Deep in a forest in Western Canada, a wounded pine tree is taking its last breath after being invaded by dozens of mountain pine beetles. The beetles chew through the tree’s wooden torso, lay their eggs underneath the bark and eventually zoom away to find their next victim.
A lengthy flight to reach the next tree is not a discouraging factor for these voracious mountain beetles looking to colonize a new tree host. Instead, researchers at the University of Alberta were surprised to find that the further female beetles travel, the stronger their ability to attract other beetles from distant populations gets.
Mountain pine beetles have invaded more than 18 million hectares of forest in Western Canada over the past three decades and are responsible for widespread pine mortality in British Columbia, reducing the growth of millions of trees.
While some pine beetles choose to colonize a healthy pine tree two metres away from their previous tree host, some travel more than 30 kilometres — a strategy that lead researcher and graduate student Kelsey Jones says can be beneficial for the beetles but devastating for the trees.
By attaching a beetle onto a computer-linked flight mill, recording the distance they flew and then allowing them to bore into trees within her lab, she found that female beetles who fly further produce higher amounts of trans-verbonel pheromone. This pheromone signals their arrival to other beetles and can facilitate mass attacks on the trees.
"It’s very beneficial for these beetles to produce more [trans-verbenol] because it attracts both males and females to the new host tree. They need to attract hundreds and thousands of beetles to a tree to be able to overcome those tree defences,” she says, explaining that a “doomed” tree is one that typically has more than 40 attacks per square metre. However, beetles can kill smaller and weaker trees with less attacks per square metre.
If mountain pine beetles produce more of this pheromone, Jones adds they have more opportunity to search for a good quality host.
“They’ll experience less offspring competition because if you are flying a short distance to your natal host, chances are there are a bunch of beetles who are also taking it easy and flying short distances,” she says. “Those closer trees aren’t going to have as healthy offspring because they’ll be so densely packed in there that they’ll have to fight for resources.”
By flying further away, Jones says these beetles find that “sweet spot” where they are still able to attract more beetles to help overcome the tree’s defences, but not so many beetles that the offspring will then suffer. Instead, the beetles who are successful in choosing the riskier strategy will produce more robust offspring.
Jones points out that the beetles that are playing it safe by flying shorter distances are still beneficial to the population. Their chances of not running out of resources are better and the risk of predation during flight is much lower, which means they can focus on host colonization on nearby trees.
Though more research still needs to be done, there are some possible explanations for why more trans-verbenol pheromones are released the more the beetles fly. Since all beetles are exposed to the same amount of pheromone precursor when they are born, Jones suggests that the fat that is burned during the beetle’s flight is linked to the release of these pheromones.
This is the opposite of what she expected.
“I looked at my results and was not expecting it whatsoever. The first thing I did was look at the data to make sure I didn’t input numbers wrong,” she laughed. “I was very convinced they would produce less of this pheromone because they use so much energy during flight.”
Though Jones is not currently working on follow-up research to this study, she says it would be interesting in the future to study how longer flight distances influences egg and offspring production.
As many people are concerned about large populations of mountain pine beetles moving eastward into Canada’s boreal forest where forest stands are thinner, Caroline Whitehouse, a forest health specialist for Alberta's Ministry of Forestry, says this is useful research when discussing mitigation strategies.
“This isn’t alarming news, but rather it gives us a mechanism so we better understand the reasons why they behave this way,” she says, adding that she was excited to hear about this research. “Understanding mountain pine beetle dynamics and pheromone communication is important for us when we think about monitoring these mountain pine beetles.”
There are provincial mountain pine beetle monitoring programs that collaborate with researchers to understand how mountain pine beetle populations move and expand their populations. With this research, the government is now looking at how they can continue to tweak their pheromone monitoring program.
“It’s so important for the government to support research like this,” she says. “We can discover new things which helps us better understand population dynamics and can help us make science-based decisions that affect our provincial operations and policies.”