Tree-Killing Worm, Please Make Your Way To The Boarding Gate

Meet the microscopic worm that can kill forests. It’s called the pinewood nematode (Bursaphelenchus xylophilus) and though each one is barely a millimetre long, they can collectively bring down a towering pine tree.

Once it gets into a tree, the nematode spreads through the trunk, branches, and roots. It feeds on the cells lining the pine’s resin canals, causing resin to leak into other vessels that carry water around the tree. Cut off from their water supply, the pine’s needles turn yellow and brown. The change is dramatic. Within just a few months, the tree wilts and dies. This pine wilt disease has claimed millions of pines, especially in Europe and East Asia, causing huge problems for ecosystems and the forestry industry alike.

But the nematode can’t spread to new trees on its own. Instead, it hitches a ride on pine sawyer beetles—substantial, finger-sized insects with long antennae and sharp mandibles.

The nematodes hide inside the breathing tubes that carry oxygen around the beetles’ bodies. They emerge only to enter the open wounds caused when the insects nibble on pine twigs. They spread through the tree and kill it—that’s perfect for the beetles, which lay their eggs in dead or dying wood. Their larvae eat the wood until they transform into adults and fly off in search of fresh pines. And before they do, they’re boarded by nematodes.

These two partners enjoy a tight, mutually beneficial relationship. The beetle brings the nematode to fresh victims, while the nematode creates the conditions that the beetle needs to reproduce.

But timing is everything. The nematode goes through four larval stages and it can only colonise beetles in the last of these—L4. The beetles, meanwhile, can only host the nematodes as adults. Fortunately, the duo have a way of synchronising their complex life cycles.

Lilin Zhao from the Chinese Academy of Sciences has found that the beetles produce a chemical signal when they transform into adults. This tells the nematode that its ride will soon be ready, and that it should enter the L4 stage. It’s a molecular announcement that says the insect will soon be ready for boarding.

Zhao’s team first noticed that although the beetle grub is surrounded by nematodes inside a pine tree, very few of these worms are in the L4 stage. They only transform en masse when the insect reaches the last stages of pupation and is about to emerge as an adult.

To work out why, the team analysed the chemicals on the beetle’s surface at different life stages. They found four that are mass-produced at the end of its pupal stage. When the adults emerge, they give off waves of this chemical stew. And when Zhao dabbed the cocktail onto stage-three nematodes, they quickly transformed into stage-four.

The substances in question are all fatty acid esters—long chains of carbon and hydrogen atoms with a couple of oxygen atoms at the end. Insects commonly use these chemicals as pheromones, but the pine sawyers make four very specific ones—ethyl palmitate, ethyl stearate, ethyl oleate and ethyl linoleate—and they do so in bulk.

These signals are very specific. Similar esters don’t trigger the nematodes’ transformation, nor do those from beetles other than the pine sawyers. That’s important—many beetles bore into wood and it would be counter-productive for the nematodes to get a boarding announcement from an insect that can’t serve as its host.

It’s possible that this discovery could be used to save pine trees from the nematodes, by somehow disrupting the worms’ ability to synchronise their lives with that of their beetles. But for now, it serves as yet another example of the intimate partnerships that exist between different animals, and the signals that seal those alliances.

Reference: Zhao, Zhang, Wei, Hao, Zhang, Butcher & Sun. 2013. Chemical Signals Synchronize the Life Cycles of a Plant-Parasitic Nematode and Its Vector Beetle. Current Biology http://dx.doi.org/10.1016/j.cub.2013.08.041