If you weren’t born before the 1980s, you probably don’t know what an entire street lined with elm trees looks like, because Dutch Elm Disease spread through both Europe and North America in the early and middle 1900s and decimated elm populations. Pockets of elms still persist in places like Amsterdam and Winnipeg, but it is a never-ending battle to keep those trees disease free.
So, which highly virulent pathogen is responsible for totally reshaping temperate tree communities? The culprit is a fungus (well, a few fungal species, actually) that is vectored by a tiny and totally adorable beetle (well, a few beetle species, actually). Ladies and gentlemen, I present to you the horrible, terrifying, death-spreading elm bark beetle:
Joking aside, these beetles are completely amazing. They have symbiotic relationships with fungi, where the fungi range from weak parasites to commensalists to mutualists depending on the beetle species, the fungus species, and perhaps environmental conditions. In the simplest cases, bark beetles act as transport vessels for the fungi, without getting anything in return for their dispersal services. In contrast, ambrosia beetles cultivate fungus gardens in the galleries that they excavate in dead trees, and the beetles consume the fungus as their sole source of nutrition. When the young beetles emerge from their natal galleries to disperse, they take fungal spores with them to their new galleries. Isn’t that cool?! Whereas fungus farming has evolved just once each in ants and termites, it has evolved many times in the ambrosia beetles (Hulcr and Dunn 2011). And get this: at least one species of ambrosia beetle is eusocial!
If bark beetles and ambrosia beetles typically only excavate in dead trees, why did the elm bark beetle go rogue and start attacking live elms? Well, it turns out that it wasn’t an isolated event. There are more than a dozen examples of the beetles shifting from dead to live trees in recent history, with catastrophic results for some of the tree species that are being attacked (Hulcr and Dunn 2011). Humans are unintentionally shipping these beetles and their fungal associates to novel regions around the globe. And in novel regions, bark beetles searching for dead trees by following volatile cues might mistake living trees for dead trees. Or, as Hulcr and Dunn (2011) put it, some living trees might smell dead. Even if the beetles realize their mistake and don’t completely excavate a gallery in a living tree – choosing instead to go search for a dead tree – their initial boring activities might inoculate the tree with the fungal symbionts.
But here’s a conundrum: if a fungal symbiont transported by bark beetles doesn’t really affect trees in the beetle’s native range, why should the fungus be highly virulent in the introduced range? After all, the beetles can only introduce a little fungal innoculum into a giant living tree. Well, it may be that the majority of the tree pathology is caused by the tree’s response to the pathogen, rather than the direct actions of the fungus, just like the animal immune response to pathogens is often worse for the host than the actual damage caused by the pathogen. It may be that trees wildly overreact to the novel fungal pathogen by expanding the walls of the xylem so much that the tree ends up dying.
Anyways, bark beetles are really cool vectors, and I think disease ecologists should pay more attention to them.
Hulcr, J., and R.R. Dunn. 2011. The sudden emergence of pathogenicity in insect–fungus symbioses threatens naive forest ecosystems. Proceedings of the Royal Society B 278: 2866–2873.