated: August 2010January 23, 2013. Retrieved January 26, 2016.
-Bent, Barbara et al. 2010."Climate Change and Bark Beetles of the Western US and Canada: Direct and Indirect Effects”.BioScience. Vol 60. No. 8.602-612.
-Bateman, Maddie. "Bark Beetles are Decimating Our Forests. that Might Actually be a Good Thing”. 2015. Mother Jones. http://www.motherjones.com/environment/2015/03/bark-pine-beetles-climate-change-diana-six
Harrington, Thomas. "Ecology and Evolution of Mycophagous Bark Beetles and their Fungal Partners". 2005. Ecological and Evolutionary Advances in Insect-Fungal Associations, F. E. Vega and M. Blackwell, eds. Oxford University Press. Pages 257-291
-Klepzig, K.D and D.L Six. 2004. “Bark Beetle-Fungal Symbiosis: Context Dependency in Complex Associations”. Symbiosis. Vol 189-205
-USDA Forest Service Pacific Southwest Region. 2015. "Bark Beetles in California Conifers: Are your trees Susceptible?”Forest Health Protection-California. Pg.1-12. http://learnmoreaboutclimate.colorado.edu/uploads/model-lessons/mountain-pine-beetles/bark-beetles-brochure.pdf.
-Rocky Mountain Forests at Risk: Confronting Climate-Driven Impacts from Insects, Wildfires, Heat and Drought. Executive Summary. 2014. Union of Concerned Scientists and The Rocky Mountain Climate Organization. http://www.ucsusa.org/sites/default/files/attach/2014/09/Rocky-Mountain-Forests-at-Risk-Executive-Summary.pdf.
-Bark Beetles: Field Guide to Insects and Diseases of Arizona and New Mexico Forests. Retrieved January 2016. http://www.fs.fed.us/r3/resources/health/field-guide/bb/
-University of Nevada, Reno, Cooperative Extension: Bark Beetles in the Lake Taho Basin. https://www.unce.unr.edu/publications/files/ho/other/fs9840.pdf. Retrieved January 16 ,2015
-Lee. J.C. et al. "Invasive Bark Beetles". Forest Insect and Disease. US Department of Agriculture. US Forest Service. Pg 1-12. http://www.na.fs.fed.us/pubs/fidls/invasive_bark_beetles/inv_bark_beetles.pdf
Davis, Rayn. Bark Beetles. Utah Pests Fact Sheet. ENT-165-12 . pg1-9. http://extension.usu.edu/files/publications/factsheet/bark-beetles2012.pdf