Bark beetle competition with its own ‘mutualist’
Beetles represent 40% of all the insects, and 25% of all the animals living on Earth today.
J. B. S. Haldane was a famous evolutionary biologist in the early and mid 1900s, who still held onto mysteries about the natural world. One of those mysteries, was why beetle diversity was so outrageously abundant. Once he exclaimed “…the Creator must be inordinately fond of beetles.” When the theory of evolution was new, it was met with outrage from the religious community, which led to tongue-in-cheek comments like the one above from individuals that understand evolutionary processes. With around 400,000 species of described beetles, these insects within the Order Coleoptera represent nearly 40% of all of the insects, and 25% of all of animal life on Earth!
The Red turpentine beetle (Dendroctonus valens)-the insect of focus in todays post.
Some morphologies, like the structures beetles possess, enhance a species fitness so much, those traits are preserved for millennia. A process called adaptive radiation ensues, which results in a large diversity of closely related species with similar traits. Already, I have talked about beetles more than once (1, 2, 3). With so many of them, they were just bound to interact with the forest floor. Today, I learned that a species of bark beetle that has evolved a close ‘symbiotic’ relationship with a fungus, may have a more convoluted relationship with the fungus than we previously thought.
Bo Wang and his team of scientists back in 2012 set off to examine this relationship a bit closer. They initially wanted to analyze this species interaction because some observations in the field showed more of an antagonistic relationship between the fungus and the beetle larvae. Additionally, bark beetles are among the most destructive forest inhabitants, as they can greatly reduce an area’s economic value and ecological functioning. For these reasons, funding for this type of research is easily acquired.
Utter destruction from pine beetles on Cascade mountain in the Adirondacks.
The spores of ophiostomatoid fungi associated with bark beetles hitch a ride within structures called mycangia. These fungi are inoculated into tree phloem, once female beetle adults bore their way in to lay a clutch of eggs. If all goes well, the fungal spores germinate and a network of mycelia grows, decomposing the phloem tissues. When the beetle eggs hatch, they begin feeding on the growing mat of fungus, until they’re ready to bore out of the comfort of their woody home, to find a mate and another suitable tree. The fungus is vertically transmitted to subsequent offspring, as spores collect in mycangium before it exits the tree, allowing the cycle to (again, hopefully) repeat itself.
Fungal spores hitching a ride in a pit mycangia.
The beetle of focus in this study is the destructive Dendroctonus valens. In its native range of North America, Guatemala and Honduras it’s not too much of an ecological nuisance. But since its invasion to Chinese forests, the beetle has lay to waste more than 10 million Chinese pines! So, before studying how the fungi interact with larval health, these researchers had to find out which species of fungi where associated with the beetle. They found nearly eleven species of fungi that allied with D. valens. They then created a uniform phloem based agar media and grew the different species of fungi on separate plates. After a few days, fungal mycelia covered the plates and beetle larvae was placed on the different agars. Control plates contained no growing fungi, so these larvae fed on the phloem agar directly.
Hyphal strands of ophiostomatoid fungi, breaking down its woody substrate.
The amount of time feeding and larval weight change was recorded after 6 days. Additionally, the nutrient content of each plate was analyzed and paired with the larval results. All of the fungi used in the experiment dramatically reduced both the fructose and glucose within the media. Seen by the reduced weight in larvae that fed on fungus inoculated agar, the fungi associated with this beetle represents an antagonistic relationship. Here, we can see that these two species compete for phloem sugars within their host trees.
All the fungus tested reduced larval health, as the fungi reduced the amount of available sugars. Wang et al. 2012.
Today I learned that there are ophiostomatoid fungi that have evolved an antagonistic ecology rather than a symbiotic one. Now, one can speculate that this relationship between the different associated fungi and this species of beetle may have started as a mutualism, only to later turn into more of a parasitism, with the fungus taking the upper hand. Beetles are ancient organisms that made their way onto Earth’s scene during the Carboniferous period, along with woody trees. We know that Earth’s atmosphere was different, so maybe these trees produced more sugars, promoting both ophiostomatoid fungi and larval growth. Whatever the case, today, these fungi have a negative impact on Dendroctonus valens. But even so, the beetle is wreaking havoc on Chinese forests. Just think of the utter annihilation if D. valens paired with a fungal species that actually enhance its fitness..